Promoted Zirconia Solid Acid Catalysts for Organic

118

Current Organic Chemistry, 2008, 12, 118-140

Promoted Zirconia Solid Acid Catalysts for Organic Synthesis Benjaram M. Reddy* and Meghshyam K. Patil Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad – 500 007, India Abstract: This review deals with the catalytic performance of sulfate, molybdate and tungstate ion promoted zirconia solid acid catalysts for various acid-catalyzed organic synthesis and transformation reactions in the liquid phase. These promoted zirconia catalysts exhibit superacidity which mainly depends on the preparation conditions. In particular, the sulfated zirconia catalyst exhibits very strong solid acidity and excellent catalytic activity not only for simple acylation, condensation and esterification reactions but also for other important reactions such as synthesis of aromatic gem-dihalides, stereocontrolled glycosidation, regioselective ring opening of aziridines, production of diaryl sulfoxides and so on. Molybdate and tungstate promoted zirconia catalysts that also exhibit good catalytic activity for various organic reactions of practical importance.

1. INTRODUCTION Acid catalysts are extensively employed in chemical and petrochemical industries. They are claimed to be responsible for producing more than 1 108 mt/year of products. Among the first acid catalysts, the most commonly used were HF, H2SO4, HClO4 and H3PO4 (in liquid form or supported on Keiselguhr). Since 1940 the tendency has been to replace, when possible, these liquid acids by solid acids, which present clear advantages with respect to the former. These advantages include along with their handling requirements, simplicity and versatility of process engineering, catalyst regeneration, decreasing reactor and plant corrosion problems, and environmentally safe disposal. In 1979, Arata and co-investigators [1,2] 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. The catalytic performance is quite unique compared to the typical solid acid catalysts, such as zeolites that show no activity at such a low temperature. Using Hammett indicators, Hino and Arata [2] observed that sulfated zirconia (SZ) is an acid 104 times stronger than 100% sulfuric acid. Acids stronger than 100% sulfuric acid are generally referred to as superacids [3,4]. 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 H0 for 100% sulfuric acid is 12. Therefore, SZ catalyst with H0 = 16 is considered as the strongest halide-free solid superacid [5,6]. Recent investigations reveal that sulfate-free ZrO2-based solid superacids could be synthesized by incorporating molybdate or tungstate promoters under certain preparation conditions [5,7]. The typical H0 values reported for SO42/ZrO2 (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 [3,8].

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o F t o N

*Address correspondence to this author at the Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad – 500 007, India; Fax: +91 40 2716 0921; E-mail: [email protected], [email protected] 1385-2728/08 $55.00+.00

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In view of its significance, many large-volume applications based on SZ catalysts are reported in the literature, especially in the petroleum industry for alkylation, isomerization and cracking reactions [4-6,8,9]. In recent times inorganic solid acid-catalyzed organic transformations are gaining more attention due to the proven advantage of heterogeneous catalysts, simplified product isolation, mild reaction conditions, high selectivity, ease in recovery and reuse of the catalysts, and reduction in the generation of wasteful byproducts [10-12]. In that connection we were interested in investigating various industrially important organic reactions aimed at replacing toxic and corrosive reagents, noxious or expensive solvents and multistep processes, with single-step solvent-free ones by using environmentally benign solid acid catalysts. Interestingly, sulfate, molybdate and tungstate promoted zirconia catalysts exhibit excellent activity for a wide range of organic synthesis and transformation reactions. In this review we tried to cover most of the papers reported so far on the application of promoted ZrO2 catalysts for various organic synthesis and transformation reactions in the liquid phase. Preparation and physicochemical characterization aspects of the promoted zirconia catalysts have also been briefly dealt in this review.

