Zirconia-Based Solid Acids: Green and Heterogeneous Catalysts for ...

Current Organic Chemistry, 2011, 15, 3961-3985

3961

Zirconia-Based Solid Acids: Green and Heterogeneous Catalysts for Organic Synthesis Meghshyam K. Patil,a Avvari N. Prasad,b and Benjaram M. Reddyb* a

Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Sub-Campus Osmanabad 413 501, Maharashtra, India b

Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Hyderabad - 500 607, India Abstract: This review highlights the application of sulfated, molybdated and tungstated zirconia solid acid catalysts, and their modified forms for variety of organic synthesis and transformation reactions in the liquid phase. Most of these catalysts offer significant improvements in various organic reactions with regard to the yield of products, simplicity in the operation, reusability of the catalysts and green features by avoiding toxic conventional catalysts. Preparation of various zirconia-based solid acid catalysts has been briefly described. Characterization of these catalysts by different techniques has also been presented. Most of these catalysts are highly promising for numerous organic reactions in the liquid phase which include condensation, isomerization, esterification and transesterification, muticomponent reactions and so on.

Keywords: Superacid, sulfated zirconia, molybdated zirconia, tungstated zirconia, tetragonal phase, organic synthesis, solvent-free. 1. INTRODUCTION Solid acids and superacids have been the subjects of everlasting interest owing to their numerous applications in many areas of the chemical industry. The application of acid catalysts is very rampant in the chemical and refinery industries, and those technologies employing highly corrosive, hazardous and polluting conventional liquid acids and Lewis acids such as H2SO4, HCl, HF, HClO4, H3PO4, AlCl3, BF3, ZnCl2 and SbF5 are being replaced with solid acids such as clays, zeolites, heteropolyacids, ion exchange resins (Amberlyst and Nafion-H) and metal oxides. Some of these solid acids and superacids are characterized by various advantages which include easy handling, simplicity and versatility of process engineering, catalyst regeneration, decreasing reactor and plant corrosion problems and environmental safe disposal [1-15]. Till date, a number of organic syntheses and transformation reactions have been conducted with solid acids leading to better regio- and stereo-selectivity/specificity which depend on the strength of the acid and type of acidity (Brønsted or Lewis). Over the past few decades, zirconia (ZrO2) based solid acids have received much attention, among other solid acids, due to their superior catalytic activity for hydrocarbon conversions at mild conditions [8,14-18]. These catalysts are finding numerous applications in oil refinery and petrochemical industries. Among the promoted zirconia-based solid acid catalysts, the SO42/ZrO2 (SZ) catalyst become more popular from 1979 when Arata and co-investigators [16,19] reported that zirconia, upon proper treatment with sulfuric acid or ammonium sulfate exhibits extremely strong acidity and is capable of catalyzing the isomerization of n-butane to isobutane at room temperature. Over the period of time the SZ catalyst has also been reported to be very active for various organic syntheses and transformation reactions including multicomponent reactions, isomerization, alkylation, acetylation, esterification, glycosidation and some other commercially useful reactions [20-26]. However, a major disadvantage associated with SZ catalyst is its rapid *

Address correspondence to this author at the Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Hyderabad–500 607, India; Fax: +9140 2716 0921; E-mail: [email protected], [email protected] 1385-2728/11 $58.00+.00

deactivation under both high temperature and in reducing atmosphere owing to the formation of SOx and H2S, respectively. Also it forms sulfuric acid if there is water in the reaction medium. Many efforts were made to improve the activity and stability of the SZ catalyst which include promotion of the catalyst with transition metals such as Fe, Mn and Cr, and with noble metal Pt, as well as with carbon molecular sieves [27]. In 1988 Arata and Hino reported that solid superacids could be synthesized by incorporating WOx or MOx into the Zr- or Ti-hydroxides under certain preparation conditions [28,29]. Our extensive investigations also confirmed their findings, and we have explored some of these catalysts for various organic reactions during the last fifteen years. Using Hammett indicators, Hino and Arata [16] observed that SZ is an acid 104 times stronger than 100% sulfuric acid. Acids stronger than 100% sulfuric acid are generally referred to as superacids [2,17,18]. The strength of an acid can be characterized by the so-called Hammett acidity function, H0. The greater the value of the function, the stronger is its acidity. The value of H 0 for 100% sulfuric acid is 12. Therefore, SZ catalyst with H0 = 16 is considered as the strongest halide-free solid superacid [4,30-32]. Subsequent investigations revealed that sulfate-free ZrO2-based solid superacids could be synthesized by incorporating molybdate or tungstate promoters under certain preparation conditions [4,3038]. The typical H0 values reported for SZ (650 °C), WOx/ZrO2 (800 °C) and MoOx/ZrO2 (800 °C) catalysts calcined at different temperatures are 16.1, 14.6 and 13.3, respectively, which reveal the superacidic character [17,18,38,39]. There were some doubts in the literature whether SZ is ‘‘just” as strong acid as H-zeolites, or a true strong superacid [40-46]. More recently, a concerted study from Japan confirmed the superacidity of the SZ catalyst beyond doubt [47]. The primary objective of this review was to summarize recent studies on organic synthesis and transformation reactions catalyzed by various zirconia-based solid acid catalysts, namely, SZ, molybdated zirconia (MZ), tungstated zirconia (WZ) and other modified-ZrO2 catalysts. Preparation and physicochemical characterization of these catalysts have also been briefly discussed in this review. © 2011 Bentham Science Publishers

3962 Current Organic Chemistry, 2011, Vol. 15, No. 23

2. PREPARATION OF CATALYSTS Promoted zirconia solid acid catalysts could be prepared by different methods. The catalytic properties generally depend on the method of preparation, nature of precursors, precipitating agents, promoting agents, method of impregnation, calcination temperature and so on. Also, the activity of catalysts depends primarily on the activation temperature. In this section, methods of preparation of zirconia-based catalysts, namely, sulfate, molybdate, tungstate ion promoted zirconia catalysts and other modified catalysts are presented in brief. 2.1. Preparation of Sulfated, Molybdated and Tungstated Zirconia SZ catalysts are prepared mostly by adopting a two-step or a single step method. Both the preparative methods are equally exploited for the synthesis of this catalyst. In the two-step method, zirconium hydroxide is prepared first followed by impregnation with a sulfating agent [16,19]. To prepare zirconium hydroxide, different zirconium compounds such as zirconium nitrate, zirconium chloride, zirconium oxychloride and zirconium isopropoxide were hydrolyzed with aqueous ammonia or urea [4850]. Sulfate impregnation is carried out by using various sulfating agents such as H2SO4, (NH4)2SO4 [16,19,48-51], SO3 [52] and ClSO3H [53]. The resultant sulfated zirconium hydroxide is then calcined in air at 550-650 °C to generate acidity. Molybdate and tungstate ion promoted zirconia catalysts are also prepared by using the same procedure. Molybdate and tungstate ion impregnation is carried out by using ammonium heptamolybdate and ammonium metatungstate, respectively. Resultant impregnated zirconium hydroxide is then calcined in air at 650-800 °C in order to generate the acidity. SZ and WZ catalysts are also commercially available; Ramos et al. [54] used SZ and WZ catalysts purchased from MEL Chemicals. SZ catalysts are also prepared by a simple one-step sol-gel method, in which organo-zirconium precursors such as zirconium n-propoxide and zirconium isopropoxide were used. In this procedure, to the alcoholic solution of zirconia-precursor, certain amount of H2SO4 solution was added slowly under vigorous stirring until a viscous solution was obtained. The gel was heated at 80 °C to evaporate excess alcohol and calcined at 600 °C for 7 h in air to get the white SZ solid. SZ is also prepared by thermal decomposition of Zr(SO4)2 [55,56]. However, this method did not attract much attention because it does not allow the control of sulfate content. Many groups working in this area have carried out modifications to the above-described methods in order to get better surface area, mesoporous structure and more tetragonal zirconia phase [57-66]. Morterra et al. [60] prepared SZ catalyst by hydrothermal precipitation route, and Rubio et al. [67] synthesized SZ using ammonium zirconium carbonate. Also, various surfactants such as triblock copolymer and cetyl trimethyl ammonium bromide (CTAB) were also used in order to generate high surface area. 2.2. Preparation of Other Zirconia-Based Solid Acids As mentioned earlier, zirconia-based catalysts were modified by using appropriate salts of Fe, Mn, Cr, and Pt, as well as with carbon molecular sieves in order to improve the physicochemical and catalytic properties [27]. Occelli et al. [68] prepared Cu, Fe, Mn and Fe-Mn promoted SZ catalysts by adopting a two-step precipitation-impregnation method by employing CuSO4, Fe2(SO4)3, or MnSO4 precursors. Many research groups are

Patil et al.

working in this area owing to the commercial significance, and reported the preparation of Al, Fe, Mn, Cr, and Pt modified SZ catalysts [69-73]. Interestingly, some mixed oxides also exhibited strong surface acidity (Brønsted or Lewis) due to the generation of excess negative or positive charge in the model structure of the binary oxides. Mixed oxides such as SiO2-ZrO2 [74] and Al2O3-ZrO2 (AZ) [75] lead to very strong acidic properties, whereas TiO2-ZrO2 (TZ) was not only a strong acid but also had a distinct basicity [76,77]. Reddy and co-workers synthesized sulfate promoted CeO2-ZrO2 and sulfate, molybdate and tungstate ion promoted TZ and AZ catalysts using two-step procedure i.e., co-precipitation followed by impregnation method [75,78,79]. Various research groups working in this area also reported preparation of sulfated AZ, sulfated TZ and sulfated ceria-zirconia (Ce-ZrO2/SO42) catalysts [80-85]. Also, Negrón-Silva et al. [86] prepared MCM-41(Mobil Composition of Matter No.41) supported SZ catalyst from siliceous MCM-41 and zirconium sulfate. Guo et al. [87] reported SZ catalyst supported

on multi-walled carbon nanotubes. Zhao et al. [88] reported synthesis of SZ supported on mesostructured -Al2O3 and other combinations. 3. CATALYST CHARACTERIZATION Modified or promoted zirconia solid acid catalysts were extensively characterized by various spectroscopic and nonspectroscopic techniques such as X-ray diffraction (XRD), Xray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), Raman spectroscopy (RS), differential thermal analysis/thermogravimetric analysis (DTA/TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and other methods in order to understand the bulk and surface properties. All characterization results revealed that the incorporated promoter ions show a strong influence on the surface and bulk properties of the ZrO2. Especially, XRD and Raman results suggested that impregnated sulfate, molybdate and tungstate ions stabilize the metastable tetragonal phase of ZrO2 at ambient conditions [49,89]. NH3-Temperature programmed desorption (TPD) and Brunauer-Emmett-Teller (BET) surface area results indicated that the sulfated catalyst exhibits enhanced acid strength and higher specific surface area than other promoted and unpromoted samples [49]. Recently, Park et al. [90] reported a

highest acid strength for 20 wt.% tungstate ion promoted zirconia catalyst among other catalysts having different loadings of tungstate-ion from NH3-TPD study. Also, there are various other methods, which are used to determine the surface acidity of the catalysts such as Hammett indicator method, TPD of various base molecules, model test reactions and so on [4,30,31]. Yu et al. [91] investigated the acidity of modified zirconia catalysts using solid state NMR technique. Sommer and co-workers [92,93] exploited another method where the rate of H/D exchange for methane has been used to measure the relative acidity and reactivity of SZ catalysts. However, all these methods are not versatile for different types of solid acids. Various spectroscopic techniques were extensively used to find out the exact structure of the SZ and other solid acid catalysts. Various structures are proposed for the SZ catalyst as shown in Fig. 1. The structures (a) and (b) were proposed by Jin et al. [94] and Ward and Ko [95], respectively, using various characterization techniques namely, XPS, IR spectroscopy, in situ and ex situ diffuse reflectance infrared Fourier transform (DRIFT) and XRD. Kustov et al. [96] suggested the structures (c), (d) and (e) based on diffuse reflectance IR spectroscopy; structures (c) and (d) are responsible

Current Organic Chemistry, 2011, Vol. 15, No. 23 3963

Green and Heterogeneous Catalysts for Organic Synthesis

(a)

O

O

(b)

O

* OH

S O

Zr

O

Zr

O

O

Zr

(e)

Zr

O

O H S O Zr

O O

O

Zr

O

O

H

OH HO S

S

O Zr

Zr

S

O

Zr

S

O

O

O

# Zr

O

(f) O Zr O O

O

Zr Zr

Zr

O (j)

O

O

O

O

(i)

(h)

O

O H

(c)

Zr

O O

O

Zr

O

O

Zr

O

Zr

O

O

O

O

O

(g)

O Zr

O

S

S

O

O

O

O

Zr

Zr Zr

Zr

O

(k)

S

O S

O

S

Zr Zr O O O O * Brönsted acid site; # Lewis acid site

(d) O

O S

O O

O S

Zr

O

O

H

H

O O

Zr

O

Fig. (1). Structures suggested for sulfated zirconia by various groups.

for Brønsted acidity where as structure (e) is responsible for Lewis acidity. Structural models (f) and (g) were suggested by Saur et al. [97]; (h) and (i) were proposed by Rosenberg and Anderson [98]. On the other hand, structural model (j) was suggested by Vedrine et al. [99] and (k) by White et al. [100]. Likewise, based on several physicochemical characterization results the surface structure of WZ has also been proposed as shown in Fig. 2 [101]. It is normally believed that tungsten oxide could exist on the zirconia surface in the form of polyoxotungstate clusters as presented in Fig. 2. Sohn et al. [102] prepared solid superacid catalyst Ce-ZrO2/SO42 and suggested the structural model presented in Fig. 3 based on IR spectroscopy. O Zr

O O

O W

O

Zr

O

ZrO2 Support

O

O W

W O

O

O

W

ZrO2 Support

Fig. (2). Proposed mono-tungstate and poly-tungstate on ZrO2 support in WOx/ZrO2 catalysts. O O O O S S * HO O O O O # Zr Zr Ce Zr Zr+ O O O O O * Brönsted acid site; # Lewis acid site Fig. (3). Proposed structural model for Ce-ZrO2/SO42 catalyst.

4. CATALYTIC ACTIVITY OF SULFATED ZIRCONIA Many industrially as well as scientifically important reactions have been investigated employing SZ catalyst because of its strong or superacidic character. Several advantages associated with it include easy handling, non-corrosive nature, water tolerance, easy preparation, and easy recovery and reusability, which make this catalyst highly versatile for numerous applications. After its discovery, it was exploited mainly for vapour phase reactions such as tert-butylation of phenol [103], Beckmann rearrangement of cyclohexanone oxime, and isomerization of butane, pentane and other hydrocarbons [38,104-109]. Recently, Alemán-Vázquez et al. [110] studied heptane isomerization with SZ catalyst having different concentrations of sulfate. Interestingly, the SZ catalyst was found to be very active for various organic synthesis and transformation reactions in the liquid phase, thus facilitating

synthesis of several fine and specialty chemicals. Recently, Thirupathi et al. carried out cyanosilylation of aldehydes [111], Tyagi et al. [112] studied synthesis of acetyl salicylic acid and Sasiambarrena et al. [113] investigated sulfonylamidomethylation of benzylsulfonamides and 2-phenylethanesulfonamides using SZ catalyst in the liquid phase. In this section application of SZ catalyst for several liquid phase reactions is discussed briefly. 4.1. Multicomponent Reactions Multicomponent reactions (MCRs) play a significant role in combinatorial chemistry because of their ability to yield small druglike molecules with several degrees of structural diversity. A MCR is a chemical reaction where three or more compounds react to form a single product [114]. MCRs have been known for over 150 years. The first reported MCR was the Strecker synthesis of aminocyanides in 1850 from which -aminoacids could be derived. As mentioned by I. Marek (the Guest Editor for a special issue of Tetrahedron Symposium-in-Print on MCRs) [115], “the practical construction techniques available to prepare elaborate products are still woefully inadequate. A seemingly trivial but rather serious limitation in practice is set by the mere number of steps accumulating in linear sequences”. MCRs are thus becoming an increasingly significant class of reactions as they allow a number of starting materials to be combined to form a single compound and in a one-pot operation [116]. They, therefore, exhibit an economy of steps and often atom economy, most of the incoming atoms being linked together in a single product. Several MCRs were attempted by using SZ catalyst. Combining the advantages associated with MCRs with heterogeneity and catalysis of SZ reinforces the “greenness” for such reactions. MCRs catalyzed by SZ include Strecker, Biginelli and Hantzsch reaction, and synthesis of acetamido carbonyl compounds. Also other zirconia-based catalysts, namely sulfated ceria-zirconia and WZ, are also used for multicomponent Mannich reaction between aldehyde, ketone and amine. Some of these reactions as reported in the literature are summarized in the following sub-sections. 4.1.1. Strecker Synthesis As discussed above, Strecker reaction is the first reported MCR, discovered in 1850. -Aminonitriles, often synthesized by Strecker reaction, are highly useful synthons for the synthesis of aminoacids [117-122], nitrogen containing heterocycles such as


Patil et al.

an enolizable ketone or -ketoester, and acetyl chloride. Acetamido carbonyl compounds are useful building blocks for a number of biologically and pharmaceutically valuable compounds [133]. Variety of catalysts are used for this MCR such as CoCl2 [134], Cu(OTf)3/Sc(OTf)3 [135] and BiCl3 generated in situ from BiOCl and acetyl chloride [136], CeCl3·7H2O [137], ZrOCl2·8H2O [138], I2 [139], a few heterogeneous catalysts [140-143] and so on. All these methods while contributing towards some advantages also suffer from different negative aspects such as the use of expensive catalysts, longer reaction times, high temperature and low yields. By considering all these aspects, Das et al. [144] studied this MCR by using SZ catalyst. SZ is found to be most efficient in offering good yields (81-94%) of various -acetamido carbonyls within short period of time (1-3 h) (Schemes 3 and 4). In addition, 1,3-diketones (ethyl acetoacetate and methyl acetoacetate) were found to afford the corresponding -acetamido ketoesters with high diastereoselectivity (Scheme 4) and in most of the cases, anti isomer was the major product.

imidazoles and thiadiazoles [123,124], and other biologically useful molecules such as saframycin A [125]. The efficiency of the reaction has been increased by the use of catalysts, and reactive cyanide ion sources. Trimethylsilyl cyanide (TMSCN) is a safer, more effective and more easily handled anion source compared to other sources. The reaction can be catalyzed by indium (III) iodide [126], rhodium (III) iodide hydrate [127], 2-iodoxybenzoic acid (IBX) and tetrabutylammonium bromide (TBAB) [128], and also few supported versions or versions based on heterogeneous catalysts [129-132] that could be found in the literature. Recently, Reddy et al. [20] utilized various zirconia-based solid acid catalysts, namely SZ, MZ and WZ for the MCR. Among them the SZ catalyst was found to be most efficient for this reaction. These reactions proceeded efficiently and various -aminonitriles were produced from the corresponding aldehydes and amines in good to excellent yields within short reaction times (20-360 min) using dry THF as the solvent under N2 atmosphere (Scheme 1). In order to evaluate the possibility of 1,3-asymmetric induction, Reddy et al. extended this MCR to the preparation of optically active -aminonitriles from (R)-(+)-methylbenzylamine, aldehydes and the TMSCN. The reaction was found to produce a mixture of diastereoisomers with both aryl as well as alkyl aldehydes, with one diastereoisomer predominating (Scheme 2). R O+

H2N

R1

H R and R1 = Alkyl, Aryl

H

TMSCN, THF SZ, N2, r.t. 20-360 min

R

C

H N

4.1.3. Biginelli and Hantzsch Reaction The Biginelli reaction is another MCR that creates 3,4dihydropyrimidinones from an aldehyde, -ketoester and urea [145,146]. Dihydropyrimidinones are widely used in the pharmaceutical industry as calcium channel blockers, antihypertensive agents and 1a-antagonists [147]. This reaction was reported for the first time by Pietro Biginelli in 1893. The reaction can be catalyzed by Brønsted acids and/or Lewis acids, and many reagents such as lanthanum chloride [148], Montmorillonite KSF [149], heteropolyacids [150], metal triflates [151], silica supported sulfuric acid [152] and so on were used. In order to find better alternatives and to overcome the drawbacks associated with these catalysts, lot of work has been

R1

CN Yield = 52-93%

Scheme 1.

4.1.2. Synthesis of -Acetamido Carbonyl Compounds Another important MCR involves synthesis of -acetamido carbonyl compounds by reaction of aromatic aldehyde, acetonitrile, O R

CH3 +

H

NH2

Ph

CH3

TMSCN, THF Ph

SZ, N2, r.t.

CH3

CN

N R H (R, R)

+

Ph

CN

N H (R,S)

R

(R, R):(R, S) Yield (%) R ________________________________ Phenyl 4-MeO-Ph i-propyl n-butyl

24:76 08:92

80 78

36:64 37:63

90 85

Scheme 2.

NHAc O CHO

R

O

R1

SZ, r.t. CH3CN R CH3COCl X R1 1-3 h R = 4-Br, 3-NO2, H etc., X = Me, Ar etc. Yield = 72-95 % R1 = Ph, 4-NO2C6H4, 4-BrC6H4, C6H10O, Me +

X

Scheme 3.

NHAc O CHO R

O

R = 4-Cl, 3-NO2, 4-Br, 4-Me Scheme 4.

O

+ R2 = Me, Et

SZ, r.t. CH3CN CH3COCl 2 OR 2-3 h

NHAc O OR2

R

+

OR2

R

O Anti

Yield = 80-88%

O Syn



R1

O H

O +

O OR2

R1

O

X +

SZ, solvent-free H2N

NH2

R2

100 0C

NH N X H Yield = 80-92 %

X = O, S

R1 = H, 4-NO2, 4-OH R2 = Me, Et Scheme 5.

R

R

EtO2C

O H

+

O O

+

H2N

EtO2C

SZ NH2

solvent-free

CO2Et

Me

4h

N H

Me

Scheme 6.

carried out using SZ catalyst. In 2005, Reddy et al. [48] reported synthesis of dihydropyrimidinones by condensation reaction between an aldehyde, -ketoester and urea or thiourea under solvent-free conditions employing SZ catalyst at 100 °C (Scheme 5). These reactions were found to proceed efficiently and various dihydropyrimidinones were produced in excellent yields (80-92%) in short reaction times (40-60 min). Thereafter, Gopalakrishnan et al. [153] and Kumar et al. [154-156] studied Biginelli reaction using SZ catalyst under microwave irradiation. Further, Angeles-Beltrán et al. [157] made a competitive study of Biginelli versus Hantzsch reaction using SZ catalyst and urea or thiourea as amine under solvent-free conditions. Reaction was studied at different temperatures (60, 80, 100 and 150 °C) for 4 h. It was found that formation of Hantzsch product (Scheme 6) increases and Biginelli product decreases with increase in temperature from 60-150 °C, and at 60 °C no Hantzsch products were obtained; at 80 °C, traces of Hantzsch products were noted. 4.2. Protection of Alcohols, Phenols and Aromatic Aldehydes Normally, a protecting group is introduced into a molecule by chemical modification of a functional group, and that protecting group allows overcoming the problems of chemoselectivity in the subsequent chemical reaction. It plays a vital role in multistep organic synthesis [158]. In many preparations of delicate organic compounds, some specific parts of their molecules cannot survive the required reagents or chemical environments. Therefore, these parts, or groups, must be protected [158]. The SZ has been used as a promising catalyst for protection and deprotection of various groups in organic synthesis. Some of these reported examples are described in the following paragraphs. 4.2.1. Synthesis of 1,1-Diacetates from Aromatic Aldehydes Acetic anhydride is one of the most commonly used reagents for formation of 1,1-diacetates from aldehydes. This reaction is catalyzed by Brønsted acids and/or Lewis acids as well as heterogeneous catalysts [159-165]. Negrón et al. [166] utilized SZ catalyst for this reaction, and various 1,1-diacetates were synthesized from aromatic and heteroaromatic aldehydes in excellent yields within 5-8 h at 0 °C (Scheme 7). The deprotection of the resulting acylals was achieved using the same catalyst at 60 °C and acetonitrile as the solvent (Scheme 8). Also, in another work, S. V. N. Raju [167] reported that SZ is an efficient catalyst for the synthesis of variety of 1,1-diacetates from the corresponding aldehydes and ketones.

O

OAc H +

R

SZ, 0 0C

Ac2O

OAc R

5-8 h

R = H, 2-CH3, 4-CH3, 2-CH3, 4-OCH3 2-NO2, 4-NO2, etc. Scheme 7.

O

OAc OAc R

SZ, 60 0C R CH3CN

H

Scheme 8.

4.2.2. Methoxymethylation of Alcohols Methoxymethyl ethers are commonly used in protecting alcohols and phenols in natural product synthesis. Methoxymethylation was carried out by using chloromethyl methyl ether [168,169], which is no longer suitable due to its extreme carcinogenicity. Therefore, chloromethyl methyl ether is replaced with dimethoxymethane. This reaction is reported with various catalysts such as phosphorus pentoxide [170], expansive graphite [171], FeCl3/3 Å molecular sieves (MS) [172], Nafion-H resin [173] and so on. Nevertheless, these catalysts having advantages also associated with some drawbacks such as low product yields, requiring high reaction temperatures, tedious work-up procedures, etc. Lin et al. [174] studied this reaction by using sulfated metal oxides (SMO) namely, ZrO2, TiO2, Fe2O3, SnO2, HfO2, Al2O3 and SiO2. To identify the optimal SMO catalyst among various catalysts, they investigated methoxymethylation of cyclohexylmethanol with dimethoxymethane at ambient temperature. Among all SMO catalysts, SZ was found to be most promising. Thereafter, methoxymethylation of various alcohols was carried out with dimethoxymethane at ambient temperature using SZ catalyst (Scheme 9). It was also found that product yield followed a preferential order of primary > seconday >> tertiary alcohol due to steric effect. OCH3 R

OH

+ OCH3

SZ r.t.

R = primary, secondary, tertiary Scheme 9.

OCH3 OR Yield = 23-99%


Patil et al.

compounds such as triazolo-, oxadiazolo-, oxazino- and furanobenzodiazepines. These represent an important class of bioactive compounds. In general, benzodiazepines are synthesized by the condensation of o-phenylenediamines with ,-unsaturated carbonyl compounds, -haloketones or ketones. Many reagents have been reported for this condensation reactions which include BF3-etherate [192], polyphosphoric acid [193], Yb(OTf)3 [194] and acetic acid under microwave conditions [195]. Also this condensation is reported in an ionic liquid medium [196,197]. Considering the advantages of heterogeneous SZ catalysts, such as non-toxicity and low cost, Reddy et al. [48,198] studied the synthesis of 2,3-dihydro-1H-1,5-benzodiazepines (Scheme 12) by the condensation of o-phenylenediamine with ketones under solvent-free conditions catalyzed by SZ. The advantages associated with this methodology are easy work-up procedure, excellent product yields and recyclable catalyst.

4.2.3. Tetrahydropyranylation of Alcohols and Phenols The tetrahydropyranylation is one of the most commonly used processes to protect hydroxyl groups in organic synthesis [175177]. The OH-group can be protected under various conditions such as oxidation, oxidative alkylation, reduction, using Grignard reagents, acylation etc. Tetrahydropyranylation of alcohols and phenols was carried out by reaction with 3,4-dihydro-2H-pyran. This reaction is catalyzed by a variety of reagents such as protic acids (HCl and p-toluenesulfonic acid (p-TSA)), Lewis acids (BF3OEt2, aluminium sulfate on silica gel and sulfuric acid on silica gel), clay materials (Montmorillonite K-10 and HY-zeolite), ionexchange resins and so on [178-181]. However, many of these methods are characterized by several disadvantages such as expensive reagents, tedious workup procedure, rapid catalyst deactivation, low product yields, high reaction times, elevated reaction conditions, and disposal problems. In view of these reasons, Reddy et al. [182] studied this reaction by using SZ catalyst. The SZ catalyst was found to be extremely active for tetrahydropyranylation of various alcohols (primary, secondary, and tertiary) and phenols providing excellent product yields (82-96%) within short reaction times (Scheme 10). O OH +

R

O

SZ, r.t.

O

4.4. Transesterification In organic chemistry, transesterification is a process in which alcohol reacts with ester to provide different alcohol and different ester through interchange of alkoxy moiety. Transesterification reactions are important synthetic organic transformations in industrial as well as in academic laboratories. Transesterification is utilized in the synthesis of polyester, in which transesterification of diesters with diols forms macromolecules. For example, polyethylene terephthalate is synthesized by reaction between dimethyl terephthalate and ethylene. Methanol is produced as byproduct, which is evaporated to accelerate the rate of reaction. The reverse reaction i.e., between polyethylene terephthalate and methanol is also an example of transesterification and has been used to recycle polyesters into individual monomers. There are many catalysts available for transesterification reaction which include acid catalysts such as H2SO4, H3PO4, HCl, p-TSA, BuSn(OH)3 and Al(OR)3, and base catalysts that include NaOH, KOH and NaOCH3 and so on. However, these catalysts are toxic, corrosive, produce large amount of by-products which are difficult to separate from the reaction medium. In view of environmental concern, there is a constant search for catalysts which are easily separable from the products, less toxic and reusable. In this regard, SZ is a highly promising catalyst which has been successfully used for few transesterification reactions. For example, transesterification of triacetin with methanol and methyl salicylate with phenol gives salol that has been studied by employing SZ catalyst. Lopez et al. [199] compared the catalytic activity of a number of solid and liquid catalysts in the transesterification of triacetin with methanol (Scheme 13) at 60 °C. The order of reactivity among various acid catalysts investigated is as follows: H2SO4 > Amberlyst-15 > SZ > Nafion NR50 > WZ > supported phosphoric acid (SPA) > Zeolite H > ETS-10 (H). Only H2SO4 and Amberlyst-15 were found to show higher activity than SZ catalyst. The solid acids SZ, Amberlyst-15, Nafion NR50 and WZ exhibited 57%, 79%, 33% and 10% conversion; and H2SO4 exhibited 99%

R

solvent-free Yield = 82-96%

R = Alkyl, Aryl Scheme 10.

4.2.4. Synthesis of Silyl Ethers Silyl ethers have been developed from alcohols using variety of silylating agents namely, chlorotrimethyl silane [183], trimethylsilyl azide [184], triethylsilyl chloride [185], allylsilane [186], triethylsilyl hydride [187] and hexamethyldisilazane (HMDS) [188]. The protection of hydroxyl groups by silylation is imperative in peptide synthesis, and as lipophilicity modifiers for the peptides [189]. Several groups synthesized an anticancer drug NCX 4040 by selective silylation of 4-hydroxy benzyl alcohol as the starting material [190]. Recently, Thirupathi et al. [191] synthesized silyl ethers using oximes and alcohols employing TMSCN as the silylating agent and SZ catalyst. These silyl ethers were generated in short reaction times with excellent yields (8596%) (Scheme 11). 10 mol% SZ

R OTMS + R1CN solvent-free, r.t. R = allylic, cyclic, acyclic, aliphatic, aromatic, hetero aromatic, oxime R

OR1 + TMSCN

Scheme 11.

4.3. Synthesis of Benzodiazepines Benzodiazepines and their polycyclic derivatives are finding numerous new applications; widely used as anticonvulsant, antiinflammatory, analgesic, hypnotic, sedative, and antidepressive agents; valuable intermediates for the synthesis of fused ring

R

O

NH2 +

R

NH2 R = H, Me

Scheme 12.

R1

R R2

SZ, r.t. 2-3 h

R1 = CH3, C2H5, Ph R2 = H, CH3

H N

R1

R2 R2

R

N

R1 Yield = 84-96%



OCOCH3

H3COCO

OCOCH3

Acid or Base Catalyst + 3 CH3OH

60 0C

O

OH + 3

HO OH

H3C

OCH3

Scheme 13.

conversion respectively for transesterification of triacetin and methanol (methanol:triacetin = 6:1) at 60 °C. At 50% conversion of triacetin, SZ showed 55.1, 31.2 and 13.8% selectivity for diacetin, monoacetin and glycerol respectively. In another study, Lopez et al. [200] carried out transesterification of triglycerides and the esterification of carboxylic acids with ethanol using modified zirconias, namely SZ, WZ and TZ. Among all these catalysts, SZ is found to be the most active for both transesterification and esterification reactions, which however exhibited significant sulfur loss and was greatly reduced its long term activity. Again, there is another report on transesterification of triglycerides at 120 °C [201]. Recently, Petchmala et al. [202] investigated transesterification of palm oil and esterification of palm fatty acid in near- and super-critical methanol with SZ catalyst. The SZ catalyst was prepared with three different sulfur loadings (0.75%, 1.8% and 2.5%) and subjected to two calcination temperatures (500 °C and 700 °C). The most appropriate sulfur loading was found to be 1.8% and the optimal calcination temperature was 500 °C. Shamshuddin and Nagaraju [203] synthesized zirconia, SO42 and Mo(VI) ions modified zirconia and studied their catalytic performance in the synthesis of phenyl salicylate (Salol) via transesterification of methyl salicylate with phenol (Scheme 14) in the liquid phase with or without molecular sieves. They studied this transesterification reaction employing ZrO2, MoO3, SZ and MZ catalysts at ~189 °C. SZ and MZ were found to be effective catalysts for synthesis of salol. In case of ZrO2, MoO3 and MZ catalysts, phenyl salicylate (salol) was the only product; in case of SZ, salol was formed as a major product along with diphenyl ether. However, the formation of diphenyl ether with SZ may be attributed to the presence of ‘very strong’ acid sites [204] on its surface. O

OCH3

OH

OH

Ph

O

Acid catalyst

4.5.1. Regioselective Ring-Opening of Aziridines SZ is found to be effective catalyst for regioselective ringopening of aziridines with potassium thiocyanate and thiols [205]. Aziridines ring can be opened smoothly with KSCN in the presence of SZ to give the corresponding -aminothiocyanates in high yields. A series of -aminothiocyanates were prepared using various Ntosyl-2-aryl aziridines or N-tosyl-2-alkyl aziridines with KSCN (Scheme 15). The conversion required only 2 h at room temperature using CH3CN as the solvent and the ring-opening of the aziridines took place regioselectively in high yields. With N-tosyl-2arylaziridines, product 1 resulting from cleavage at the benzylic position, and with N-tosyl-2-alkylaziridines, product 2 resulting from the cleavage at the terminal position were formed predominantly along with minor amounts of the other regioisomers. In case of symmetrical bicyclic aziridines only product 3 is formed with trans stereochemistry (scheme 16). Also, a series of aminosulfides were prepared from different N-tosyl-2-aryl aziridines or N-tosyl-2-alkyl aziridines by treatment with various thiophenols (products 4 and 5 formed (scheme 17) same as above) in the presence of SZ. In case of symmetrical bicyclic aziridines only product 6 is formed with trans stereochemistry (Scheme 18). NHTs NTs + KSCN

SZ, CH3CN

R

r.t., 2 h

R = Aryl, Alkyl

SCN

+ SCN R NHTs 1 2 minor major when R = Aryl when R = Alkyl R

Scheme 15.

NHTs

+ KSCN SZ, CH3CN r.t., 2 h NTs

n

n SCN 3 _______________ n Yield (%) _______________ 1 89 87 2 _______________

O OH

+ Scheme 16. Scheme 14. NTs + R1SH

4.5. Ring-Opening of Aziridines and Epoxides Aziridines are important precursors for the synthesis of various nitrogen-containing bioactive molecules such as heterocycles, alkaloids and amino acids. Regioselective ring-opening of aziridines contributes mainly to their synthetic utility. Ring-opening of aziridines with KSCN provides -aminothiocyanates, which are the precursors of thiazoles or benzothiazoles having pesticidal properties; with thiols providing -aminosulfides, which are the precursors of various bioactive compounds. Ring-opening of epoxides with amines gives -aminoalcohols, which are the key intermediates to many organic compounds including biologically active natural and synthetic products and are chiral auxiliaries for asymmetric synthesis. The SZ catalyst has been utilized for the ring-opening of N-tosyl aziridines with amines, thiols and potassium thiocyanate, and ring-opening of epoxides with amines and N-heterocycles.

R R = Aryl, Alkyl

r.t., 2 h R1 = Aryl

SR1

NHTs

SZ, CH3CN

+ NHTs SR1 R 5 4 major major when R = Aryl when R = Alkyl R

Scheme 17.

n

NTs

Scheme 18.

+

R1SH

SZ, CH3CN r.t., 2 h

NHTs n SR1 6 _______________________ n Yield (%) R1 _______________________ 1 91 C6H5 2 4-Cl-C6H4 88 _______________________


Patil et al.

pharmaceuticals [209,210]. Sulfoxides have attracted much attention as important chiral auxiliaries in asymmetric synthesis and in carbon-carbon bond forming reactions [211]. Usually, sulfoxides are prepared by various methods. However, these methods have some limitations which include use of hazardous, corrosive and expensive reagents and formation of a mixture of products along with the desired sulfoxides. Reddy et al. [48] reported the synthesis of various diaryl sulfoxides from the reaction of diverse activated and non-activated arenes with thionyl chloride catalyzed by SZ under solvent-free conditions (Scheme 22). All the diaryl sulfoxides were formed in excellent yields in short reaction times.

4.5.2. Regioselective Ring-Opening of Epoxides Reddy et al. [206] carried out the regioselective ring-opening of epoxides with various aromatic amines catalyzed by SZ under solvent-free conditions to provide the corresponding aminoalcohols in high yields (70-97%). Ring-opening of styrene oxide with aromatic amines afforded the major regioisomer 7, and aliphatic oxirane (propylene oxide) with aromatic amines gave major regioisomer 8 (Scheme 19). Symmetrical oxirane (cyclohexeneoxide) with various aryl amines provided the product of racemic 2-aryl amino cyclohexanol 9, which was identified as the trans-diastereoisomer (Scheme 20). In another study, NegrónSilva et al. [86] employed SZ and SZ/MCM-41 catalysts for the regioselective synthesis of -aminoalcohols using various epoxides with aniline or benzyl amine at 60 °C (conventional heating) or microwave exposure under solvent-free conditions. Both catalysts exhibited good catalytic activity, and the catalysts were recovered and reused without any appreciable loss of activity. Also, Das et al. [207,208] exploited SZ catalyst for ring-opening of various epoxides with nitrogen heterocycles such as indoles, pyrroles and immidazoles in CH2Cl2 (Scheme 21). Reaction took place within 34.5 h offering good yields of products (42-83%). O

HN

SZ, r.t.

R

O

NH2

NH2

OH

R1 2

OH

R + R1 CHO

NHR

NR R N H H Yield = 73-85%

Scheme 23.

Scheme 20.

aldehydes/ketones in the presence of an acid catalyst to produce azafulvenium salts. These azafulvenium salts can undergo further addition reaction with another molecule of indole to produce bis(indolyl)methanes [213]. Protic acids as well as Lewis acids, and various other reagents such as lanthanide triflates, clays, ion

4.6. Synthesis of Diaryl Sulfoxides Sulfoxides and sulfones are important compounds for synthesis of various organosulfur compounds in the field of drugs and R3 N

R3

R1

R2 N

CH2Cl2, SZ, r.t., 3-4.5 h

Ar

Ar

Ar N R

CH2Cl2, SZ, r.t., 4 h

Ar = Ph

N

Yield = 42-75%

R1

N R

O

OH

Ar

R2

OH

HO

Ar N R

OH

N R

R=H

45%

16%

6%

R = CH3

42%

12%

8%

NH

CH2Cl2, SZ, r.t., 4 h Scheme 21.

H

SZ, r.t.

solventfree R1 = Aromatic, Heteroaromatic

N H R = H, Me

9

Ar

S Ar Ar Yield = 80-92%

solvent-free

Large number of natural products which contain bis(indolyl)methanes and bis(indolyl)ethanes have been isolated from both marine and terrestrial sources, and some of them were found to exhibit interesting biological activity [212]. In general, bis(indolyl)methanes are synthesized by the analogous reactions of Ehrlich test, where indoles react with aliphatic or aromatic

OH + 1 R

SZ, r.t.