2. PREPARATION OF CATALYSTS The catalytic properties of promoted zirconia catalysts significantly depend on the preparation method adopted and the activation treatment employed [5]. Various preparation methods have been reported which mainly differ in terms of precursor, promoting agent, precipitating agent, method of impregnation, calcination temperature, and so on [1,2,5,13, 14]. There are primarily two approaches to obtain the final catalysts. 2.1. Two-Step Method This method is most often used for the preparation of sulfated zirconia catalysts. In the first step, zirconium hydroxide is prepared followed by impregnation with a sulfating agent in the second stage (Scheme 1) [1,2]. To prepare zirconium hydroxide, mostly ZrOCl2 or ZrO(NO3)4 salts were hydrolyzed with aqueous ammonia or urea [13-15]. Other precur© 2008 Bentham Science Publishers Ltd.

Promoted Zirconia Solid Acid Catalysts for Organic Synthesis

Current Organic Chemistry, 2008, Vol. 12, No. 2

ZrOCl2.8H2O + NH4OH.1.4H2O

119

Zr(OH)4 + 2NH4Cl + 9.8H2O H2SO4 or (NH4)2SO4

SO42-/ZrO2

Calcination SO42-/Zr(OH)4

550-650°C

Scheme 1.

sors, namely ZrCl4, Zr(NO3)4 and Zr(OC3H7)4 were also used for this purpose [16]. The desired quantity of sulfate ion is normally impregnated by using either H2SO4 or (NH4)2SO4 [1,2,13-15,17] or SO3 [18] or ClSO3H [19]. The resultant sulfated zirconium hydroxide is then calcined in air at 550650°C to generate acidity. To impregnate molybdate (10-15 wt.%) and tungstate (10-18 wt.%) promoters, the desired quantities of ammonium heptamolybdate or ammonium metatungstate precursors dissolved in doubly distilled water are normally employed [7,20]. The resultant molybdated and tungstated zirconium hydroxide samples are then calcined in air at 650-800°C in order to generate the acidity [20]. The sulfur content of SZ catalyst strongly depends on the calcination temperature. Increasing the calcination temperature usually results in the gradual removal of sulfur from the catalyst surface, thus decreasing the sulfur content. The most commonly used calcination temperature ranges between 550650°C. The typical sulfur content of SZ catalysts calcined in this range is 0.8-3 wt.%. The calcination temperature significantly affects the catalytic performance of the resulting SZ samples.

The SZ catalysts have also been prepared in a single step by using the sol-gel method. Ward and Ko [21] and Negrón et al. [16] reported the preparation of SZ by using zirconium n-propoxide and zirconium isopropoxide, respectively, as the precursors of zirconia. In a typical procedure, 20 ml of zirconium isopropoxide (70 wt.% in 1-propanol) with 30.5 ml of propan-1-ol and 1 ml of sulfuric acid (98 wt.%) were mixed with 3.5 ml of deionized water. The acid solution was added drop wise to the alcoxide solution under vigorous stirring, until a viscous solution was obtained. The gel was heated at 80°C to evaporate excess alcohol and calcinated at 600°C for 7 h in air to get the white SZ solid. Also the preparation of SZ catalyst may also be performed in one step by thermal decomposition of Zr(SO4)2 as shown in the following equation [22,23]:

u rib

H * O

ZrO2 + 2 SO3

O

O

*

S O

O Zr

Zr O

Zr(SO4)2

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t s i D r

o F t o N 2.2. One-Step Method

of ammonia and other methods [14,24,25]. All characterization results reveal that the incorporated sulfate ions show a strong influence on the surface and bulk properties of the ZrO2. In particular, XRD and Raman results suggested that impregnated sulfate ions stabilize the metastable tetragonal phase of ZrO2 at ambient conditions [14,24]. Ammonia-TPD and BET surface area results indicated that the sulfated catalyst exhibits enhanced acid strength and higher specific surface area than other promoted and unprompted samples [14]. In general, the appearance of sulfate ion promoted catalyst differs greatly from that of unpromoted sample. The former sample is finely powdered solid, which coats the walls of the glass ampoule obscuring vision, whereas the latter is not. This is a simple way to confirm whether superacidity has been generated or not as reported by Arata and Hino [5b]. There are various other methods also to determine the surface acidity of the catalysts such as the Hammett indicator method, temperature-programmed desorption of various base molecules, model test reactions and so on [5]. However, all these methods are not versatile for different types of solid acids. Based on several characterization and catalytic studies various explanations were advocated in the literature on the origin of solid acidity in promoted zirconia catalysts [5]. The surface acidity characterization results primarily suggest that the SZ catalyst surface contains strong Brönsted as well as Lewis acid sites. The number and strength of these sites largely vary with various parameters such as sulfur concentration, activation temperature and surface area of the precursor oxide. Based on IR and XPS techniques, Ward and Ko [21] proposed the following structure (Fig. (1)) of the SZ catalyst indicating the presence of both Brönsted and Lewis acid sites:

O

O

H O

O

O S

O

# Zr

Zr

O Zr

O

O

# Zr O

* Brönsted acid site; # Lewis acid site

However, this method did not attract much attention because it does not allow the control of sulfate content.

Fig. (1). Schematic structure of the SZ catalyst.

3. CATARACTERIZATION AND CATALYST STRUCTURE The bulk and surface properties of promoted ZrO2 catalysts have been extensively examined by using several techniques such as X-ray powder diffraction, X-ray photoelectron spectroscopy, infrared spectroscopy, Raman spectroscopy, BET surface area, temperature programmed desorption

The Brönsted acid sites result from weakening of O–H bond which is bonded to a Zr atom adjacent to another Zr atom bearing a chelating sulfate group. The proton donating ability of the hydroxyl group on the zirconia surface is strengthened by electron-inductive effect of S=O double bonds in the surface group, whereas the Lewis acid sites are electronically deficient Zr4+ centers resulting from the elec-

120 Current Organic Chemistry, 2008, Vol. 12, No. 2

Reddy et al.

O O W O

O

W

Zr

O O

Zr O

O

W

ZrO2 Support

O O

O W

ZrO2 Support

Isolated mono-tungstate on zirconia support

Poly-tungstate growth on monolayer coverage

Fig. (2). Schematic surface structures of WOx/ZrO2 catalysts.

tron-withdrawing nature of the sulfate group. The XPS data also revealed that the oxidation state of sulfur in the catalysts that shows high activity in acid catalyzed reactions is S6+. Catalysts containing sulfur in a lower oxidation state are inactive [17,26-28]. Similarly, based on several physicochemical characterization results the surface structure of tungstated zirconia (WZ) has been proposed as shown in Fig. (2) [29]. It is generally believed that tungsten oxide could exist on the zirconia surface in the form of polyoxotungstate clusters as presented in the figure.

and mechanism of the reaction (Scheme 3). The reaction was carried out by using 0.07 mole of diphenyl oxide (11.9 g) and 0.01 mole of benzyl chloride (1.265 g). A catalyst load of 50 kg/m3 to that of total reactants (0.064 g) was used. It was proved from their study that external mass transfer resistance could be eliminated by providing adequate stirring and the internal mass-transfer resistance was absent. The in-situ generated HCl was desorbed from the reaction mixture and it did not catalyze the reaction. A Langmuir-HinshelwoodHougen-Watson (LHHW) model was tested which revealed that the reaction is intrinsically kinetically controlled.

4. CATALYTIC ACTIVITY OF SULFATED ZIRCONIA (SZ) The SZ is an excellent solid acid catalyst from the point of view of its catalytic activity. Additional advantages such as easy handling, non-corrosive nature, water tolerance, easy preparation, and easy recovery and reusability make this catalyst highly versatile for numerous applications. It has been widely employed for various vapour-phase reactions such as isomerization of butane [30,31], paraffin [8], pentane [32] and other hydrocarbons. More details of these vapour-phase applications could be found in various articles published in the literature [33-35]. These details are not covered in this review. The application of SZ catalyst for various liquid-phase organic synthesis and transformation reactions as reported in the literature are reviewed in the following sections.

4.2. Protection and Deprotection of Aromatic Aldehydes

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o F t o N 4.1. Alkylation of Diphenyl Oxide

Friedel-Crafts reactions are ubiquitous in fine chemical, pharmaceutical and petrochemical industries. Yadav and Sengupta [36] reported the alkylation of diphenyl oxide with benzyl chloride to produce the corresponding isomeric benzyldiphenyl oxide in excellent yields employing SZ catalyst (Scheme 2). They also investigated the effect of various parameters at 90°C in a batch reactor to establish the kinetics O

Cl +

Scheme. 2.