O

SZ, r.t.

4.7. Synthesis of Bis(indolyl)methanes

Scheme 19.

O + R

Ar

Scheme 22.

H N R1 R 45-300 min R1 8 7 R1 = Aryl, Alkyl R = Aryl major major when R1= Aryl when R1 = Alkyl +R

+H S H + Cl Cl

Ar

OH Ar

N

N Yield = 68%

OH



exchange resins and zeolites have also been studied for this reaction. Reddy et al. [48] synthesized bis(indolyl)methanes by electrophilic substitution reaction of indole with various aldehydes in the presence of SZ catalyst (Scheme 23). This reaction was carried out for an appropriate time at room temperature and the resulting bis(indolyl)methanes were produced in excellent yields. 4.8. Friedel-Crafts Alkylation Reactions The Friedel-Crafts reactions were developed by Charles Friedel and James Crafts in 1877. Now, these reactions are used extensively in the synthesis of fine chemicals and various intermediates in pharmaceutical as well as in petrochemical industries. Friedel-Crafts alkylation reactions involve the alkylation of an aromatic compound with an alkyl halide using a strong Lewis acid catalyst. The SZ catalyst has been utilized for various FriedelCrafts alkylation reactions such as that of diphenyl oxide, guaiacol, p-cresol and methoxyphenol. Yadav and Sengupta [22] carried out the alkylation of diphenyl oxide with benzyl chloride and synthesized the corresponding isomeric benzyl chloride in excellent yields using SZ catalyst (Scheme 24). In another study, Yadav and Rahuman [214] tested

various solid acid catalysts such as Filtrol-24, DTP/K-10, Deloxane ASP resin, K-10 Montmorillonite clay, and SZ for alkylation of 4methoxyphenol with methyl tert-butyl ether (MTBE) (Scheme 25). Activity of SZ catalyst was found to be the least among the tested catalysts. Though the SZ catalyst exhibited the lowest activity as compared to other solid acid catalysts, it showed maximum selectivity to monoalkylated products. Thus with 1:3 molar ratio of 4-methoxyphenol and MTBE, the SZ catalyst provided 22% conversion and 85% selectivity to monoalkylated product. Yadav et al. [215,216] investigated the O- versus C-alkylation of p-cresol and guaiacol with cyclohexene (Schemes 26 and 27) by using several solid acid catalysts including SZ. For example, alkylation of p-cresol with cyclohexene (1:1 molar ratio) provided 47% conversion and 82% O-alkylated product selectivity. On the other hand, the alkylation of guaiacol with cyclohexene (0.226:0.045 molar ratio) provided 74% conversion and 68% O-alkylated product selectivity. Apart from these studies, there are some other reports in which SZ catalyst has been used for alkylation of benzene with benzyl chloride [217], alkylation of p-cresol with isobutylene [218], and alkylation of diphenyl oxide with 1-decene [219]. O

O +

O

Cl

SZ

+

90 0C

Scheme 24.

OCH3

OCH3

OCH3 OCH3 +

H3C

CH3 CH3

SZ, 1,4-dioxane

+

150 0C, 3 h OH

OH

Conversion 28%

OH

BHA (mono)

BHA (di)

Scheme 25.

OH

OH

O SZ, Toluene +

+ 80 0C, 3 h Conversion 47%

CH3

CH3

CH3 O-alkylation (82%)

C-alkylation

Scheme 26.

OCH3

OCH3 OH +

SZ, Toluene 80 0C Conversion 74%

Scheme 27.

OCH3 OH

O + O-alkylation (68%)

C-alkylation


Patil et al.

chlorobenzophenone (Scheme 28). Deutsch et al. [221] studied the benzoylation of anisole with benzoic anhydride in presence of various solid acid catalysts including SZ, Nafion-H, Amberlyst-15, K-10, H-BEA and H-mordenite at 50 °C, and the SZ catalyst exhibited the best performance for benzoylation (71% yield in 3 h). Furthermore, benzoylation of various aromatic compounds such as anisole 10, m-xylene 11 and mesitylene 12 with benzoic anhydride and benzoyl chloride were studied at 100 °C (Scheme 29), the reactivity of aromatics was in the order: anisole > mesitylene > mxylene with both the acylating agents. On raising the temperature from 100 to 136 °C, there was no effect on the order of reactivity of aromatics. In another study, Deutsch et al. [222] used SZ catalyst for acylation of anisole and chlorobenzene with various acylating agents which include aliphatic carboxylic anhydrides, aliphatic acid chlorides and substituted benzoyl chlorides. But only benzoyl chlorides reacted with chlorobenzene in the presence of SZ. Later on, the same group reported the acylation of methoxynaphthalenes 13, dimethylnaphthalenes (14, 15), methylnaphthalenes 16, naphthalene, and anthracene with benzoic anhydride 17, benzoyl chloride 18, and acetic anhydride 19 to synthesize aromatic ketones by employing SZ catalyst (Schemes 30 and 31) [223]. In this study, it was found that the rate of product formation on SZ was dependent on the respective aromatic compound, solvent used, and the ratio of substrate to the acylating agent. Apart from these, various groups working in this fascinating area also reported interesting results. Recently, El-Sharkawy and Al-Shihry, [224] studied Friedel-Crafts acylation of toluene with acetic anhydride using various sulfated metal oxides such as

4.9. Friedel-Crafts Acylation Reactions Acylation of aromatic compounds is another class of FriedelCrafts reactions. Acylation reaction has several advantages over alkylation; the product (aromatic ketone) is less reactive than original substrate so that the possibility of multiple acylation is reduced. Aromatic ketones are intermediates or end-products employed in the formation of pharmaceuticals, cosmetics, agrochemicals, dyes and speciality chemicals. Friedel-Crafts acylation is carried out by using various Lewis/Brønsted acids such as AlCl3, BF3-HF, FeCl3, ZnCl2, SnCl4, InCl4, SbCl5, H2SO4, HCl and so on. The SZ catalyst has been exploited for various FriedelCrafts acylation reactions such as acylation of benzene, substituted benzenes, naphthalenes, anthracenes and other. Cl

O

O SZ

+

70 0C

Cl

Cl Scheme 28.

Yadav and Pujari [220] studied acylation of benzene with 4chlorobenzoyl chloride using different solid acid catalysts such as K-10 clay, Filtrol-24 clay, dodecatungstophosphoric acid (DTPA), DTPA supported on K-10, Amberlyst-15, Amberlite IR 120, Indion-130 and SZ. Among them SZ exhibited the best activity. The acylation reaction was 100% selective to 4O

R3

X R3

SZ, -HX

R1

R2 O R1

O

R2 +

isomers

+

isomers

R4 R3

O

O

R2

R4

O SZ, -CH3CO2H

R1

10 R1 = R2= R4 = H, R3 = OCH3 11 R1 = R3 = CH3, R2 = R4 = H 12 R3 = R1 = R4 = CH3, R2 = H

R4

Scheme 29.

R2

R2 O + R1

R3

X

SZ -HX

17, 18 R3 = C6H5, 13 = H, = OCH3 X = OCOC6H5/Cl 14 R1, R2 = CH3 19 R3 = CH3, 15 R1 = H, R2 = CH3 X = OCOCH3 16 R1 = CH3, R2 = H R1

R2

+ isomers(s) R1 O

R3

Scheme 30.

O O +

R

SZ X

-HX

R = C6H5, CH3; X = Cl, OCOC6H5, OCOCH3 Scheme 31.

R

9-Acylanthracene



O R

H3C

X

+

H

SZ, r.t.

O

R

solvent-free 10-15 min

H3C

O

O

X

+

CH3 Yield = 85-97% O R = 2-naphthyl, 4-acylphenyl, benzyl, cyclopentane etc. X = O, NH

H3C

OH

Scheme 32.

glycosidation methods have been one of the major concerns in synthetic organic chemistry due to the structural complexity and the biological significance of glycosubstances. Toshima et al. working in this area for several years carried out several stereoselective glycosidation of glycosyl phosphites; manno- and 2deoxyglucopyranosyl -fluorides; 2-deoxyand 2,6dideoxyglycosyl diethyl phosphites; mannopyranosyl sulfoxides; 3,4-di-O-protected olivoses; 2-deoxy--D-glucopyranosyl fluoride; O-benzylated 1-hydroxy sugars with various alcohols using solid acid catalysts including SZ [25,228-235]. In an interesting investigation, Toshima et al. [25] reported stereo-controlled glycosidation of totally benzylated 2-deoxy--Dglucopyranosyl fluoride 20 with various alcohols (21-26) using SZ catalyst to synthesize both the 2-deoxy--and -Dglucopyranosides. They examined the glycosidation reaction of 20 with cyclohexyl methanol 21 using SZ catalyst with or without 5

sulfated tin oxide, SZ containing different amounts of sulfate and Al2O3-SZ. Yadav and Pimparkar [225] carried out the synthesis of 2,5-dimethoxyacetophenone by acylation of 1,4-dimethoxybenzene with acetic anhydride over various solid acid catalysts including SZ. 4.10. Acylation of Phenols, Alcohols and Amines Acylation of phenols, alcohols and amines is of synthetic importance. Acylation of phenol and alcohol gives an ester which corresponds to an important family of intermediates widely employed in the synthesis of fine chemicals, drugs, plasticizers, perfumes, food preservatives, pharmaceuticals and chiral auxillaries [226,227]. SZ was found to be a promising catalyst for acylation of phenols, alcohols and amines with acetic anhydride as an acylating agent under solvent-free conditions at room temperature (Scheme 32). In this study, acetylation of diverse alcohols, phenols and OBn

OBn O

BnO BnO

SZ, MS 5A

OR

Et2O, 00C

OBn

O

BnO BnO

+ R OH F

20

SZ, 250C CH3CN OH

HO 22

23

25

24

OBn O

O

MeO MeO

n-C8H17-OH

21

OR O

21-26

OH HO

O

BnO BnO

Me

O OMe

OH

26

N3

Scheme 33.

molecular sieves under various conditions. They observed that glycosidation using 5 wt.% SZ in CH3CN at 25 °C for 1 h gives 2deoxyglucopyranoside in high yield with high -stereoselectivity. On the other hand, by using a 100 wt.% SZ in the presence of 5 Å molecular sieves (500 wt.%) in Et2O at 0 °C, the corresponding 2deoxyglucopyranoside was obtained with -stereoselectivity (Scheme 33). In another study, Toshima et al. [235] studied the stereocontrolled glycosidation of manno- and 2-deoxyglucopyranosyl -

amines was carried out with acetic anhydride; products were formed in excellent yields (85-97%) within short period of time (10-15 min). 4.11. Stereo-Controlled Glycosidation Glycosubstances including glycoconjugates, glycolipids, glycoproteins, oligosaccharides and many antibiotics continue to be the central focus of research both in chemistry and biology. Therefore, the development of efficient and stereoselective BnO BnO BnO

X

O

SZ (100wt%) OR

Et2O, 25 0C MS 5 A (100wt%)

X

OBn 28

BnO O

+

R

20 X = H F 27 X = OBn

OH

SZ (5wt%) CH3CN, 40 0C

OBn O

BnO HO BnO

O OBn

OMe OH

29

N3

BnO BnO

X

O OR

21-24, 28-30 O

OH O

BnO BnO

Scheme 34.

BnO BnO BnO

30

OMe


BnO BnO BnO

OBn O

Patil et al.

Y +

BnO BnO BnO solvent 25 0C, 3 h

HO

X 31 X = S(O)Ph, Y = H

O 33

21

32 X = H, Y = S(O)Ph

OBn O

OH n-C8H17-OHHO OH 22

OBn O

SZ

23

24

O BnO BnO

OH O OBn OMe

N3 34

29

Scheme 35.

fluorides using several heterogeneous solid acids, namely Montmorillonite K-10, Nafion-H and SZ. Their study revealed that SZ gives better yields for this reaction. Also, SZ provides maximum -selectivity when CH3CN is employed as the solvent at 40 °C, and -selectivity in Et2O as solvent at 25 °C when used along with molecular sieves 5 Å (100 wt.%). By taking these conditions in hand, they carried out manno- and 2deoxyglucopyranosyl -fluorides (20, 27) with several alcohols (21-24, 28-30) and synthesized both - and -manno- and 2deoxyglucopyranosides selectively (Scheme 34). In another study, Nagai et al. [232] initially performed glycosidation of mannopyranosyl sulfoxides 31 and 32 with cyclohexylmethanol by employing several solid acids such as Montmorillonite K-10, Nafion-H and SZ in the presence of 5 Å molecular sieves and different solvents at 25 °C for 3 h. From the initial study, the glycosidation of -mannopyranosyl sulfoxide 31 and cyclohexylmethanol with 100 wt% Nafion-H and 100 wt.% 5 Å MS in CH3CN at 25 °C for 3 h exclusively gave the mannopyranoside 33 in high yield (97%) with high stereoselectivity (/ = 97/3). On the other hand, the glycosidation of -mannopyranosyl sulfoxide 32 with 300 wt.% SZ and 300 wt.% 5 Å MS in Et2O proceeded smoothly and selectively give the mannopyranoside 33 in high yield (99%) with high stereoselectivity (/ = 19/81). By taking these conditions in hand, glycosidation of 31 and 32 with several alcohols 21-24, 29, and 34 (Scheme 35) was carried out resulting in high yields and selectivity of products.

literature showing a high activity of SZ catalyst for isomerization reactions in the liquid phase. Satoh et al. [236,237] studied skeletal isomerization of cycloalkanes such as cycloheptane, cyclooctane, cyclodecane and cyclododecane with SZ in liquid phase at 50 °C. The main product of methylcyclohexane was formed from cycloheptane along with small amounts of trans 1,2-dimethylcyclopentane, cis and trans 1,3dimethylcyclopentanes, 1,1-dimethylcyclopentane and ethylcyclopentane. The major product from cyclooctane was ethylcyclohexane along with other products namely, cis 1,3dimethylcyclohexane, small amounts of trans 1,2-, 1,3-, 1,4dimethylcyclohexanes, 1,1-dimethylcyclohexane and methylcycloheptane. Cyclodecane was dehydrogenated into cis- or transdecaline with the evolution of dihydrogen, and no isomerization occurred. Cyclododecane was converted into more than 30 products resulting from processes of isomerization, dehydrogenation and cracking. The isomerization of -pinene produces bicyclic and monocyclic compounds such as camphene 35, tricyclene 36, fenchene 37, bornylene 38, and monocyclic compounds such as terpinene 39, limonene 40, -terpinene 41, terpinolene 42, pcymene 43, etc (Scheme 36). This isomerization reaction was carried out in the presence of acid catalysts and acidity of catalyst was directly related to the activity as well as camphene yield. SZ catalysts were briefly studied for isomerization of -pinene [238241]. Comelli et al. [238] examined the isomerization of -pinene with SZ catalyst and compared with ZrO2 and H2SO4. Also to understand the effect of pre-treatment on the catalytic activity, the SZ catalyst was treated in a muffle furnace for 2 h at 250, 350 and 550 °C; these catalysts were designated as SZ250, SZ350 and SZ500. In the case of ZrO2 catalyst that possesses only Lewis acid sites therefore no activity was observed. On the other hand Brønsted acid H2SO4 exhibited barely any activity. SZ possesses

4.12. Isomerization Reactions SZ catalyst came to the focus from isomerization reaction: nbutane to iso-butane. Thereafter, it was used for various reactions such as the isomerization of butane, pentane and other hydrocarbons in vapour phase. A few reports were found in the

35

39 Scheme 36.

36

40

38

37

41

42

43



both Brønsted as well as Lewis acid sites, it showed good activity. It was also found that the treatment temperature of SZ played a major role, the SZ250 catalyst exhibited a high activity and the ratio between bicyclic and monocyclic compounds was maximum. Various catalysts, namely, SZ, SZ250, SZ350, SZ500 and H2SO4, after 2 h reaction exhibited a conversion of 85.8, 88.2, 78.4, 56.5 and 1.4% and selectivities for camphene were 58.9, 67.4, 56.8, 59.3 and 12.0%, respectively. In another study, Grzona et al. [241] prepared SZ catalyst with three different sulfur loadings (5, 10, and 20 wt.% H2SO4) and utilized for -pinene isomerization. All three SZ catalysts were found to be active for obtaining camphene. With increasing the amount of catalyst loading the catalytic activity increased while the selectivity for camphene decreased. A catalyst concentration of 1 wt.% was found to be the most suitable for high activity and selectivity. Flores-Moreno et al. [242] studied the isomerization of pinene oxide 44 to campholenic aldehyde 45 using sulfated alumina, titania and zirconia in the liquid phase at 0 °C (Scheme 37). They have found that sulfated alumina produces campholenic aldehyde (76% yield) at full conversion of the reactant where as SZ produced it with 37% yield again at full conversion.

anhydride with 2-ethylhexanol in the presence of solid acid catalysts such as natural zeolites, synthetic zeolites (ZEOKAR-2, ASHNCH-3), heteropolyacids (H4Si(W3O10)4) and SZ. These reactions were carried out under solvent-free conditions. The SZ catalyst showed the best activity and efficiency among the investigated catalysts (Scheme 39). They have also studied the esterification of trans-(2-hexenyl)succinic anhydride and dibasic acids such as sebacic acid, adipic acid and caproic acid with various alcohols such as 2-ethylhexanol, diethyleneglycol and pentaerythritol using SZ catalyst. In another study, Ardizzone et al. [248] studied the esterification reaction of benzoic acid to methyl benzoate with methanol by employing different SZ catalysts. Excellent product yields under mild reaction conditions were reported. O

O O +2 R

OH

Solid acid

OR

-H2O

OR

O

O

Scheme 39.

O SZ, 0 0C

CHO

100% conversion 45 37%

44 Scheme 37.

Tyagi et al. [21] reported the isomerization of longifolene 46, decahydro-4,8,8-trimethyl-9-methylene-1-4-methanoazulene to isolongifolene 47, 2,2,7,7-tetramethyltricycloundec-5-ene, (Scheme 38) employing nano-crystalline SZ catalyst obtained by sol-gel technique in acidic medium using one-step as well as in basic and neutral medium using two-step procedure. Almost all catalysts exhibited excellent selectivity (90-93%) with 100% conversion at 180 °C reaction temperature in solvent-free isomerization of longifolene to iso-longifolene. In order to obtain the maximum conversion, the reaction was carried out at different temperatures in the range of 120-200 °C. It was observed that conversion increases from 120 to 180 °C then it does not change until 200 °C. Apart from the above applications, the SZ catalyst was also used for isomerization of citronellal to isopulegol [243].

4.14. Synthesis of Aromatic ,-Dihalobenzyl Derivatives ,-Dihalo aromatic compounds (gem-dihalides) are important intermediates in the pharmaceutical, agricultural and dye industries [249-251]. These are also used as starting materials for several C-C coupling reactions and for the synthesis of imines as well as parent raw materials for the preparation of the corresponding amines, acids and alcohols [252,253]. Wolfson et al. [254] investigated the synthesis of aromatic gem-dihalides from their corresponding aromatic aldehydes by using various acid catalysts including both homogeneous and heterogeneous Lewis and Brønsted acids. They observed that AlCl3 and SZ are the most active homogeneous and heterogeneous catalysts, respectively. Benzoyl chloride was more reactive than acetyl and propionyl chloride. Replacing the chloride with bromide also resulted in increased activity. Performance of the reaction in polar solvent and in neat benzaldehyde resulted in higher product yields. The SZ catalyst provided benzal chloride in 22% yield (Scheme 40), which is highest among the solid acids used. Also they have carried out the oxidative regeneration of spent SZ catalyst in air at 550 °C and fully recovered its catalytic activity that allowed multiple catalysts recycling. O

O H

catalyst

+

SZ 46

47

Cl

Scheme 38.