SZ

The use of protecting groups is very important in organic synthesis, being often the key for the success of many synthetic enterprises [37]. Negrón et al. [16] employed the SZ catalyst obtained by the sol-gel method for the chemoselective synthesis of acylals from aromatic aldehydes and their deprotection. Different aromatic (Scheme 4 and 5) and hetero-aromatic aldehydes (Scheme 6) were converted into their corresponding 1,1-diacetates in CH3CN or under solvent-free conditions using acetic anhydride as acylating agent in the presence of catalytic amounts of SZ resulting in high product yields (75-98%) at short reaction times (5-8 h) (Table 1 and 2). Ketones and aliphatic aldehydes were unaffected under the reaction conditions investigated. The deprotection of acylals to the corresponding aromatic aldehydes was carried out using CH3 CN as solvent at 60oC. The SZ catalyst was reused in two cycles without lose of its activity. 4.3. Regioselective Ring Opening of Aziridines Aziridines behave as carbon electrophiles capable of reacting with different nucleophiles and their ability to undergo regioselective ring-opening reaction contributes mainly to their synthetic utility [38]. The regioselective ring opening of aziridines with KSCN and thiols gives aminothiocyanates and -aminosulfides, respectively. O

O +


O

O


O

S

121

O S

O

O

O

Zr

Cl

H H

OZr Cl

+

+ CH2 O

O Cl

H H

C H H

+ H

O

O S

O O

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O O

S O

O

-HCl

O

O

S O

Zr

OH Zr Cl

+

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Scheme 3. OAc CHO

OZr Cl

0°C

OAc

D or R1

Scheme 4.

F t o N

t s i O

SZ ,Ac2O

R1

O

Table 1. Conversion of Aromatic Aldehydes R1

Time (h)

Yield (%)

H

6

97

o-CH3

5

p-CH3

5

94

o-OCH3

5

97

p-OCH3

5

95

o-NO2

7

86

p-NO2

6

90

p-PhO

6.5

92

99

Scheme 5.

R1

O

O

SZ , Ac2O 0°C, 9 h

OAc

OAC

Yield=75.3%

SZ

CHO

Ac2O, 0 °C

OAc

R1

O OAc

Scheme 6. Table 2. Conversion of Hetero-Aromatic Aldehydes R1

Time (h)

Yield (%)

H

5

85

CH3

8

98

NO2

8

95

NHTs N

aminothiocyanates are the precursors of thiazoles or benzothiazoles having pesticidal properties [39] and -aminosulfides are the precursors of various bioactive compounds [40]. Das et al. [41] reported aziridine ring opening with KSCN in the presence of SZ catalyst to offer the corresponding -aminothiocyanates in high yields (Scheme 7). Reactions were complete within 2 h at room temperature and the ring-opening of the aziridines took place regioselectively to provide the products. In the case of symmetrical bicyclic aziridines, products were formed (Scheme 8 (1)) with trans stereochemistry. Also similar results were observed for ring

CHO

O

R

Ts + KSCN

SCN

SZ, CH3CN r.t., 2h

+ R

SCN major when R= Aryl

R NHTs major when R= Alkyl

Scheme 7.

opening of aziridines with thiols to form -aminosulfides (Schemes 8 (2) and 9). 4.4. Synthesis of Aromatic , -Dihalobenzyl Derivatives ,-Dihalo aromatic compounds (gem-dihalides) are important intermediates in the pharmaceutical, agricultural and


Reddy et al.

dihalides from their corresponding aromatic aldehydes by using various acid catalysts. 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 benzaldehyde as selfsolvent resulted in higher product yields. The SZ catalyst provided 22% benzal chloride yield (Scheme 10), which is highest of all the solid acids used. They have also carried out the oxidative regeneration of spent SZ catalyst in air at 550°C and fully recovered its catalytic activity that allowed multiple catalyst recycling.