Cl

100 0C, 1 h

Cl H

+ (C6H5-CO)2O Yield = 22%

4.13. Esterification Reactions Esters have a fruity odour and are prepared in large quantities for various purposes such as artificial fruit essences, flavorings and components of perfumes. Lot of research has been carried out on esterification reactions by using SZ catalysts such as esterification of acetic acid (with ethanol [244] and butanol [245]), palm fatty acid distillate [246], long-chain fatty acids [202], 4methoxyphenylacetic acid (with dimethyl carbonate) [64], phenylacetic acid (with p-cresol) [247] and so on. Sejidov et al. [24] investigated the synthesis of di-2ethylhexylphthalate (DOP) via esterification reaction of phthalic

2

Scheme 40.

4.15. Knoevenagel Condensation The Knoevenagel condensation of aldehydes with active methylene compounds is one of the important C-C bond forming reactions in organic synthesis. Knoevenagel condensation has been extensively investigated in view of its significance [255] and has been commonly employed in the synthesis of numerous speciality chemicals and chemical intermediates. This condensation reaction is generally catalyzed by bases, acids or catalysts containing both


Patil et al.

acid-base sites [256], and numerous acid-base reagents or catalysts are reported. Reddy et al. [257] investigated this reaction by employing SZ catalyst under reflux and solvent-free conditions. Various aliphatic, aromatic and heterocyclic aldehydes with malononitrile produced corresponding Knoevenagel product (Scheme 41) in short period of time (0.5-6 h) and in excellent yields (78-98%). O H

CN + CN

R

CN SZ Reflux 30-300 min

SO42/Fe2O3, SO42/HfO2, SO42/SnO2, H-form of mordenite (CBV21A , CBV90A and CBV10A ), and -zeolite (CP811E-75, CP811E-150 and CP811E-300). Aniline (45.1 mmol) was mixed with trimethyl orthoformate (130.2 mmol), 0.3 g of catalyst was added and the solution was stirred at 40 °C for 1 h. The SZ is found to be excellent as compared to other studied catalysts in view of better selectivity (98%) and conversion (35%) (Scheme 44). Intrestingly, methyl-N-phenylformimidate yield was less than 3 % and no N-methylformanilide was found in the product mixture. H3CO Ar

CN R Yield = 78-98%

NH2 + H3CO C H3CO

SZ

ArN=CH-NHAr + 3 CH3OH 40 0C, 1 h 98% conversion 35%

H

Scheme 41.

Scheme 44.

4.16. Aza-Michael Reaction

4.18. Cyclodehydration of Some Diols

The aza-Michael addition is one of the most important organic reactions especially for the synthesis of C-N heterocycles containing -aminocarbonyl functionality. The product from azaMichael reaction not only constitutes a component of biologically active natural product but also serves as an essential intermediate in the synthesis of -aminoketones, -aminoacids and -lactam antibiotics. Various catalysts are reported in the literature for this fascinating reaction. Reddy et al. [258] employed SZ for azaMichael reaction of ,-unsaturated carbonyl compounds with aromatic amines. A variety of ,-unsaturated carbonyl compounds such as methyl vinyl ketone, methyl acrylate, ethyl acrylate and cyclohexenone underwent 1,4-addition with a wide range of aliphatic, aromatic, heterocyclic amines and primary as well as secondary amines in the presence of SZ catalyst under solvent-free conditions at room temperature to provide the corresponding aminoketones in high yields (70-95%) in short reaction times (Scheme 42). To check the selectivity of the reaction, a mixture (1:1) of morpholine and aniline was treated with an excess methyl vinyl ketone in the presence of SZ, only the morpholine adduct was formed as the product (Scheme 43).

Cyclodehydration of diols is highly useful method to obtain oxygen heterocycles such as tetrahydrofuran, 2,5-dihydrofuran, 1,4dioxane and so on. Oxygen heterocycles are mainly used as intermediates in the production of drugs, pesticides and other chemicals on industrial scale [260]. Several catalysts are reported in the literature for the reactions of diols in the liquid or gaseous phases. Wali and Pillai [261] carried out a comparative study with SZ and H-ZMS-5 catalysts for cyclodehydration reaction. Initially, cyclodehydration of diethylene glycol was studied with SZ and HZMS-5 at 200 °C, and SZ was found to be better than H-ZSM-5 with respect to recyclability, yield and selectivity. They also compared the results of SZ with other catalysts (Al3+Montmorillonite K10, HMPT and Al2O3) reported in the literature. Further, SZ has also been successfully employed for cyclodehydration of various diols such as butane-1,4-diol, pentane1,5-diol, hexane-1,6-diol, cyclohexane-1,4-diol and triethylene glycol to produce the corresponding oxygen heterocycles with high conversion, selectivity and yield.

R4

R1

SZ, r.t.

NH + R2

R3

EWG solvent-free 15-120 min

R1

R3

N R2

EWG

R4 Yield = 70-95%

Scheme 42.

4.17. Synthesis of Formamidine Lin et al. [259] studied the reaction between aniline and trimethyl orthoformate to produce formamidine by using various solid acid catalysts, namely, SZ, SO42/TiO2, SO42/Al2O3,

4.19. Synthesis of Coumarins Pechmann’s reaction is a simple and easy procedure to synthesize coumarins by reacting phenols and -ketoesters in the presence of acid catalysts. Several catalysts are reported in the literature for this reaction that include Lewis acids, ionic liquids, zirconium tetrachloride, sulfated ceria-zirconia, H2SO4 promoted silica gel, SZ and so on [262-267]. Tyagi et al. [26] prepared nanocrystalline SZ catalysts by one-step as well as two-step sol-gel techniques and exploited for synthesis of 7-substituted 4-methyl coumarins under different conditions such as increasing phenol to substrate weight ratio, solvent-free/nitrobenzene or toluene as solvent, and thermal/microwave irradiation. 7-Amino-4-methyl coumarin was synthesized from the reaction of m-aminophenol with

O

NH2 O O

NH +

+

SZ, r.t. solvent-free 45 min

N 100%

O O

N H 0% (constitution of products) Scheme 43.



R

O +

HO

OH

O

R1

OEt

R2 R1 = CH3, CH2Cl R2 = H, Cl

R = H, OH

SZ, 80 0C or 60-65 0C 24 h, solvent-free or small amount of EtOH

HO

O

O R2

R

R1 Yield = 52-93%

Scheme 45.

ethyl acetoacetate, and 7-hydroxy 4-methyl coumarin from the reaction of m-hydroxyphenol (resorcinol) with ethyl acetoacetate. Amino- derivative was formed in excellent yield (~100%) and selectivity (~100%) in solvent-free condition and also in the presence of nitrobenzene solvent. Reaction was very fast in solventfree conditions and the complete conversion was attained at comparatively lower temperature within few minutes than with nitrobenzene. The solvent-free microwave-assisted synthesis was found to be the most suitable way to synthesize the hydroxy derivative giving excellent yields at lower temperatures and in much lesser times as compared to thermal heating. In another study, Tyagi et al. [268] used nanocrystalline SZ catalyst for microwaveassisted solvent-free synthesis of hydroxy derivatives of 4-methyl coumarins by Pechmann reaction. The nanocrystalline catalyst showed good activity for activated resorcinol substrates, such as phloroglucinol and pyrrogallol with ethyl acetoacetate for the synthesis of 5,7-dihydroxy-4-methyl coumarin and 7,8-dihydroxy4-methyl coumarin, respectively, showing significant yields (7885%) within short reaction times (5-20 min) at 130 °C. On the other hand, phenol and m-methylphenol were found to be inactive for the synthesis of 4-methylcoumarin and 4,7-dimethylcoumarin respectively, under the same experimental conditions. Apart from these, Rodríguez-Domínguez and Kirsch [269] also used small amounts of SZ catalyst (1wt.%) for the synthesis of hydroxycoumarins via Pechmann reaction without solvent or in some cases using a small amount of ethanol with rapid stirring for 24 h at 80 °C or 60-65 °C, and synthesized various coumarins (Scheme 45) in moderate to good yields. 4.20. Synthesis of -Amino-,-Unsaturated Ketones and Esters -Amino-,-unsaturated esters and ketones find synthetic importance, particularly in the construction of heterocyclic compounds such as dihydropyridines, pyridines, pyrimidines, indoles and isothiazoles. Different methods for the synthesis of amino-,-unsaturated esters and ketones have been reported in the literature [270,271]. The condensation of 1,3-dicarbonyl compounds with amines is one of the most simple and straightforward synthetic routes. This transformation has been catalyzed by variety of catalysts such as HCl, H2SO4, p-TSA, trimethylsilyl trifluoromethanesulfonate (TMSTf), Montmorillonite K-10, I2, BF3-OEt2, Al2O3, silica gel, Zn(ClO4)2·6H2O, CeCl3·7H2O, NaAuCl4, Bi(OTf)3, natural clays and so on. Zhang and Song [272] exploited SZ catalyst for synthesis of -amino-,unsaturated esters and ketones from 1,3-dicarbonyl compounds and amines under solvent-free conditions at room temperature (Scheme O

O

R1 R1 R2

R3 R2

+ H2N

R

SZ, r.t. 10-360 min

= CH3, Ph R = Alkyl, Aryl =H R3 = OMe, OEt, CH3

Scheme 46.

R R1

NH

46). Initially, they investigated the reaction between 2-bromoaniline and acetylacetone in order to optimize the reaction conditions. It was found that without catalyst after 24 h, 90% of 2-bromoaniline was recovered from the reaction mixture. On the other hand, using SZ as catalyst -enaminone product has been isolated with 86% yield under the same reaction conditions. Various amines (primary, benzylic and aromatic amines) reacted with acetylacetone effectively to afford the corresponding -enaminone in good to better yields (78-95%) within short reaction times (10-360 min). 4.21. Mannich-Type Reaction As discussed earlier, carbon-carbon bond forming reactions continue to be the central focus of research in synthetic organic chemistry. One of the important carbon-carbon bond forming reactions is the Mannich-type reaction of ketene silyl acetals and aldimines to produce corresponding -aminoesters in a single step which is of considerable importance for synthesizing biologically active molecules containing nitrogen atom [272-275]. Wang et al. [276] extensively studied this reaction by using SZ catalysts. To optimize the reaction conditions, the Mannich-type reaction between aldimine 48 and ketene silyl acetal 49 (Scheme 47) was studied in different solvents and by varying the quantity of catalyst. It was found that the reaction proceeded smoothly when 50 or 100 wt.% SZ in CH3CN at room temperature (25 °C) was used; with 150 wt.% the reaction time was shortened (from 20 to 5 h) to give the -aminoester 50 in high yield, and CH3CN was found to be superior to other solvents (ethanol and toluene). By using these excellent reaction conditions (i.e., catalyst: 150 wt.%, solvent: acetonitrile, reaction time: 5 h), they explored the Mannich-type reaction of various ketene silyl acetals with a range of aldimines to give the corresponding -aminoesters in good to excellent yields. OMe

SZ

+

OMe

N Ph

PMP

OTMS

O

25 0C Ph

49

48

NH

OMe 50

Scheme 47.

4.22. Bamberger Rearrangement Bamberger rearrangement of phenylhydroxylamine (PHA) to paminophenol (PAP) was investigated at 80 °C, with water as solvent, employing various solid acid catalysts such as -zeolite, Montmorillonite K10 clay, sulfonated silica and SZ [277]. Both activity and selectivity were affected by the choice of the catalyst. NH2

NHOH

O R3

R

R2 Yield = 78-95%

SZ

R = H, 3-OH, 3-NH2 Scheme 48.

R

80 0C OH


Patil et al.

The selectivity for PAP was found to be 17% for K10 clay,70% for sulfonated silica, 84% for -zeolite and >90% for SZ respectively. The SZ catalyst calcined at 650 °C was found to be the promising catalyst for this reaction. Also, it showed good activity and selectivity for Bamberger rearrangement of substituted phenylhydroxylamines (Scheme 48).

of triazenes [284], cyclization of citronellal to isopulegol [285], rearrangement of allyl-2,4-di-tert-butylphenyl ether to 6-allyl-2,4di-tert-butylphenol [286], acetylation of benzo crown ethers [287], synthesis of conjugated nitroalkenes [288] and isomerization and arylation of oleic acid [289]. Interested readers are advised to see these original publications for more details.

4.23. Synthesis of Dypnone from Acetophenone

5. REACTIONS WITH TUNGSTATED ZIRCONIA (WZ)

Dypnone is a highly useful intermediate for the production of a large range of compounds such as softening agents, plastisizers and perfumery bases. Venkatesan and Singh [278] investigated the synthesis of dypnone from acetophenone by utilizing SZ catalyst. They initially compared the activity of several acid catalysts, namely SO42/TiO2, H-, AlCl3 and SZ for dypnone synthesis and it was found that SZ exhibits highest acetophenone conversion (40 wt.%) and highest selectivity for dypnone (86%) among various catalysts investigated (Scheme 49). Latter on, they studied the effect of SZ concentration and reaction temperature. The conversion of acetophenone was found to increase noticeably up to 2 h of reaction time with an increase of SZ/acetophenone ratio from 0.05 to 0.15. However, for longer reaction times (5 h), the conversion of acetophenone was same at all catalyst concentrations. Also, they studied the effect of reaction temperature on the conversion of acetophenone. The reaction temperature was varied from 403 to 443 K. With increasing the reaction temperature the conversion increased.

The WZ is an excellent and environmentally benign solid acid catalyst. There are many reports in the literature on WZ catalysts dealing with its preparation, characterization and catalytic acidity [290-293]. The WZ catalyst has been mostly employed for various vapour phase reactions, namely, tert-butylation of p-cresol [294], production of biodiesel [295] and isomerization [38,39]. This catalyst also exhibits excellent activity for various liquid phase organic reactions as compiled in this section. A major advantage associated with this catalyst is that it is highly stable and does not undergo deactivation unlike SZ where sulfate loss during the course of reaction is normally expected under severe heat and reducing conditions. Acetylation of alcohols, phenols and amines was carried out by using WZ catalyst and acetic anhydride as the acylating agent to give O-acylated products in case of alcohols and phenols (Scheme 51), and N-acylated products in case of amines (Scheme 52) [35]. In a typical reaction procedure a mixture of 1:2 molar amounts of alcohol and acetic anhydride along with the catalyst (0.4 g) were taken in a round bottom flask and refluxed for required times. Reaction took place efficiently providing excellent yields (89-98%) of products within short reaction times (1-4 h). In another study, Sakthivel et al. [36] prepared WZ catalysts with 5, 10, 15, 19 and 25 wt.% WO3 and exploited for Friedel-Crafts acetylation of anisole with acetic anhydride to make 2- and 4methoxyacetophenone. In this study, it was found that catalyst having 19 wt.% WO3 is the best among studied catalysts with regard to conversion and selectivity. Bordoloi et al. [37] employed different WZ catalysts (having different WO3 loading 5-30 wt.%) for acetylation of veratrole with acetic anhydride to produce 3,4dimethoxy acetophenone and toluene alkylation with 1-dodecene. The catalyst with 15 wt.% WO3 calcined at 800 °C was found to be the most active in acylation and alkylation reactions. This most active catalyst showed 67% acetic anhydride conversion in veratrole acylation (veratrole/acetic anhydride molar ratio 2 and 4 h time) at 70 °C, and 99% dodecene conversion with >99% monododecyl toluene selectivity at 100 °C (toluene/1-dodecene molar ratio 10 and time 1 h). The advantages of WZ catalyst are easy operation and simplicity in the work-up, which involves mere filtration of the catalyst, and its reusability. Same groups reported [296] the alkylation of phenol with 1-octene, 1-decene and 1dodecene employing WZ catalyst. As in alkylation and acetylation reactions, WZ catalyst with 15 wt.% WO3 calcined at 800 °C exhibited a maximum conversion at 70 °C reaction temperature. The selectivity to phenyldodecyl ether was high at low reaction temperatures (51% at 70 °C) and decreased with an increase in temperature and totally disappeared at 120 °C.

O

O CH3

2

CH3

SZ

+ H2O

H

140 0C

Scheme 49.

4.24. Cyanosilylation of Aldehydes Thirupathi et al. [111] employed solid acid catalysts, namely SZ, MZ and WZ for cyanosilylation of aldehydes with TMSCN. Initially, to find out better catalyst and suitable conditions, they studied reaction between benzaldehyde and TMSCN using different solvents and different amounts of catalysts. The SZ catalyst, under solvent-free condition at room temperature was found to be best for this reaction. By using the optimized conditions, they performed the reaction by using variety of aldehydes with TMSCN to synthesize corresponding cyanohydrin silyl ethers (Scheme 50). Most of the reactions proceeded smoothly under these mild conditions and produced cyanohydrin silyl ethers in good to excellent yields within short reaction times. Also, they carried out mechanistic study of this methodology by using XPS and FTIR techniques. O

OTMS H

SZ, TMSCN R H sovent-free R = Aryl, Heteroaromatic etc. N2, r.t. 50-300 min

R

CN

Yield = 75-96%

Scheme 50.

There are some more examples of synthesis and transformation reactions catalyzed by SZ in the literature exhibiting moderate to good catalytic activity, which include sulfonylamidomethylation of benzylsulfonamides and 2-phenylethanesulfonamides [113], synthesis of N,N’-diphenylenediamines [279], cyclization of 1phenyl-2-propen-1-ones into 1-indanones [280], nitration of chlorobenzene [281], dehydration of fructose to 5hydroxymethylfurfural [282], Koch carbonylation [283], synthesis

R

OH + Ac2O

WZ 2-4 h

R

OAc + AcOH Yield = 89-97%

Scheme 51.

R

NH2 + Ac2O

WZ

R = Benzyl, Cyclohexyl Scheme 52.

1h

R

NHAc + AcOH Yield = 95-98%



OH O

HO

O

+

R

O

O

WZ, Toluene OEt

OH

110 0C, 5 h

R

R = CH3, OH

CH3 Yield = 50-80%

Scheme 53. NH2 O Ar

CHO +

+

WZ

R

CH3

R = Ph or CH3

NH

r.t., N2 solvent-free

O

Ar

R

Scheme 54.

Ar'

O Ar

CHO + Ar'

Ar'

O

Ar

WZ

NH2 +

NH

+

r.t., N2 solvent-free

NH

O

Ar

anti

syn

Scheme 55.

Ramu et al. [297] reported the esterification of palmitic acid with methanol using WZ catalysts. They investigated a series of WZ catalysts containing 2.5 to 25 wt.% WO3 and found that the catalyst with 5 wt.% loading shows maximum catalytic activity. This is a good example of the utility of WZ catalyst in the liquid phase large-volume applications apart from its several vapour phase catalytic utilities. Reddy et al. [298] reported the synthesis of substituted coumarins from resorcinol and substituted resorcinol with ethyl acetoacetate and ethyl -methylacetoacetate by employing WZ catalyst resulting in good yields (50-80%) at 110 °C in toluene solvent within 5 h (Scheme 53). In another study, Reddy et al. [299] exploited WZ catalysts for multicomponent Mannich reaction between various aromatic aldehydes, amines and ketones at room temperature under solvent-free conditions (Schemes 54 and 55). Various Mannich adducts were formed in good yields (6690%) in short reaction times (1-8 h). Apart from all the applications discussed with WZ catalysts in liquid phase, there are some more examples in the literature on synthesis and transformation reactions catalyzed by WZ which include esterification of oleic acid with methanol, Beckmann rearrangement of cyclohexanone, synthesis of aryl-14Hdibenzo[a.j]xanthenes by one-pot condensation of -naphthol and aryl aldehydes, etc [300-303].