Table 3. Reaction of Aziridines with KSCN

a

R

Yield (%)a

C6H5

91(5)

o-Me-C6H4

89(7)

p-Cl-C6H4

84(9)

p-OMe-C6H4

86(6)

p-CH3CO-C 6H4

82(8)

n-C4H9

85(9)

n-C9H19

83(10)

Yields in parenthesis are for the other isomer.

( 1)

( 2) NHTs

SZ, CH3CN ( )n

N Ts +

KSCN

( )n

r.t., 2h

( )n

SCN

r.t., 2h

n=1, Yield = 89% n=2, Yield = 87%

SZ, CH3CN

t s i D r R1SH

+

R

r.t., 2h

R

SR1

+

SR1

R

major when R= Aryl

Scheme 9.

o F t o N Table 4. Reaction of Aziridines with R1SH

a

Esters have a fruity odor and are prepared in large quantities for various purposes such as artificial fruit essences, flavorings and components of perfumes. Sejidov et al. [44a] investigated the esterification of phthalic anhydride by using various solid acid catalysts including natural zeolite, synthetic zeolites (ZEOKAR-2, ASHNCH-3), H4Si(W3O10)4 and SZ catalyst (Scheme 11). Their studies conclude that SZ is the best catalyst for this reaction. They have also carried out the esterification of dibasic acids with various alcohols over SZ catalysts and the results are found to be highly promising (Table 5). The esterification reaction of benzoic acid to methyl benzoate with methanol was investigated by Ardizzone et al. [44b] by employing SZ catalysts. Excellent product yields under mild reaction conditions were reported. Since traditional fossil energy resources are limited and green-house gas emissions are becoming a greater concern, research is now being directed towards the use of alternative renewable fuels that are capable of fulfilling an increasing energy demand. One of the most promising approaches is the conversion of vegetable oils (VOs) and other feedstocks which primarily contain triglycerides (TGs) and free fatty acids (FFAs) into biodiesel. This is an attractive alternative

Yield (%)a

R1

C6H5

p-Cl-C6H4

p-Me-C6H4

C6H5

p-Cl-C6H4

C6H5

89(9)

p-Br-C6H4

p-OMe-C6H4

87(6)

p-OMe-C6H4

C6H5

93(8)

n-C4H9

C6H5

84(7)

n-C6H13

C6H5

85(10)

n-C9H19

C6H5

81(12)

90(5)

92(7)

Yields in parentheses are for the other isomer.

dye industries [42]. 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 their corresponding amines, acids and alcohols [43]. Wolfson et al. [12] investigated the synthesis of aromatic gemO

O Cl +

Cl Cl SZ

2

H +

100°C, 1h

Scheme 10.

NHTs

major when R= Alkyl

4.5. Esterification and Transesterification

R

H

SR1

u rib

NHTs Ts

n tio

( )n

n=1, Yield = 91% n=2, Yield = 88%

Scheme 8.

N

NHTs

SZ, CH3CN N Ts + R1SH

(C6H5-CO)2O



O

O O

+

were found to show more activity than that of SZ catalyst. The SZ and WZ catalysts exhibited 57% and 10% conversion respectively.

OR

Solid acid

2 ROH

OR

4.6. Stereocontrolled Glycosidation

- H2O

O

O

Highly effective, simple and environmentally acceptable glycosidations have attracted considerable attention recently in synthetic organic chemistry related to the synthesis of biological active molecules as well as functional materials [47]. For example, the direct stereocontrolled glycosidation of 2-deoxy sugar, especially -stereoselective glycosidation and stereocontrolled construction of -and -mannopyranoside is difficult. Therefore, there is a considerable attention for the stereocontrolled glycosidation in synthetic chemistry. Various stereocontrolled glycosidation reactions reported so far employing SZ catalyst are briefly discussed below.

Scheme 11.