6. REACTIONS WITH MOLYBDATED ZIRCONIA (MZ) The MZ is another very promising modified zirconia solid acid catalyst. Lot of research work is going on over this catalyst as an alternative to SZ catalyst. Mostly, it is used in the vapour phase reactions. Very little work is done by using this catalyst in the liquid phase. Reddy and Reddy [33] reported the synthesis of substituted diphenylureas in the presence of MZ catalyst (Scheme 56). Various substituted diphenylureas were synthesized in good yields (60-75%) from substituted aniline and ethyl acetoacetate under reflux conditions at 180 °C. In another study, Reddy et al. [304] carried out the esterification of -ketoester with various alcohols employing the MZ catalyst. This reaction was carried out by using variety of alcohols (aliphatic, unsaturated, aromatic and hetero aromatic) at 110 °C under reflux conditions with toluene as the solvent and excellent product yields were obtained (43-98%). After completion, the reaction mixture was separated and the wet catalyst was reused. No appreciable change in the activity was observed in several cycles. Manohar et al. [34] reported esterification of mono- and dicarboxylic acids employing the true eco-friendly MZ catalyst under reflux conditions for 1-4 h (Scheme 57). The MZ catalyst exhibited excellent product yields for various esterification reactions. Further, Reddy et al. [305] prepared Ptpromoted TZ catalyst and utilized it for selective protection of

NH2 O

O

+

H N

MZ OMe

O

180 0C, 6 h

R

H N

R Yield = 60-75%

Scheme 56.

O R1

OH

R1 = CH3, Ph Scheme 57.

R2-OH MZ

O R1

OR2

; HOOC

R2 = Bu, Pr etc.

n COOH n = 1,2

R-OH MZ

n ROOC R = CH3

COOR


Patil et al.

carbonyl compounds Variety of carbonyl compounds with ethane1,2-diol in the presence of eco-friendly Pt-Mo/ZrO2 solid acid catalyst provided corresponding 1,3-dioxolanes in excellent yields within 6 h. 7. REACTIONS CATALYSTS

WITH

OTHER

ZIRCONIA-BASED

In order to obtain better catalysts than SZ, to increase the tetragonal phase of ZrO2 and the stability of catalyst, are lots of works in the literature and several investigations are still going on. Various modified zirconia catalysts and promoted SZ catalysts were prepared, characterized and exploited for various organic synthesis and transformation reactions. Recently, Chen et al. [306] utilized SBA-15 supported SZ catalyst for regioselective oxybromination. Yadav and co-workers reported novel mesoporous superacidic zirconia-based catalysts, namely, UDCaT-4, UDCaT-5 and UDCaT-6 and their modified versions which exhibited high catalytic activity, stability and reusability. They studied alkylation of m-cresol with tert-butanol, alkylation of mesitylene with tertbutanol and alkylation of o-cresol with iso-propanol using UDCaT4, UDCaT-5 and UDCaT-6 catalysts [307-309], and acylation of 1,4-dimethoxybenzene with acetic anhydride using UDCaT-5 and other solid acids [225]. Devulapelli and Weng [64] prepared SZ catalyst by a conversional method and mesoporous-SZ by using CTAB as template and compared the catalytic activity for esterification of 4-methoxyphenylacetic acid with dimethyl carbonate. Zhao et al. [88] synthesized SZ catalyst supported on mesostructured -Al2O3 and studied Friedel-Crafts benzoylation of anisole with benzoyl chloride and dealkylation of 1,3,5-tri-tertbutyl-benzene. Also a comparison is made between SZ and MCM41 supported SZ catalysts. Reddy et al. [78,310] prepared sulfated CexZr1-xO2 solid acid catalyst by a co-precipitation method followed by sulfate impregnation using H2SO4 and utilized this catalyst for solvent-free synthesis of coumarins via Pechmann reaction and multicomponent Mannich reaction between aldehyde, ketone and amines. Very recently, Reddy et al. [311] reported for the first time the original use of solid acid catalysts (various zirconia-based catalysts) in regioselective organic synthesis for the formation of NC bond (-aminoalcohols) (Schemes 58 and 59) and CC bond (Friedel-Crafts alkylation) (Scheme 60) by using epoxides and anilines/indoles, and water as the reaction medium. Among various solid acid catalysts, the TZ mixed oxide catalyst exhibited better activity with excellent product yields (7596%) towards the regioenriched desired product. In another study, they prepared sulfate, molybdate and tungstate ion promoted TZ solid acid catalysts by co-precipitation followed by impregnation with sulfuric acid, ammonium heptamolybdate and ammonium metatungstate precursors. Physico-chemical characterization of the prepared catalysts was achieved by using XRD, BET surface area, FT-Raman and XPS methods [79]. Molybdate ion promoted TZ showed good catalytic activity for the reaction of variety of aromatic and aliphatic aldehydes with acetic anhydride to produce corresponding 1,1-diacetates [312]. In the

R1

O

Scheme 60.

R2 O +

R3 R4

N H

OH

TZ, r.t. water

N

R2

R1 Scheme 58.

R1

O

R2

R3

R6 + HN

R4

R5

R1 OH

TZ

R3 +

water, r.t. R2 R5

4 N R 6 R

R1 HO R3 R2 R4 N R5 R6

Scheme 59.

8. CONCLUDING REMARKS In spite of the expected drawbacks of the SZ catalyst in terms of deactivation due to sulfate loss, crystalline-phase transformation from tetragonal to monoclinic, and coke formation, the SZ catalyst has received tremendous interest in recent times due to its simplicity, versatility and superior performance for various organic synthesis and transformation reactions as elaborated in this review. Promoted SZ catalysts have shown good catalytic activity or sometimes better than SZ. Catalytic activity of these catalysts depends on the method of preparation, precursors used, nature of promoting agents, calcination temperature etc. All these solid acid catalysts (mainly SZ) are useful for variety of organic reactions including MCRs, condensation reactions, isomerization reactions, esterification, trans-esterification and so on. These solid acid catalysts having strong incentives to which one can replace the unfriendly H2SO4 and HF acids in many industrial processes, and in this direction there is a lot of scope and advantage to work. Many efforts were undertaken in the last decades on the catalytic activity of SZ and related catalysts, but still there is tremendous scope to study and exploit these catalysts for numerous reactions. ACKNOWLEDGEMENTS We wish to acknowledge all the researchers whose work is described in this review for their valuable contributions. A.N.P. is the recipient of the junior research fellowship of CSIR, New Delhi. M.K.P thanks to UGC, New Delhi for a research project [No: 39727/2010(SR)]. REFERENCES [1] [2]

Olah, G.A.; Prakash, G.K.S.; Sommer, J. Superacids. Science, 1979, 206, 13. Olah, G.A.; Prakash, G.K.S.; Sommer, J. Superacids. John Wiley & Sons: New York, 1985. Yamaguchi, T. Recent progress in solid superacids. Appl. Catal., 1990, 61, 1.

[3]

R1

R5

HN R1

H +

R2

same line, they prepared sulfate, molybdate and tungstate ion promoted AZ, and the surface and bulk properties of the catalysts were studied by using XRD, BET surface area, TGA/DTA, NH3TPD and XPS techniques. The catalytic activity has been evaluated for acetylation of alcohols and amines with acetic anhydride in the liquid phase. The sulfate ion promoted Al2O3-ZrO2 exhibited better product yields under very mild reaction conditions as compared to molybdate- and tungstate-promoted AZ mixed oxides [75].

TZ water, r.t.

R2

OH

R3 R4 R5 NH

R1

HO

R2 + R5

R3 R4

HN


Green and Heterogeneous Catalysts for Organic Synthesis [4] [5] [6] [7] [8]

[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

[20]

[21]

[22]

[23]

[24] [25]

[26]

[27]

[28]

[29] [30] [31]

[32] [33] [34] [35]

Arata, K. Solid superacids. Adv. Catal., 1990, 37, 165. Davis, B.H.; Keogh, R.A.; Srinivasan, R. Sulfated zirconia as a hydrocarbon conversion catalyst. Catal. Today, 1994, 20, 219. Song, X.; Sayari, A. Sulfated zirconia-based strong solid-acid catalysts: Recent progress. Catal. Rev.-Sci. Eng., 1996, 38, 329. Corma, A. Solid acid catalysts. Curr. Opin. Solid. St M., 1997, 2, 63. Yadav, G.D.; Nair, J.J. Sulfated zirconia and its modified versions as promising catalysts for industrial processes. Microporous Mesoporous Mater., 1999, 33, 1. Feller, A.; Lercher, J.A. Chemistry and technology of isobutane/alkene alkylation catalyzed by liquid and solid acids. Adv. Catal., 2004, 48, 229. Corma, A. Attempts to fill the gap between enzymatic, homogeneous, and heterogeneous catalysis. Catal. Rev.-Sci. Eng., 2004, 46, 369. Busca, G. Acid catalysts in industrial hydrocarbon chemistry. Chem. Rev., 2007, 107, 5366. Bhan, A.; Iglesia, E. A link between reactivity and local structure in acid catalysis on zeolites. Acc. Chem. Res., 2008, 41, 559. Bell, A.T. Microscopy: Watching catalysts at work. Nature (London), 2008, 456, 185. Reddy, B.M.; Patil, M.K. Promoted zirconia solid acid catalysts for organic synthesis. Curr. Org. Chem., 2008, 12, 118. Arata, K. Organic syntheses catalyzed by superacidic metal oxides: sulfated zirconia and related compounds. Green Chem., 2009, 11, 1719. Hino, M.; Arata, K. Synthesis of solid superacid catalyst with acid strength of H0?-16.04. J. Chem. Soc., Chem. Commun., 1980, 851. Gillespie, R.J. Fluorosulfuric acid and related superacid media. Acc. Chem. Res., 1968, 1, 202. Gillespie, R.J.; Peel, T.E. Superacid systems. Adv. Phys. Org. Chem., 1971, 9, 1. Hino, M.; Kobayashi, S.; Arata, K. Reactions of butane and isobutane catalyzed by zirconium oxide treated with sulfate ion. Solid superacid catalyst. J. Am. Chem. Soc., 1979, 101, 6439. Reddy, B.M.; Thirupathi, B.; Patil, M.K. Highly efficient promoted zirconia solid acid catalysts for synthesis of -aminonitriles using trimethylsilyl cyanide. J. Mol. Catal. A: Chem., 2009, 307, 154. Tyagi, B.; Mishra, M.K.; Jasra, R.V. Solvent-free isomerisation of longifolene with nano-crystalline sulphated zirconia. Catal. Commun., 2006, 7, 52. Yadav, G.D.; Sengupta, S. FriedelCrafts alkylation of diphenyl oxide with benzyl chloride over sulphated zirconia. Org. Process. Res. Dev., 2002, 6, 256. a) Ratnam, K.J.; Reddy, R.S.; Sekhar, N.S.; Kantam, M.L.; Figuéras, F. Sulphated zirconia catalyzed acylation of phenols, alcohols and amines under solvent-free conditions. J. Mol. Catal. A: Chem., 2007, 276, 230. b) Lenardão, E.J.; Botteselle, G.V.; de Azambuja, F.; Perin, G.; Jacob, R.G. Citronellal as key compound in organic synthesis. Tetrahedron Lett., 2007, 63, 6671. Reddy, B.M.; Patil, M.K. Organic syntheses and transformations catalyzed by sulfated zirconia. Chem. Rev., 2009, 109, 2185. Toshima, K.; Kasumi, K.-i.; Matsumura, S. Novel stereocontrolled glycosidations of 2-deoxyglucopyranosyl fluoride using a heterogeneous solid acid, sulfated zirconia (SO4/ZrO2). Synlett, 1999, 813. Tyagi, B.; Mishra, M.K.; Jasra, R.V. Synthesis of 7-substituted 4-methyl coumarins by Pechmann reaction using nano-crystalline sulphated-zirconia. J. Mol. Catal. A: Chem., 2007, 276, 47. Hsu, C.-Y.; Heimbuch, C.R.; Armes, C.T.; Gates, B.C. A highly active solid superacid catalyst for n-butane isomerization: A sulfated oxide containing iron, manganese and zirconium. J. Chem. Soc., Chem. Commun., 1992, 1645. Hino, M.; Arata, K. Synthesis of solid superacid of tungsten oxide supported on zirconia and its catalytic action for reactions of butane and pentane. J. Chem. Soc., Chem. Commun., 1988, 1259. Arata, K.; Hino, M. Synthesis of solid superacid of molybdenum oxide supported on zirconia and its catalytic action. Chem. Lett., 1989, 971. Arata, K.; Hino, M. Preparation of superacids by metal oxides and their catalytic action. Mater. Chem. Phys., 1990, 26, 213. Hino, M.; Arata, K. Superacids by metal oxides, IX. Catalysis of WO 3/ZrO2 mechanically mixed with Pt/ZrO2 for reaction of butane to isobutane. Appl. Catal., A, 1998, 169, 151. Misono, M.; Okuhara, T. Solid superacid catalysts. CHEMTECH, 1993, 23. Reddy, B.M.; Reddy, V.R. Mo-ZrO2 Solid acid catalyst for synthesis of substituted diphenylureas. Synth. Commun., 1999, 29, 2789. Manohar, B.; Reddy, V.R.; Reddy, B.M. Esterification by ZrO2 and Mo-ZrO2 eco-friendly solid acid catalysts. Synth. Commun., 1998, 28, 3183. Reddy, B.M.; Sreekanth, P.M. Eco-friendly WO3-ZrO2 solid acid catalyst for acetylation of alcohols and phenols. Synth. Commun., 2002, 32, 2815.

[36] [37]

[38]

[39] [40] [41] [42]

[43]

[44] [45]

[46]

[47]

[48]

[49]

[50] [51] [52] [53]

[54]

[55]

[56] [57]

[58]

[59]

[60]

[61]

Sakthivel, R.; Prescott, H.; Kemnitz, E. WO 3/ZrO2: A potential catalyst for the acetylation of anisole. J. Mol. Catal. A: Chem., 2004, 223, 137. Bordoloi, A.; Mathew, N.T.; Devassy, B.M.; Mirajkar, S.P.; Halligudi, S.B. Liquid-phase veratrole acylation and toluene alkylation over WOx/ZrO2 solid acid catalysts. J. Mol. Catal. A: Chem., 2006, 247, 58. Corma, A.; Serra, J.M.; Chica, A. Discovery of new paraffin isomerization catalysts based on SO42/ZrO2 and WOx/ZrO2 applying combinatorial techniques. Catal. Today, 2003, 81, 495. Corma, A. Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions. Chem. Rev., 1995, 95, 559. Fraenkel, D.; Jentzsch, N.R.; Starr, A.C.; Nikrad, P.V. Acid strength of solids probed by catalytic isobutane conversion. J. Catal., 2010, 274, 29. Umansky, B.; Engelhardt, J.; Hall, W.K. On the strength of solid acids. J. Catal., 1991, 127, 128. Ebitani, K.; Tsuji, J.; Hattori, H.; Kita, H. Dynamic modification of surface acid properties with hydrogen molecule for zirconium oxide promoted by platinum and sulfate ions. J. Catal., 1992, 135, 609. Cheung, T.-K.; Lange, F.C.; Gates, B.C. Propane conversion catalyzed by sulfated zirconia, iron- and manganese-promoted sulfated zirconia, and USY zeolite. J. Catal., 1996, 159, 99. Grago, R.S.; Kob, N. Acidity and reactivity of sulfated zirconia and metaldoped sulfated zirconia. J. Phys. Chem. B, 1997, 101, 3360. Arena, F.; Dario, R.; Parmaliana, A. A characterization study of the surface acidity of solid catalysts by temperature programmed methods. Appl. Catal., A, 1998, 170, 127. Vartuli, J.C.; Santiesteban, J.G.; Traverso, P.; Cardona-Martinez, N.; Chang, C.D.; Stevenson, S.A. Characterization of the acid properties of tungsten/zirconia catalysts using adsorption microcalorimetry and n-pentane isomerization activity. J. Catal., 1999, 187, 131. Matsuhashi, H.; Nakamura, H.; Ishihara, T.; Iwamoto, S.; Kamiya, Y.; Kobayashi, J.; Kubota, Y.; Yamada, T.; Matsuda, T.; Matsushita, K.; Nakai, K.; Nishiguchi, H.; Ogura, M.; Okazaki, N.; Sato, S.; Shimizu, K.; Shishido, T.; Yamazoe, S.; Takeguchi, T.; Tomishige, K.; Yamashita, H.; Niwa, M.; Katada, N. Characterization of sulfated zirconia prepared using reference catalysts and application to several model reactions. Appl. Catal., A, 2009, 360, 89. Reddy, B.M.; Sreekanth, P.M.; Lakshmanan, P. Sulfated zirconia as an efficient catalyst for organic synthesis and transformation reactions. J. Mol. Catal. A: Chem., 2005, 237, 93. Reddy, B.M.; Sreekanth, P.M.; Reddy, V.R. Modified zirconia solid acid catalysts for organic synthesis and transformations. J. Mol. Catal. A: Chem., 2005, 225, 71. Yamaguchi, T.; Tanabe, K.; Kung, Y.C. Preparation and characterization of ZrO2 and SO42-promoted ZrO2. Mater. Chem. Phys., 1986, 16, 67. Sohn, J.R.; Kim, H.W. Catalytic and surface properties of ZrO2 modified with sulfur compounds. J. Mol. Catal., 1989, 52, 361. Li, X.; Nagaoka, K.; Olindo, R.; Lercher, J.A. Synthesis of highly active sulfated zirconia by sulfation with SO3. J. Catal., 2006, 238, 39. Yadav, G.D.; Pathre, G.S. Novelties of a superacidic mesoporous catalyst UDCaT-5 in alkylation of phenol with tert-amyl alcohol. Appl. Catal., A, 2006, 297, 237. Ramos, F.S.; Duarte de Farias, A.M.; Borges, L.E.P.; Monteiro, J.L.; Fraga, M.A.; Sousa-Aguiar, E.F.; Appel, L.G. Role of dehydration catalyst acid properties on one-step DME synthesis over physical mixtures. Catal. Today, 2005, 101, 39. Arata, K.; Hino, M.; Yamaguta, N. Acidity and catalytic activity of zirconium and titanium sulfates heat-treated at high temperature. Solid superacid catalysts. Bull. Chem. Soc. Jpn., 1990, 63, 244. Platero, E.E.; Mentruit, M.P. IR characterization of sulfated zirconia derived from zirconium sulfate. Catal. Lett., 1994, 30, 31. Sun, Y.; Ma, S.; Du, Y.; Yuan, L.; Wang, S.; Yang, J.; Deng, F.; Xiao, F.-S. Solvent-free preparation of nanosized sulfated zirconia with Brønsted acidic sites from a simple calcination. J. Phys. Chem. B, 2005, 109, 2567. Risch, M.A.; Wolf, E.E. Characterization and n-butane isomerization activity of high surface area sulfated zirconia catalysts. Appl. Catal., A, 1998, 172, L1. Risch, M.A.; Wolf, E.E. Effect of the preparation of a mesoporous sulfated zirconia catalyst in n-butane isomerization activity. Appl. Catal., A, 2001, 206, 283. Morterra, C.; Cerrato, G.; Ardizzone, S.; Bianchi, C.L.; Signoretto, M.; Pinna, F. Surface features and catalytic activity of sulfated zirconia catalysts from hydrothermal precursors. Phys. Chem. Chem. Phys., 2002, 4, 3136. Sun, Y.; Yuan, L.; Wang, W.; Chen, C.-L.; Xiao, F.-S. Mesostructured sulfated zirconia with high catalytic activity in n-butane isomerization. Catal. Lett., 2003, 87, 57.

3980 Current Organic Chemistry, 2011, Vol. 15, No. 23 [62]

[63]

[64]

[65]

[66]

[67]

[68]

[69]

[70]

[71]

[72] [73] [74]

[75]

[76] [77]

[78]

[79]

[80] [81]

[82] [83]

[84]

[85]

[86]

[87]

Yang, X.; Jentoft, F.C.; Jentoft, R.E.; Girgsdies, F.; Ressler, T. Sulfated zirconia with ordered mesopores as an active catalyst for n-butane isomerization. Catal. Lett., 2002, 81, 25. Suh, Y.-W.; Lee, J.-W.; Rhee, H.-K. Skeletal isomerization of 1-butene over sulfate-promoted zirconia with large surface area prepared by an atrane route. Appl. Catal., A, 2004, 274, 159. Devulapelli, V.G.; Weng, H.-S. Esterification of 4-methoxyphenylacetic acid with dimethyl carbonate over mesoporous sulfated zirconia. Catal. Commun., 2009, 10, 1711. He, C.X.; Yue, B.; Cheng, J.F.; Hua, W.M.; Yue, Y.H.; He, H.Y. Preparation and characterization of mesoporous tetragonal sulfated zirconia. Chinese Chem. Lett., 2009, 20, 869. Das, S.K; Bhunia, M.K.; Sinha, A.K.; Bhaumik, A. Self-assembled mesoporous zirconia and sulfated zirconia nanoparticles synthesized by triblock copolymer as template. J. Phys. Chem. C, 2009, 113, 8918. Rubio, E.; Rodriguez-Lugo, V.; Rodriguez, R.; Castaño, V.M. Nano zirconia and sulfated zirconia from ammonia zirconium carbonate. Rev. Adv. Mater. Sci., 2009, 22, 67. Occelli, M.L.; Schiraldi, D.A.; Auroux, A.; Keogh, R.A.; Davis, B.H. Effects of copper on the activity of sulfated zirconia catalysts for n-pentane isomerization. Appl. Catal., A, 2001, 209, 165. Delahay, G.; Coq, B.; Ensuque, E.; Figuéras, F. Interaction between nitric oxide and copper on copper zirconia sulfated catalysts. Langmuir, 1997, 13, 5588. Delahay, G.; Ensuque, E.; Coq, B.; Figuéras, F. Selective catalytic reduction of nitric oxide by n-decane on Cu/sulfated-zirconia catalysts in oxygen rich atmosphere: Effect of sulfur and copper contents. J. Catal., 1998, 175, 7. Ardizzone, S.; Bianchi, C.L.; Cappelletti, G.; Annunziata, R.; Cerrato, G.; Morterra, C.; Scardi, P. Liquid phase reactions catalyzed by Fe- and Mnsulphated ZrO2. Appl. Catal., A, 2009, 360, 137. Nagaraju, N.; Shamshuddin, S.Z.M. Vapour phase pinacol rearrangement over modified sulphated zirconia. Indian J. Chem. Techn., 2007, 14, 47. Soylu, G.S.P.; Iik, A.B.; Boz, I. Dehydroisomerization of n-butane over metal promoted sulfated zirconia. Turk. J. Eng. Environ. Sci., 2009, 33, 273. Rosenberg, D.J.; Coloma, F.; Anderson, J.A. Modification of the acid properties of silica-zirconia aerogels by in situ and ex situ sulfation. J. Catal., 2002, 210, 218. Reddy, B.M.; Sreekanth, P.M.; Yamada, Y.; Kobayashi, T. Surface characterization and catalytic activity of sulfate-, molybdate- and tungstatepromoted Al2O3-ZrO2 solid acid catalysts. J. Mol. Catal. A: Chem., 2005, 227, 81. Shibta, K.; Kiyoura, T.; Kitagawa, J.; Sumiyoshi, T.; Tanabe, K. Acidic properties of binary metal oxides. Bull. Chem. Soc. Jpn., 1973, 46, 2985. Arata, K.; Akutagawa, S.; Tanabe, K.; Epoxide rearrangement III. Isomerization of 1-methylcyclohexene oxide over TiO2-ZrO2, NiSO4 and FeSO4. Bull. Chem. Soc. Jpn., 1976, 49, 390. Reddy, B.M.; Sreekanth, P.M.; Lakshmanan, P.; Khan, A. Synthesis, characterization and activity study of SO42/CexZr1xO2 solid superacid catalyst. J. Mol. Catal. A: Chem., 2006, 244, 1. Reddy, B.M.; Sreekanth, P.M.; Yamada, Y.; Xu, Q.; Kobayashi, T. Surface characterization of sulfate, molybdate, and tungstate promoted TiO2-ZrO2 solid acid catalysts by XPS and other techniques. Appl. Catal., A, 2002, 228, 269. Zhao, J.; Yue, Y.; Hua, W.; Gao, Z. Catalytic activities and properties of mesoporous sulfated Al2O3-ZrO2. Catal. Lett., 2007, 116, 27. Guevara-Franco, M.L.; Robles-Andrade, S.; García-Alamilla, R.; SandovalRobles, G.; Domínguez-Esquivel, J.M. Study of n-hexane isomerization on mixed Al2O3-ZrO2/SO42 catalysts. Catal. Today, 2001, 65, 137. Gao, Z.; Xia, Y.; Hua, W.; Miao, C. New catalyst of SO42/Al2O 3-ZrO2 for nbutane isomerization. Top. Catal., 1998, 6, 101. Lónyi, F.; Valyon, J. Thermally effected structural and surface transformations of sulfated TiO2, ZrO2 and TiO2-ZrO2 catalysts. J. Therm. Anal., 1996, 46, 211. Azambre, B.; Zenboury, L.; Weber, J.V.; Burg, P. Surface characterization of acidic ceria-zirconia prepared by direct sulfation. Appl. Surf. Sci., 2010, 256, 4570. Mejri, I.; Younes, M.K.; Ghorbel, A.; Eloy, P.; Gaigneaux, E.M. Characterization and reactivity of aerogel sulfated zirconia-ceria catalyst for n-hexane isomerization. J. Porous Mater., 2010, 17, 545. Negrón-Silva, G.; Hernández-Reyes, C.X.; Angeles-Beltrán, D.; LomasRomero, L.; González-Zamora, E. Microwave-enhanced sulphated zirconia and SZ/MCM-41 catalyzed regioselective synthesis of -amino alcohols under solvent-free conditions. Molecules, 2008, 13, 977. Guo, D.-J.; Qiu, X.-P.; Zhu, W.-T.; Chen, L.-Q. Synthesis of sulfated ZrO2/MWCNT composites as new supports of Pt catalysts for direct methanol fuel cell application. Appl. Catal., B, 2009, 89, 597.

Patil et al. [88]

[89] [90]

[91]

[92]

[93] [94] [95] [96] [97]

[98] [99]

[100]

[101]

[102]

[103] [104] [105]

[106]

[107]

[108]

[109] [110]

[111]

[112]

[113]

Zhao, J.; Yue, Y.; Hua, W.; He, H.; Gao, Z. Catalytic activities and properties of sulfated zirconia supported on mesostructured -Al2O3. Appl. Catal., A, 2008, 336, 133. Reddy, B.M.; Reddy, V.R. Influence of SO42, Cr2O 3, MoO3, and WO3 on the stability of ZrO2-tetragonal phase. J. Mater. Sci. Lett., 2000, 19, 763. Park, Y.-M.; Lee, J.Y.; Chung, S.-H.; Park, I.S.; Lee, S.-Y.; Kim, D.-K.; Lee, J.-S.; Lee, K.-Y. Esterification of used vegetable oils using the heterogeneous WO3/ZrO2 catalyst for production of biodiesel. Bioresour. Technoly., 2010, 101, S59. Yu, H.; Fang, H.; Zhang, H.; Li, B.; Deng, F. Acidity of sulfated tin oxide and sulfated zirconia: A view from solid-state NMR spectroscopy. Catal. Commun., 2009, 10, 920. Hua, W.; Goeppert, A.; Sommer, J. H/D Exchange and isomerization of small alkanes over unpromoted and Al2O3-promoted SO42/ZrO2 catalysts. J. Catal., 2001, 197, 406. Hua, W.; Goeppert, A.; Sommer, J. Methane activation in the presence of Al2O3-promoted sulfated zirconia. Appl. Catal., A, 2001, 219, 201. Jin, T.; Yamaguchi, T.; Tanabe, K. Mechanism of acidity generation on sulfur-promoted metal oxides. J. Phys. Chem., 1986, 90, 4794. Ward, D.A.; Ko, E.I. One-step synthesis and characterization of zirconiasulfate aerogels as solid superacids. J. Catal., 1994, 150, 18. Kustov, L.M.; Kazansky, V.B.; Figuéras, F.; Tichit, D. Investigation of the acidic properties of ZrO2 modified by SO42anions. J. Catal., 1994, 150, 143. Saur, O.; Bensitel, M.; Saad, A.B.M.; Lavalley, J.C.; Tripp, C.P.; Morrow, B.A. The structure and stability of sulfated alumina and titania. J. Catal., 1986, 99, 104. Rosenberg, D.J.; Anderson, J.A. On determination of acid site densities on sulfated oxides. Catal. Lett., 2002, 83, 59. Babou, F.; Bigot, B.; Coudurier, G.; Sautet, P.; Vedrine, J.C. Sulfated zirconia for n-butane isomerization experimental and theoretical approaches. Stud. Surf. Sci. Catal., 1994, 90, 519. White, R.L.; Sikabwe, E.C.; Coelho, M.A.; Resasco, D.E. Potential role of penta-coordinated sulfur in the acid site structure of sulfated zirconia. J. Catal., 1995, 157, 755. Baertsch, C.D.; Soled, S.L.; Iglesia, E. Isotopic and chemical titration of acid sites in tungsten oxide domains supported on zirconia. J. Phys. Chem. B, 2001, 105, 1320. Sohn, J.R.; Lim, J.S.; Lee, S.H. A new solid superacid catalyst prepared by doping ZrO2 with Ce and modifying with sulfate simultaneously. Chem. Lett., 2004, 33, 1490. Sakthivel, A.; Saritha, N.; Selvam, P. Vapour phase tertiary butylation of phenol over sulfated zirconia catalyst. Catal. Lett., 2001, 72, 225. Li, X.; Nagaoka, K.; Simon, L.J.; Olindo, R.; Lercher, J.A. Mechanism of butane skeletal isomerization on sulfated zirconia. J. Catal., 2005, 232, 456. Li, X.; Nagaoka, K.; Lercher, J.A. Labile sulfates as key components in active sulfated zirconia for n-butane isomerization at low temperatures. J. Catal., 2004, 227, 130. Funamoto, T.; Nakagawa, T.; Segawa, K. Isomerization of n-butane over sulfated zirconia catalyst under supercritical conditions. Appl. Catal., A, 2005, 286, 79. Essayem, N.; Taaˆrit, Y.B.; Feche, C.; Gayraud, P.Y.; Sapaly, G.; Naccache, C. Comparative study of n-pentane isomerization over solid acid catalysts, heteropolyacid, sulfated zirconia, and mordenite: Dependence on hydrogen and platinum addition. J. Catal., 2003, 219, 97. Rezgui, S.; Jentoft, R.E.; Gates, B.C. n-Pentane isomerization and disproportionation catalyzed by promoted and unpromoted sulfated zirconia. Catal. Lett., 1998, 51, 229. Adeeva, V.; Liu, H.-Y.; Xu, B.-Q.; Sachtler, W.M.H. Alkane isomerization over sulfated zirconia and other solid acids. Top. Catal., 1998, 6, 61. Alemán-Vázquez, L.O.; Mariel-Reyes, P.R.; Cano-Domínguez, J.L. The effect of sulfates concentration in sulfated zirconia (SZ) catalysts n-heptane isomerization. Petrol. Sci. Tech., 2010, 28, 374. Thirupathi, B.; Patil, M.K.; Reddy, B.M. Activation of trimethylsilyl cyanide and mechanistic investigation for facile cyanosilylation by solid acid catalysts under mild and solvent-free conditions. Appl. Catal., A, 2010, 384, 147. Tyagi, B.; Mishra, M.K.; Jasra, R.V. Solvent free synthesis of acetyl salicylic acid over nano-crystalline sulfated zirconia solid acid catalyst. J. Mol. Catal. A: Chem., 2010, 317, 41. Sasiambarrena, L.D.; Mendez, L.J.; Ocsachoque, M.A.; Cánepa, A.S.; Bravo, R.D.; González, M.G. Sulfated zirconia as an efficient catalyst for sulfonylamidomethylation of benzylsulfonamides and 2phenylethanesulfonamides: Effect of catalyst thermal treatment. Catal. Lett., 2010, 138, 180.


Green and Heterogeneous Catalysts for Organic Synthesis [114]

[115] [116] [117]

[118] [119] [120]

[121] [122] [123] [124]

[125] [126]

[127]

[128]

[129]

[130]

[131]

[132]

[133]

[134]

[135]

[136]

[137]

[138]

[139]

Armstrong, R.W.; Combs, A.P.; Tempest, P.A.; Brown, S.D.; Keating, T.A. Multiple-component condensation strategies for combinatorial library synthesis. Acc. Chem. Res., 1996, 29, 123. Marek, I. Multicomponent reactions. Tetrahedron, 2005, 61, 11309. Zhu, J.; Bienaymé, H. Multicomponent Reactions, Wiley-VCH: Weinheim, 2005. Huang, X.; Huang, J.; Wen, Y.; Feng, X. Asymmetric Strecker reaction of ketoimines catalyzed by a novel chiral bifunctional N,N-dioxide. Adv. Synth. Catal., 2006, 348, 2579. March, J. Advanced organic chemistry, 4th ed.; Wiley: New York, 1999, p. 65. Dyker, G. Amino acid derivatives by multicomponent reactions. Angew. Chem., Int. Ed., 1997, 36, 1700. González-Vera, J.A.; García-López, M.T.; Herranz, R. Molecular diversity via amino acid derived -aminonitriles: Synthesis of spirocyclic 2,6dioxopiperazine derivatives. J. Org. Chem., 2005, 70, 3660. Shafran, Y.M.; Bakulev, V.A.; Mokrushin, V.S. Synthesis and properties of -aminonitriles. Russ. Chem. Rev., 1989, 58, 148. Enders, D.; Shilvock, J.P. Some recent applications of -amino nitrile chemistry. Chem. Soc. Rev., 2000, 29, 359. Weinstock, L.M.; Davis, P.; Handelsman, B.; Tull, R.A. General synthetic system for 1,2,5-thiadiazoles. J. Org. Chem., 1967, 32, 2823. Matier, W.L.; Owens, D.A.; Comer, W.T.; Deitchman, D.; Ferguson, H.C.; Seidehamel, R.J.; Young, J.R. Antihypertensive agents. Synthesis and biological properties of 2-amino-4-aryl-2-imidazolines. J. Med. Chem., 1973, 16, 901. Duthaler, R.O. Recent developments in the stereoselective synthesis of aminoacids. Tetrahedron, 1994, 50, 1539. Shen, Z.-L.; Ji, S.-J.; Loh, T.-P. Indium(III) iodide-mediated Strecker reaction in water: An efficient and environmentally friendly approach for the synthesis of -aminonitrile via a three-component condensation. Tetrahedron, 2008, 64, 8159. Majhi, A.; Kim, S.S.; Kadam, S.T. Rhodium(III) iodide hydrate catalyzed three-component coupling reaction: Synthesis of -aminonitriles from aldehydes, amines, and trimethylsilyl cyanide. Tetrahedron, 2008, 64, 5509. Fontaine, P.; Chiaroni, A.; Masson, G.; Zhu, J. One-pot three-component synthesis of -iminonitriles by IBX/TBAB-mediated oxidative Strecker reaction. Org. Lett., 2008, 10, 1509. Fetterly, B.M.; Jana, N.K.; Verkade, J.G. [HP(HNCH 2CH 2)3N]NO3: An efficient homogeneous and solid-supported promoter for aza and thiaMichael reactions and for Strecker reactions. Tetrahedron, 2006, 62, 440. Yadav, J.S.; Reddy, B.V.S.; Eeshwaraiah, B.; Srinivas, M. Montmorillonite KSF clay catalyzed one-pot synthesis of -aminonitriles. Tetrahedron, 2004, 60, 1767. Karimi, B.; Safari, A.A. One-pot synthesis of -aminonitriles using a highly efficient and recyclable silica-based scandium (III) interphase catalyst. J. Organomet. Chem., 2008, 693, 2967. Heydari, A.; Arefi, A.; Khaksar, S.; Shiroodi, R.K. Guanidine hydrochloride: An active and simple catalyst for Strecker type reaction. J. Mol. Catal. A: Chem., 2007, 271, 142. Casimir, J.R.; Turetta, C.; Ettouati, L.; Paris, J. First application of the Dakin-West reaction to fmoc chemistry: Synthesis of the ketomethylene tripeptide fmoc-N-Asp(tBu)-(R,S Tyr(tBu)(CO-CH2)Gly-OH. Tetrahedron Lett., 1995, 36, 4797. Rao, I.N.; Prabhakaran, E.N.; Das, S.K.; Iqbal, J. Cobalt-catalyzed one-pot three-component coupling route to -acetamido carbonyl compounds: A general synthetic protocol for -lactams. J. Org. Chem., 2003, 68, 4079, and references therein. Pandey, G.; Singh, R.P.; Garg, A.; Singh, V.K. Synthesis of Mannich type products via a three-component coupling reaction. Tetrahedron Lett., 2005, 46, 2137. Ghosh, R.; Maiti, S.; Chakraborty, A. One-pot multicomponent synthesis of -acetamido ketones based on BiCl3 generated in situ from the procatalyst BiOCl and acetyl chloride. Synlett, 2005, 115. Khan, A.T.; Choudhury, L.H.; Parvin, T.; Ali, M.A. CeCl3 ·7H2O: An efficient and reusable catalyst for the preparation of -acetamido carbonyl compounds by multicomponent reactions (MCRs). Tetrahedron Lett., 2006, 47, 8137. Ghosh, R.; Maiti, S.; Chakraborty, A.; Chakraborty, S.; Mukherjee, A.K. ZrOCl2·8H2O: An efficient Lewis acid catalyst for the one-pot multicomponent synthesis of -acetamido ketones. Tetrahedron, 2006, 62, 4059. Das, B.; Reddy, K.R.; Ramu R.; Thirupathi, P.; Ravikanth, B. Iodine as an efficient catalyst for one-pot multicomponent synthesis of -acetamido ketones. Synlett, 2006, 1756.

[140]

[141]

[142]

[143]

[144]

[145] [146] [147] [148]

[149]

[150]

[151]

[152]

[153]

[154]

[155]

[156]

[157]

[158] [159]

[160]

[161]

[162]

Bahulayan, D.; Das, S.K.; Iqbal, J. Montmorillonite K10 clay: An efficient catalyst for the one-pot stereoselective synthesis of -acetamido ketones. J. Org. Chem., 2003, 68, 5735. Khodaei, M.M.; Khosropour, A.R.; Fattahpour, P. A modified procedure for the Dakin-West reaction: An efficient and convenient method for a one-pot synthesis of -acetamido ketones using silica sulfuric acid as catalyst. Tetrahedron Lett., 2005, 46, 2105. Rafiee, E.; Shahbazi, F.; Joshaghani, M.; Tork, F. The silica supported H3PW12O40 (a heteropoly acid) as an efficient and reusable catalyst for a onepot synthesis of -acetamido ketones by Dakin-West reaction. J. Mol. Catal. A: Chem., 2005, 242, 129. Rafiee, E.; Tork, F.; Joshaghani, M. Heteropoly acids as solid green Brønsted acids for a one-pot synthesis of -acetamido ketones by Dakin-West reaction. Bioorg. Med. Chem. Lett., 2006, 16, 1221. Das, B.; Krishnaiah, M.; Laxminarayana, K.; Reddy K.R. A simple and efficient one-pot synthesis of -acetamido carbonyl compounds using sulfated zirconia as a heterogeneous recyclable catalyst. J. Mol. Catal. A: Chem., 2007, 270, 284. Zaugg, H.E.; Martin, W.B. -Amidoalkylations at carbon. Org. React. 1965, 14, 88. Kappe, C.O. 100 years of the Biginelli dihydropyrimidine synthesis. Tetrahedron, 1993, 49, 6937. Kappe, C.O. Biologically active dihydropyrimidones of the Biginelli-type — a literature survey. Eur. J. Med. Chem., 2000, 35, 1043. Lu, J.; Bai, Y.; Wang, Z.; Yang, B.; Ma, H. One-pot synthesis of 3,4dihydropyrimidin-2(1H)-ones using lanthanum chloride as a catalyst. Tetrahedron Lett., 2000, 41, 9075. Bigi, F.; Carloni, S.; Frullanti, B.; Maggi, R.; Sartori, G. A revision of the Biginelli reaction under solid acid catalysis. Solvent-free synthesis of dihydropyrimidines over montmorillonite KSF. Tetrahedron Lett., 1999, 40, 3465. Salehi, P.; Dabiri, M.; Zolfigol, M.A.; Fard, M.A.B. Silica sulfuric acid: An efficient and reusable catalyst for the one-pot synthesis of 3,4dihydropyrimidin-2(1H)-ones. Tetrahedron Lett., 2003, 44, 2889. Ma, Y.; Qian, C.; Wang, L.; Yang, M. Lanthanide triflate catalyzed Biginelli reaction. One-pot synthesis of dihydropyrimidinones under solvent-free conditions. J. Org. Chem., 2000, 65, 3864. Yadav, J.S.; Reddy, B.V.S; Sridhar, P.; Reddy, J.S.S.; Nagaiah, K.; Lingaiah, N.; Saiprasad, P.S. Green protocol for the Biginelli three-component reaction: Ag3PW12O 40 as a novel, water-tolerant heteropolyacid for the synthesis of 3,4-dihydropyrimidinones. Eur. J. Org. Chem., 2004, 552. Gopalakrishnan, M.; Sureshkumar, P.; Kanagarajan, V.; Thanusu, J.; Govindaraju, R.; Ezhilarasi, M.R. Microwave-promoted facile and rapid solvent-free synthesis procedure for the efficient synthesis of 3,4dihydropyrimidin-2(1H)-ones and -thiones using ZrO2/SO42- as a reusable heterogeneous catalyst. Lett. Org. Chem., 2006, 3, 484. Kumar, D.; Sundaree, M.S.; Mishra, B.G. Sulfated zirconia-catalyzed onepot benign synthesis of 3,4-dihydropyrimidin-2(1H)-ones under microwave irradiation. Chem. Lett., 2006, 35, 1074. Kumar, D.; Mishra, B.G.; Rao, V.S. An environmentally benign protocol for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones using solid acid catalysts under solvent-free conditions. Indian J. Chem., Sect. B, 2006, 45, 2325. Kumar, D.; Mishra, B.G. Sulfated zirconia and phosphotungstic acid catalyzed synthesis of some biologically potent heterocyclic compounds. Bull. Catal. Soc. India, 2006, 5, 121. Angeles-Beltrán, D.; Lomas-Romero, L.; Lara-Corona, V.H.; GonzálezZamora, E.; Negrón-Silva G. Sulfated zirconia-catalyzed synthesis of 3,4dihydropyrimidin-2(1H)-ones (DHPMs) under solventless conditions: Competitive multicomponent Biginelli vs. Hantzsch reactions. Molecules, 2006, 11, 731. Warren, S. Organic synthesis: The disconnection approach, Wiley: 2009, p. 67. Carrigan, M.C.; Eash, K.J.; Oswald, M.C.; Mohan, R.S. An efficient method for the chemoselective synthesis of acylals from aromatic aldehydes using bismuth triflate. Tetrahedron Lett., 2001, 42, 8133. Aggen, D.H.; Arnold, J.N.; Hayes, P.D.; Smoter, N.J.; Mohan, R.S. Bismuth compounds in organic synthesis. Bismuth nitrate catalyzed chemoselective synthesis of acylals from aromatic aldehydes. Tetrahedron, 2004, 60, 3675. Smitha, G.; Reddy, Ch.S. A facile and efficient ZrCl4 catalyzed conversion of aldehydes to geminal-diacetates and dipivalates and their cleavage. Tetrahedron, 2003, 59, 9751, and references therein. Gowravaram, S.; Abraham S.; Ramalingam, T.; Yadav, J.S. A facile method for the preparation and cleavage of 1,1-diacetates of aldehydes. J. Chem. Res., 2002, 3, 144.


[164]

[165] [166]

[167] [168]

[169]

[170]

[171]

[172]

[173] [174] [175] [176] [177] [178]

[179]

[180]

[181]

[182]

[183] [184] [185]

[186]

[187]

[188]

Li, T.-S.; Zhang, Z.-H.; Gao, Y.-J.; A rapid preparation of acylals of aldehydes catalysed by Fe3 -montmorillonite. Synth. Commun., 1998, 28, 4655. Olah, G.A.; Mehrotra, A.K. Catalysis by solid superacids; 161. Improved Nafion-H catalyzed preparation of 1,1-diacetates from aldehydes. Synthesis, 1982, 962. ubo, J.; Komadel, P. Metal cation-exchanged montmorillonite catalyzed protection of aromatic aldehydes with Ac2O. J. Catal., 2003, 218, 227. Negrón, G.E.; Palaciosa, L.N.; Angelesa, D.; Lomasb, L.; Gaviño, R. A mild and efficient method for the chemoselective synthesis of acylals from aromatic aldehydes and their deprotections catalyzed by sulfated zirconia. J. Braz. Chem. Soc., 2005, 16, 490. Raju, S.V.N. Sulfated zirconia: An efficient catalyst for the synthesis of 1,1diacetates from aldehydes and ketones. J. Chem. Res., 1996, 68. Ranu, B.C.; Majae, A.; Das, A.R. Surface-mediated solid phase reaction. PART 7. A simple and convenient procedure for the methoxymethylation of alcohols with methoxymethyl chloride on the surface of alumina. Synth. Commun., 1995, 25, 363. Kumar, P.; Raju, S.V.N.;. Reddy, R.S; Pandey, B. Na-Y Zeolite, an efficient catalyst for the methoxymethylation of alcohols. Tetrahedron Lett., 1994, 35, 1289. Fugi, K.; Nakano, S.; Fujita, E. An improved method for methoxymethylation of alcohols under mild acidic conditions. Synthesis, 1975, 276. Jin, T.-S.; Li, T.-S.; Gao, Y.-T. A facile preparation of methoxymethyl ethers of primary and secondary alcohols with dimethoxymethane catalyzed by expansive graphite. Synth. Commun., 1998, 28, 837. Patney, H.K. Anhydrous iron(III) chloride dispersed on 3A molecular sieves, a mild and efficient catalyst for methoxymethylation of alcohols. Synlett, 1992, 567. Kantam, M.L.; Santhi, P.L. Molybdenum catalyzed methoxymethylation of alcohols. Synlett, 1993, 429. Lin, C.-H.; Wan, M.-Y.; Huang, Y.-M. Methoxymethylation of alcohols catalyzed by sulfated metal oxides. Catal. Lett., 2003, 87, 253. Fieser, L.F.; Fieser, M. Reagents for Organic Synthesis: Wiley: New York, 1967, 1, 256. Greene, T.W.; Wuts, P.G.M. Protective groups in organic synthesis, 2nd Ed.; Wiley: New York, 1991, and references therein. Kocienski, P.J. Protecting groups: George Thieme Verlag: New York, 1994. Hoyer, S.; Laszlo, P. Catalysis by acidic clay of the protective tetrahydropyranylation of alcohols and phenols. Synthesis, 1986, 655, and references therein. Nishiguchi, T.; Kawamine, K. Selective monoetherification of 1, n-diols catalyzed by aluminium sulphate supported on silica gel. J. Chem. Soc., Chem. Commun., 1990, 1766. a) Heravi, M.M.; Ajami, D.; Ghassemzadeh, M. Solvent-free tetrahydropyranylation of alcohols and phenols over sulfuric acid adsorbed on silica gel. Synth. Commun., 1999, 29, 1013; b) Gajare, A.S.; Sabde, D.P.; Shingare, M.S.; Wakharkar, R.D. Microwave accelerated tetrahydropyranylation and detetrahydropyranylation of alcohols, phenols, and thiols catalyzed by hydrated zirconia. Synth. Commun., 2002, 32, 1549. Kumar, P.; Dinesh, C.U.; Reddy, R.S.; Pandey, B. H-Y Zeolite: A mild and efficient catalyst for the tetrahydropyranylation of alcohols. Synthesis, 1993, 1069. Reddy, B.M.; Sreekanth, P.M. An efficient zirconia catalyst for solvent-free tetrahydropyranylation of alcohols and phenols. Synth. Commun., 2002, 32, 3561. Langer, S.H.; Connell, S.; Wender, I. Preparation and properties of trimethylsilyl ethers and related compounds. J. Org. Chem., 1958, 23, 50. Sinou, D.; Emziane, M. Trimethylsilylazide: A highly reactive silylating agent for primary and secondary alcohols. Synthesis, 1986, 1045. Jacqueline, D.H.; Chan, W.H.; Ashley, D.L.; William, R.R. Concerning the selective protection of (Z)-1,5-syn-ene-diols and (E)-1,5-anti-ene-diols as allylic trimethylsilyl ethers. Org. Lett., 2007, 9, 5621. Morita, T.; Okamoto, Y.; Sakurai, H. Use of allysilanes as a new type of silylating agent for alcohols and carboxylic acids. Tetrahedron Lett., 1980, 21, 835. Hajime, I.; Katsuhiro, T.; Takahiro, M.; Masaya, S. Gold(I)phosphine catalyst for the highly chemoselective dehydrogenative silylation of alcohols. Org. Lett., 2005, 7, 3001. Firouzabadi, H.; Iranpoor, N.; Amani, K.; Nowrouzi, F. Tungstophosphoric acid (H3PW12O 40) as a heterogeneous inorganic catalyst. Activation of hexamethyldisilazane (HMDS) by tungstophosphoricacid for efficient and selective solvent-free O-silylation reactions. J. Chem. Soc., Perkins Trans 1, 2002, 23, 2601.

Patil et al. [189]

[190]

[191]

[192]

[193]

[194]

[195]

[196]

[197]

[198]

[199]

[200]

[201]

[202]

[203] [204]

[205]

[206]

[207]

[208]

[209] [210]

[211] [212]

[213]

John, S.D.; Clement, L.H.; Tremeer, E.J.; Charles, B.; Richard, C.T. Protection of hydroxy groups by silylation: Use in peptide synthesis and as lipophilicity modifiers for peptides. J. Chem. Soc., Perkin Trans., 1992, 1, 3043. Mei Xiao, H.Y.; Suzane, M.K.; Christian, M.M.; William, L.S.; Yu, L.J. Facile synthesis of anticancer drug NCX 4040 in mild conditions. Lett. Org. Chem., 2008, 5, 510. Thirupathi, B.; Prasad, A.N.; Srinivas, R.; Reddy, B.M. Sulfated zirconia: An efficient catalyst for solvent-free synthesis of silyl ethers under mild conditions. Synth. Commun., 2011, 41, 2064. Herbert, J.A.L.; Suschitzky, H. Syntheses of heterocyclic compounds. Part XXIX. Substituted 2,3-dihydro-1H-1,5-benzodiazepines. J. Chem. Soc., Perkin Trans. 1, 1974, 2657. Dai-Il, J.; Tae-wonchoi, C.; Yun-Young, K.; In-Shik, K.; You-Mi, P.; YongGyun, L.; Doo-Hee, J. Synthesis of 1,5-benzodiazepine derivatives. Synth. Commun., 1999, 29, 1941. Curini, M.; Epifano, F.; Marcotullio, M.C.; Rosati, O. Ytterbium triflate promoted synthesis of 1,5-benzodiazepine derivatives. Tetrahedron Lett., 2001, 42, 3193. Pozarentzi, M.; Stephanidou-Stephanatou, J.; Tsoleridis, C.A. An efficient method for the synthesis of 1,5-benzodiazepine derivatives under microwave irradiation without solvent. Tetrahedron Lett., 2002, 43, 1755. Jarikote, D.V.; Siddiqui, S.A.; Rajagopal, R.; Daniel, T.; Lahoti, R.J.; Srinivasan, K.V. Room temperature ionic liquid promoted synthesis of 1,5benzodiazepine derivatives under ambient conditions. Tetrahedron Lett., 2003, 44, 1835. Yadav, J.S.; Reddy, B.V.S.; Eshwaraiah, B.; Anuradha, K. Amberlyst-15®: A novel and recyclable reagent for the synthesis of 1,5-benzodiazepines in ionic liquids. Green Chem., 2002, 4, 592. Reddy, B.M.; Sreekanth, P.M. An efficient synthesis of 1,5-benzodiazepine derivatives catalyzed by a solid superacid sulfated zirconia. Tetrahedron Lett., 2003, 44, 4447. López, D.E.; Goodwin Jr., J.G.; Bruce, D.A.; Lotero, E. Transesterification of triacetin with methanol on solid acid and base catalysts. Appl. Catal., A, 2005, 295, 97. López, D.E.; Goodwin Jr., J.G.; Bruce, D.A.; Furuta, S. Esterification and transesterification using modified-zirconia catalysts. Appl. Catal., A, 2008, 339, 76. Suwannakarn, K.; Lotero, E.; Goodwin Jr., J.G.; Lu, C. Stability of sulfated zirconia and the nature of the catalytically active species in the transesterification of triglycerides. J. Catal., 2008, 255, 279. Petchmala, A.; Laosiripojana, N.; Jongsomjit, B.; Goto, M.; Panpranot, J.; Mekasuwandumrong, O.; Shotipruk, A. Transesterification of palm oil and esterification of palm fatty acid in near- and super-critical methanol with SO4-ZrO2 catalysts. Fuel, 2010, 89, 2387. Shamshuddin, S.Z.M.; Nagaraju, N. Transesterification: Salol synthesis over solid acids. Catal. Commun., 2006, 7, 593. Kirumakki, S.R.; Nagaraju, N.; Murthy, K.V.V.S.B.S.R.; Narayanan, S. Esterification of salicylic acid over zeolites using dimethyl carbonate. Appl. Catal., A, 2002, 226, 175. Das, B.; Ramu, R.; Ravikanth, B.; Reddy, K.R. Regioselective ring-opening of aziridines with potassium thiocyanate and thiols using sulfated zirconia as a heterogeneous recyclable catalyst. Tetrahedron Lett., 2006, 47, 779. Reddy, B.M.; Patil, M.K.; Reddy, B.T.; Park, S.-E. Efficient synthesis of aminoalcohols by regioselective ring-opening of epoxides with anilines catalyzed by sulfated zirconia under solvent-free conditions. Catal. Commun., 2008, 9, 950. Das, B.; Thirupathi, P.; Kumar, R.A.; Reddy, K.R. Efficient synthesis of 3alkyl indoles through regioselective ring-opening of epoxides catalyzed by sulfated zirconia. Catal. Commun., 2008, 9, 635. Das, B.; Thirupathi, P.; Kumar, R.A. Sulfated zirconia as an efficient recyclable heterogeneous catalyst for selective aminolysis of epoxides and N-tosyl aziridines under solvent-free condition. Indian J. Heterocycl. Chem., 2008, 17, 339. Holland, H.L.; Chiral sulfoxidation by biotransformation of organic sulfides. Chem. Rev., 1988, 88, 473. Block, E. The organosulfur chemistry of the genus Allium-Implications for the organic chemistry of sulphur. Angew. Chem., Int. Ed. Engl., 1992, 31, 1135. Carreno, M.C. Applications of sulfoxides to asymmetric synthesis of biologically active compounds. Chem. Rev., 1995, 95, 1717. Garbe, T.R.; Kobayashi, M.; Shimizu, N.; Takesue, N.; Ozawa, M.; Yukawa, H. Indolyl carboxylic acids by condensation of indoles with -keto acids. J. Nat. Prod., 2000, 63, 596. Ehrlich, P. üeber die Dimethylamidobenzaldehydreaction. Medicin Woche, 1901, 151.



[215]

[216]

[217]

[218] [219] [220] [221]

[222]

[223] [224]

[225]

[226] [227]

[228]

[229]

[230]

[231]

[232]

[233]

[234] [235]

[236]

[237]

Yadav, G.D.; Rahuman, M.S.M.M. Efficacy of solid acids in the synthesis of butylated hydroxy anisoles by alkylation of 4-methoxyphenol with MTBE. Appl. Catal., A, 2003, 253,113. Yadav, G.D.; Ramesh, P. Selectivity engineering in the O- versus Calkylation of p-cresol with cyclohexene over sulfated zirconia. Can. J. Chem. Eng., 2000, 78, 917. Yadav, G.D.; Pathre, G.S. Chemoselective catalysis by sulphated zirconia in O-alkylation of guaiacol with cyclohexene. J. Mol. Catal. A: Chem., 2005, 243, 77. Katada, N.; Endo, J.-i.; Notsu, K.-i.; Yasunobu, N.; Naito, N.; Niwa, M. Superacidity and catalytic activity of sulfated zirconia. J. Phys. Chem. B, 2000, 104, 10321. Yadav, G.D.; Thorat, T.S. Kinetics of alkylation of p-Cresol with isobutylene catalyzed by sulfated zirconia. Ind. Eng. Chem. Res., 1996, 35, 721. Yadav, G.D., Kundu, B. Friedel-Crafts alkylation of diphenyl oxide with 1decene over sulfated zirconia as catalyst. Can. J. Chem. Eng., 2001, 79, 805. Yadav, G.D.; Pujari, A.A. Friedel-Crafts acylation using sulfated zirconia catalyst. Green Chem., 1999, 69. Deutsch, J.; Trunschke, A.; Müller, D.; Quaschning, V.; Kemnitz, E.; Lieske, H. Acetylation and benzoylation of various aromatics on sulfated zirconia. J. Mol. Catal. A: Chem., 2004, 207, 51. Deutsch, J.; Trunschke, A.; Müller, D.; Quaschning, V.; Kemnitz, E.; Lieske, H. Different acylating agents in the synthesis of aromatic ketones on sulfated zirconia. Catal. Lett., 2003, 88, 9. Deutsch, J.; Prescott, H.A.; Müller, D.; Kemnitz, E.; Lieske, H. Acylation of naphthalenes and anthracene on sulfated zirconia. J. Catal., 2005, 231, 269. El-Sharkawy, E.A.; Al-Shihry, S.S. Friedel-Crafts acylation of toluene using superacid catalysts in a solvent-free medium. Monatshefte fur Chemie, 2010, 141, 259. Yadav, G.D.; Pimparkar, K.P. Insight into Friedel-Crafts acylation of 1,4dimethoxybenzene to 2,5-dimethoxyacetophenone catalyzed by solid acidsmechanism, kinetics and remedies for deactivation. J. Mol. Catal. A: Chem., 2007, 264, 179. Green, T.W.; Wuts, P.G. Protective groups in organic synthesis, 3rd ed.; Wiley: New York, 1999, p. 150. a) Sano, T.; Ohashi, K.; Oriyama, T. Remarkably fast acylation of alcohols with benzoyl chloride promoted by TMEDA. Synthesis, 1999, 7, 1141; b) Kumar, P.; Pandey, R.K.; Bodas, M.S.; Dagade, S.P.; Dongare, M.K.; Ramaswamy, A.V. Acylation of alcohols, thiols and amines with carboxylic acids catalyzed by yttria-zirconia-based Lewis acid. J. Mol. Catal. A: Chem., 2002, 181, 207. Nagai, H.; Sasaki, K.; Matsumura, S.; Toshima, K. Environmentally benign -stereoselective glycosidations of glycosyl phosphites using a reusable heterogeneous solid acid, montmorillonite K-10. Carbohydrate Research, 2005, 340, 337. Toshima, K.; Nagai, H.; Kasumi, K.-I.; Kawahara, K.; Matsumura, S. Stereocontrolled glycosidations using a heterogeneous solid acid, sulfated zirconia, for the direct syntheses of - and -manno- and 2deoxyglucopyranosides. Tetrahedron, 2004, 60, 5331. Nagai, H.; Matsumura, S.; Toshima, K. Environmentally benign and stereoselective construction of 2-deoxy and 2,6-dideoxy--glycosidic linkages employing 2-deoxy and 2,6-dideoxyglycosyl phosphites and montmorillonite K-10. Chem. Lett., 2002, 11, 1100. Nagai, H.; Matsumura, S.; Toshima, K. Environmentally benign and stereoselective formation of -O-glycosidic linkages using benzyl-protected glucopyranosyl phosphite and montmorillonite K-10. Tetrahedron Lett., 2002, 43, 847. Nagai, H.; Kawahara, K.; Matsumura, S.; Toshima, K. Novel stereocontrolled - and -glycosidations of mannopyranosyl sulfoxides using environmentally benign heterogeneous solid acids. Tetrahedron Lett., 2001, 42, 4159. Jyojima, T.; Miyamoto, N.; Ogawa, Y.; Matsumura, S.; Toshima, K. Novel stereocontrolled glycosidations of olivoses using montmorillonite K-10 as an environmentally benign catalyst. Tetrahedron Lett., 1999, 40, 5023. Toshima, K.; Nagai, H.; Matsumura, S. Novel dehydrative glycosidations of 1-hydroxy sugars using a heteropoly acid. Synlett, 1999, 9, 1420. Toshima, K.; Kasumi, K.-I.; Matsumura, S. Novel stereocontrolled glycosidations using a solid acid, SO4/ZrO2, for direct syntheses of -and mannopyranosides. Synlett, 1998, 643. Satoh, D.; Matsuhashi, H.; Nakamura, H.; Arata, K. Isomerizations of cycloheptane to methylcyclohexane and cyclooctane to ethylcyclohexane catalyzed by sulfated zirconia. Catal. Lett., 2003, 89, 105. Satoh, D.; Matsuhashi, H.; Nakamura, H.; Arata, K. Isomerization of cycloheptane, cyclooctane, and cyclodecane catalyzed by sulfated zirconia— comparison with open-chain alkanes. Phys. Chem., Chem. Phys., 2003, 5, 4343.

[238]

[239]

[240]

[241]

[242] [243]

[244]

[245]

[246]

[247]

[248]

[249]

[250] [251]

[252]

[253]

[254]

[255] [256] [257]

[258]

[259]

Comelli, N.A.; Ponzi, E.N.; Ponzi, M.I. -Pinene isomerization to camphene: Effect of thermal treatment on sulfated zirconia. Chem. Eng. J., 2006, 117, 93. Comelli, N.A.; Ponzi, E.N.; Ponzi, M.I. Isomerization of -pinene, limonene, -terpinene, and terpinolene on sulfated zirconia. J. Am. Oil Chem. Soc., 2005, 82, 531. Comelli, N.A.; Masini, O.; Carrascull, A.L.; Ponzi, E.N.; Ponzi, M.I. Production of camphene by isomerization reaction on sulfated ZrO2. Chin. J. Chem. Eng., 2000, 8, 64. Grzona, L.; Comelli, N.; Masini, O.; Ponzi, E.; Ponzi, M. Liquid phase isomerization of -pinene. Study of the reaction on sulfated ZrO2. React. Kinet. Catal. Lett., 2000, 69, 271. Flores-Moreno, J.L.; Baraket, L.; Figuéras, F. Isomerisation of -pinene oxide on sulfated oxides. Catal. Lett., 2001, 77, 113. Yadav, G.D.; Nair, J.J. Novelties of eclectically engineered sulfated zirconia and carbon molecular sieve catalysts in cyclisation of citronellal to isopulegol. Chem. Commun., 1998, 21, 2369. Yu, G.X.; Zhou, X.L.; Li, C.L.; Chen, L.F.; Wang, J.A. Esterification over rare earth oxide and alumina promoted SO42/ZrO2. Catal. Today, 2010, 148, 169. Sert, E.; Atalay, F.S. Kinetic study of the esterification of acetic acid with butanol catalyzed by sulfated zirconia. Reaction Kinetics, Mechanisms and Catalysis 2010, 99, 125. Mongkolbovornkij, P.; Champreda, V.; Sutthisripok, W.; Laosiripojana, N. Esterification of industrial-grade palm fatty acid distillate over modified ZrO2 (with WO3-, SO4-and TiO2-): Effects of co-solvent adding and water removal. Fuel Process. Tech., 2010, 91, 1510. Yadav, G.D.; Lande, S.V. Ion-exchange resin catalysis in benign synthesis of perfumery grade p-cresylphenyl acetate from p-cresol and phenylacetic acid. Org. Process. Res. Dev., 2005, 9, 288. Ardizzone, S.; Bianchi, C.L.; Ragaini, V.; Vercelli, B. SO4-ZrO2 catalysts for the esterification of benzoic acid to methylbenzoate. Catal. Lett., 1999, 62, 59. Halpert, J.R.; Balfour, C.; Miller, N.E.; Kaminsky, L.S. Dichloromethyl compounds as mechanism-based inactivators of rat liver cytochromes P-450 in vitro. Mol. Pharmacol., 1996, 30, 19. Oliván, M.; Caulton, K.G. C(Halide) Oxidative addition routes to ruthenium carbenes. Inorg. Chem., 1999, 38, 566. Ishino, Y.; Mihara, M.; Nishihama, S.; Nishiguchi, I. Zinc metal-promoted stereoselective olefination of aldehydes and ketones with gem-dichloro compounds in the presence of chlorotrimethylsilane. Bull. Chem. Soc. Jpn., 1998, 71, 2669. Varshaver, E. V.; Kruglova, T. P.; Uskach, Ya. L. Manufacture of chlorinecontaining aromatic compounds from dibenzyl ether, RU2152381, July 10, 2000; Chem. Abstr., 2002, 136, 264824. Xu, F.; Savary, K.; Williams, J.M.; Grabowski, E.J.J.; Reider, P.J. Novel synthesis of sulfones from ,-dibromomethyl aromatics. Tetrahedron Lett., 2003, 44, 1283. Wolfson, A.; Shokin, O.; Tavor, D. Acid catalyzes the synthesis of aromatic gem-dihalides from their corresponding aromatic aldehydes. J. Mol. Catal. A: Chem., 2005, 226, 69. G. Jones, Inorganic Reactions, Wiley: New York, 1967, 15, p. 204. Reeves, R.L. Patai, S. (Eds.). The chemistry of carbonyl compounds, Interscience Publishers, New York, 1996, p. 567. a) Reddy, B.M.; Patil, M.K.; Rao, K.N.; Reddy, G.K. An easy-to-use heterogeneous promoted zirconia catalyst for Knoevenagel condensation in liquid phase under solvent-free conditions. J. Mol. Catal. A: Chem., 2006, 258, 302.; b) Perin, G.; Jacob, R.G.; Botteselle, G.V.; Kublik, E.L.; Lenardão, E.J.; Cellab, R.; dos Santos, P.C.S. Clean and atom-economic synthesis of -phenylselenoacrylonitriles and -phenylseleno-,-unsaturated esters by Knoevenagel reaction under solvent-Free conditions. J. Braz. Chem. Soc., 2005, 16, 857. a) Reddy, B.M.; Patil, M.K.; Reddy, B.T. An efficient protocol for azaMichael addition reactions under solvent-free condition employing sulfated zirconia catalyst. Catal. Lett., 2008, 126, 413.; b) Lenardão, E.J.; Trecha, D.O.; Ferreira, P.C.; Jacob, R.G.; Perin, G. Green Michael addition of thiols to electron deficient alkenes using KF/alumina and recyclable solvent or solvent-free conditions. J. Braz. Chem. Soc., 2009, 20, 93.; c) Lenardão, E.J.; Ferreira, P.C.; Jacob, R.G.; Perina, G.; Leiteb, F.P.L. Solvent-free conjugated addition of thiols to citral using KF/alumina: Preparation of 3thioorganylcitronellals, potential antimicrobial agents. Tetrahedron Lett., 2007, 48, 6763. Lin, C.-H.; Tsai, C.-H.; Chang, H.-C. A convenient preparation method for synthesizing formamidine utilizing sulfated zirconia. Catal. Lett., 2005, 104, 135.


[261] [262]

[263] [264]

[265] [266]

[267]

[268]

[269]

[270]

[271] [272]

[273] [274] [275] [276]

[277]

[278]

[279]

[280]

[281]

[282]

[283]

[284]

[285]

Matsuno, H.; Odaka, K. Process for preparation of 2,5-dihydrofuran by cyclization of cis-2-butene-1,4-diol, JP 09110850, April 28, 1997; Chem. Abstr., 1997, 127, 34109. Wali, A.; Pillai, S.M. Cyclodehydration of some 1,n-diols catalysed by sulfated zirconia. J. Chem. Res., 1999, 326. Kaufman, K.D.; Kelly, R.C.; Eaton, D.C. Synthetic furocoumarins. VIII. The Pechmann condensation of 2-alkylhydroquinones. J. Org. Chem., 1967, 32, 504. Miyano, M.; Dorn, C.R. Mirestrol. I. Preparation of the tricyclic intermediate. J. Org. Chem., 1972, 37, 259. Bose, D.S.; Rudradas, A.P.; Babu, M.H. The indium(III) chloride-catalyzed von Pechmann reaction: A simple and effective procedure for the synthesis of 4-substituted coumarins. Tetrahedron Lett., 2002, 43, 9195. De, S.K.; Gibbs, R.A. An efficient and practical procedure for the synthesis of 4-substituted coumarins. Synthesis, 2005, 8, 1231. Potdar, M.K.; Mohile, S.S.; Salunkhe, M.M. Coumarin syntheses via Pechmann condensation in Lewis acidic chloroaluminate ionic liquid. Tetrahedron Lett., 2001, 42, 9285. Reddy, B.M.; Thirupathi, B.; Patil, M.K. One-pot synthesis of substituted coumarins catalyzed by silica gel supported sulfuric acid under solvent-free conditions. The Open Catalysis Journal, 2009, 2, 33. Tyagi, B.; Mishra, M.K.; Jasra, R.V. Microwave-assisted solvent-free synthesis of hydroxy derivatives of 4-methyl coumarin using nano-crystalline sulfated-zirconia catalyst. J. Mol. Catal. A: Chem., 2008, 286, 41. Rodríguez-Domínguez, J.C.; Kirsch, G. Sulfated zirconia, a mild alternative to mineral acids in the synthesis of hydroxycoumarins. Tetrahedron Lett., 2006, 47, 3279. Negri, G., Kascheres, C., Kascheres, A.J. Recent development in preparation reactivity and biological activity of enaminoketones and enaminothiones and their utilization to prepare heterocyclic compounds. J. Het. Chem., 2004, 41, 461. Ferraz, H.M.C.; Gonçalo, E.R.S. Recent preparations and synthetic applications of enaminones. Quim. Nova, 2007, 30, 957. Zhang, Z.-H.; Song, L.-M. A solvent-free synthesis of -amino-,unsaturated ketones and esters catalyzed by sulfated zirconia. J. Chem. Res., 2005, 817. Trost, B.M.: Fleming, I., Eds. Comprehensive organic synthesis. Oxford: Pergamon, 1991, 2, p. 893. Arend, M.; Westermann, B.; Risch, N. Modern variants of the Mannich reaction Angew. Chem., Int. Ed., 1998, 37, 1045. Kobayashi, S.; Ishitani, H. Catalytic enantioselective addition to imines. Chem. Rev., 1999, 99, 1069. Wang, S.; Matsumura, S.; Toshima, K. Sulfated zirconia (SO4/ZrO2) as a reusable solid acid catalyst for the Mannich-type reaction between ketene silyl acetals and aldimines. Tetrahedron Lett., 2007, 48, 6449. Ratnam, K.J.; Reddy, R.S.; Sekhar, N.S.; Kantam, M.L.; Deshpande, A.; Figuéras, F. Bamberger rearrangement on solid acids. Appl. Catal., A, 2008, 348, 26. Venkatesan, C.; Singh, A.P. Condensation of acetophenone to ,unsaturated ketone (dypnone) over solid acid catalysts. J. Mol. Catal. A: Chem., 2002, 181, 179. Kumbhar, P.S.; Yadav, G.D. Catalysis by sulfur-promoted superacidic zirconia: Condensation reactions of hydroquinone with aniline and substituted anilines. Chem. Eng. Sci., 1989, 44, 2535. Koltunov, K.Y.; Walspurger, S.; Sommer, J. Cyclization of 1-phenyl-2propen-1-ones into 1-indanones using H-zeolite and other solid acids. The role of mono- and dicationic intermediates. Tetrahedron Lett., 2005, 46, 8391. Yadav, G.D.; Nair, J.J. Selectivity engineering in the nitration of chlorobenzene using eclectically engineered sulfated zirconia and carbon molecular sieve catalysts. Catal. Lett., 1999, 62, 49. Qi, X.; Watanabe, M.; Aida, T.M.; Smith Jr., R.L. Sulfated zirconia as a solid acid catalyst for the dehydration of fructose to 5-hydroxymethylfurfural. Catal. Commun., 2009, 10, 1771. Mori, H.; Wada, A.; Xu, Q.; Souma, Y. Novel application of a solid super acid, sulfated zirconia, as a catalyst for Koch carbonylation reaction. Chem. Lett., 2000, 136. Dabbagh, H.A.; Teimouri, A.; Chermahini, A.N. Environmentally friendly efficient synthesis and mechanism of triazenes derived from cyclic amines on clays, HZSM-5 and sulfated zirconia. Appl. Catal., B, 2007, 76, 24. a) Chuah, G.K.; Liu, S.H.; Jaenicke, S.; Harrison, L.J. Cyclisation of citronellal to isopulegol catalyzed by hydrous zirconia and other solid acids. J. Catal., 2001, 200, 352.; b) Jacob, R.G.; Perin, G.; Botteselle , G. V.; Lenardão, E.J. Clean and atom-economic synthesis of octahydroacridines: application to essential oil of citronella. Tetrahedron Lett., 2003, 44, 6809.; c) Jacob, R.G.; Perin, G.; Loi, L. N.; Pinnoa, C.S.; Lenardão, E.J. Green

Patil et al.

[286]

[287] [288]

[289] [290]

[291]

[292]

[293]

[294] [295]

[296]

[297]

[298] [299]

[300]

[301]

[302]

[303]

[304] [305]

[306]

[307]

[308]

[309]

synthesis of ()-isopulegol from (+)-citronellal: application to essential oil of citronella. Tetrahedron Lett., 2003, 44, 3605. Yadav, G.D.; Lande, S.V. Selective Claisen rearrangement of allyl-2,4-ditert-butylphenyl ether to 6-allyl-2,4-di-tert-butylphenol catalyzed by heteropolyacid supported on hexagonal mesoporous silica. J. Mol. Catal. A: Chem., 2006, 243, 31. Biro, K.; Békássy, S.; Ágai, B.; Figuéras, F. Heterogeneous catalysis for the acetylation of benzo crown ethers. J. Mol. Catal. A: Chem., 2000, 151, 179. Bandgar, B.P.; Kasture, S.P. Microwave-induced solvent-free, rapid and efficient synthesis of conjugated nitroalkenes using sulfated zirconia. Indian J. Chem., Sect. B, 2001, 40, 1239. Zhang, S.; Zhou, J.; Zhang, Z.C. Isomerization and arylation of oleic acid on anion modified zirconia catalysts. Catal. Lett., 2009, 127, 33. Wongmaneenil, P.; Jongsomjit, B.; Praserthdam, P. Solvent effect on synthesis of zirconia support for tungstated zirconia catalysts. J. Ind. Eng. Chem., 2010, 16, 327. Kourieh, R.; Bennici, S.; Auroux, A. Study of acidic commercial WOx/ZrO2 catalysts by adsorption microcalorimetry and thermal analysis techniques. J. Therm. Anal. Calorim., 2010, 99, 849. Rodriguez-Gattorno, G.; Galano, A.; Torres-García, E. Surface acid-basic properties of WOx-ZrO2 and catalytic efficiency in oxidative desulfurization. Appl. Catal., B, 2009, 92, 1. Brei, V.V.; Melezhyk, A.V.; Prudius, S.V.; Oranskaya, E.I. Study of surfacebulk distribution of tungsten in WO3/ZrO2 oxides prepared by different methods. Pol. J. Chem., 2009, 83, 537. Sarish, S.; Devassy, B.M.; Halligudi, S.B. tert-Butylation of p-cresol over WOx/ZrO2 solid acid catalysts. J. Mol. Catal. A: Chem., 2005, 235, 44. Furuta, S.; Matsuhashi, H.; Arata, K. Biodiesel fuel production with solid superacid catalysis in fixed bed reactor under atmospheric pressure. Catal. Commun., 2004, 5, 721. Sarish, S.; Devassy, B.M.; Böhringer, W.; Fletcher, J.; Halligudi, S.B. Liquid-phase alkylation of phenol with long-chain olefins over WOx/ZrO2 solid acid catalysts. J. Mol. Catal. A: Chem., 2005, 240, 123. Ramu, S.; Lingaiah, N.; Devi, B.L.A.P.; Prasad, R.B.N.; Suryanarayana, I.; Prasad, P.S.S. Esterification of palmitic acid with methanol over tungsten oxide supported on zirconia solid acid catalysts: Effect of method of preparation of the catalyst on its structural stability and reactivity. Appl. Catal., A, 2004, 276, 163. Reddy, B.M.; Reddy, V.R.; Giridhar, D. Synthesis of coumarins catalyzed by eco-friendly W/ZrO2 solid acid catalyst. Synth. Commun., 2001, 31, 3603. Reddy, B.M.; Patil, M.K.; Reddy, B.T. An efficient and ecofriendly WOxZrO2 solid acid catalyst for classical Mannich reaction. Catal. Lett., 2008, 125, 97. Jiménez-Morales, I.; Santamaría-González, J.; Maireles-Torres, P.; JiménezLópez, A. Zirconium doped MCM-41 supported WO3 solid acid catalysts for the esterification of oleic acid with methanol. Appl. Catal., A, 2010, 379, 61. Shiju, N.R.; Anilkumar, M.; Hoelderich, W.F.; Brown, D.R. Tungstated zirconia catalysts for liquid-phase Beckmann rearrangement of cyclohexanone oxime: Structure-activity relationship. J. Phys. Chem. C, 2009, 113, 7735. Nagarapu, L.; Baseeruddin, M.; Kumari, N.V.; Kantevari, S.; Rudradas, A.P. Efficient synthesis of aryl-14H-dibenzo[a.j]xanthenes using NaHSO4-SiO2 or 5%WO3/ZrO2 as heterogeneous catalysts under conventional heating in a solvent-free media. Synth. Commun., 2007, 37, 2519. Ngaosuwan, K.; Mo, X.; Goodwin Jr., J.G.; Praserthdam, P. Effect of solvent on hydrolysis and transesterification reactions on tungstated zirconia. Appl. Catal., A, 2010, 380, 81. Reddy, B.M.; Reddy, V.R.; Manohar B. Mo-ZrO2 Solid acid catalyst for transesterification of -ketoesters. Synth. Commun., 1999, 29, 1235. Reddy, B.M.; Reddy, V.R.; Giridhar, D. Eco-friendly Pt-Mo/Zro2 solid acid catalyst for selective protection of carbonyl compounds. Synth. Commun., 2001, 31, 1819. Chen, A.-J., Chen, X.-R., Mou, C.-Y. Environmental friendly and highly regioselective oxybromination in an aqueous system using SBA-15 supported sulfated zirconia catalyst. Materials Research Society Symposium Proceedings, 2010, 1217, 107. Yadav, G.D.; Pathre, G.S. Novel mesoporous solid superacids for selective C-alkylation of m-cresol with tert-butanol. Microporous and Mesoporous Materials, 2006, 89, 16. Yadav, G.D.; Kamble, S.B. Friedel-Crafts alkylation of mesitylene with tertbutyl alcohol over novel solid acid catalyst UDCaT-5. International Journal of Chemical Reactor Engineering, 2009, 7, art. no. A12 Yadav, G.D.; Kamble, S.B. Synthesis of carvacrol by Friedel-Crafts alkylation of o-cresol with isopropanol using superacidic catalyst UDCaT-5. J. Chem. Technol. Biotechnol., 2009, 84, 1499.



[311]

Reddy, B.M.; Patil, M.K.; Lakshmanan, P. Sulfated CexZr1xO2 solid acid catalyst for solvent free synthesis of coumarins. J. Mol. Catal. A: Chem., 2006, 256, 290. Thirupathi, B.; Srinivas, R.; Prasad, A.N.; Kumar, J.K.P.; Reddy, B.M. Green progression for synthesis of regioselective -amino alcohols and chemoselective alkylated indoles. Org. Process. Res. Dov., 2010, 14, 1457.

Received: 27 October, 2010

[312]

Revised: 06 April, 2011

Reddy, B.M.; Sreekanth, P.M.; Khan, A. Facile synthesis of 1,1-diacetates from aldehydes using environmentally benign solid acid catalyst under solvent-free conditions. Synth. Commun., 2004, 34, 1839.

Accepted: 27 April, 2011