(or extender) to petrodiesel fuel due to several well-known advantages [45]. Lopez et al. [46] compared the catalytic activity of a number of solid and liquid catalysts in the transesterification of triacetin with methanol (Scheme 12) 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

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Table 5. Esterification Using SZ Catalyst

Acid

Alcohol

Temperature (°C)

Sebacic acid

2-Et-Hexanol

110–170

Trans-(2-hexenyl)succinic anhydride

2-Et-Hexanol

Adipic acid

Diethyleneglycol

Caproic acid

Pentaerythritol

F t o Stepwise reactions

O

O

N

O

O

O

Time (Min.)

t s i 110–160

110–160

110–190

CH3

120

120

CH3OH

95.6 97.7

150

96.5

O O

H3C

H3C

+

OCH3

O CH3

CH3 O Diacetin

Triacetin Catalyst

CH3OH

O

O OH OCH3

97.3

OH

O

H3C

96.4

90

O

Catalyst

+

Conversion (%)

u rib 90

110–180

D or 2-Et-Hexanol

Caproic acid

H3C

+

O

Catalyst

OH H3C

HO OH

O

H3C

+

CH3OH

OCH3

OH

Overall reaction O CH3

O O H3C

+

3 CH3OH

HO

O +

OH CH3

O

OH

Catalyst

O O

Scheme 12.

123

3 H3C

OCH3


Reddy et al.

O

BnO BnO

OBn

OBn

OBn SZ, MS 5A

OR

BnO BnO

Et2O, 0°C

SZ, 25°C

O + ROH

O

BnO BnO

CH3CN

OR

F

1

O

OH HO

n-C8H17-OH

HO

Me O OMe 4

3

2

O

MeO MeO

HO

OBn O

5

OH

6

N3 7

Scheme 13. Table 6. Glycosidation of 1 with Alcohols Using SZ Catalyst SZ, MS 5A, Et 2O

SZ, CH 3CN

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Alcohol / Ratio

Yield (%)

/ Ratio

19:81

98

2

88:12

15:85

96

3

84:16

20:80

97

19:81

99

27:73

85

33:67

56

Yield (%)

u rib

t s i D r

o F t o N

98 92

4

83:17

5

82:18

6

82:18

90

7

80:20

80

4.6.1. Glycosidation of 2-Deoxyglucopyranosyl Fluorides

The direct stereocontrolled glycosidation of 2-deoxy sugars, in particular the -stereoselective glycosidation is not easy due to lack of stereodirecting anchimeric assistance from the C-2 position and low stability of the glycosidic bond of the 2-deoxyglycoside under acidic conditions. Therefore, stereocontrolled glycosidation of 2-deoxy sugars in an ecofrendly way is of particular interest. Toshima et al. [48] reported direct stereo-controlled glycosidation of totally benzylated 2-deoxy--D-glucopyranosyl fluoride with alcohols employing SZ catalyst to synthesize both the 2-deoxy-and -D-glucopyranosides. In this study they examined first the glycosidation of the totally benzylated 2-deoxy-glucopyranosyl fluoride 1 with cyclohexyl methanol 2 using SZ catalyst with or without 5A molecular sieves under various conditions. They observed that glycosidation of 1 employing 5 wt.% SZ in CH3CN at 25°C for 1 h gives 2deoxyglucopyranoside in high yield with high -stereoselectivity. On the other hand, the corresponding 2-deoxyglucopyranoside was selectively (-stereo-selectivity) obtained by employing a 100 wt.% SZ in the presence of 5A molecular sieves (500 wt.%) in Et2O at 0°C (Scheme 13). The obtained results by using various alcohols are summarized in Table 6. 4.6.2. Glycosidation of Mannopyranosyl Sulfoxides Glycosubstances including glycolipids, glycoproteins and many antibiotics continue to be the central focus of research

92 97

both in chemistry and biology. Since - and - mannopyranosides frequently appear in many naturally occurring bioactive substances, the stereocontrolled construction of and -mannopyranosides is of considerable importance in synthetic organic chemistry [47]. Therefore, highly stereocontrolled synthesis of both the - and -mannopyranosides in an environmentally friendly manner is again of particular interest. Nagai et al. [49] investigated the stereocontrolled glycosidation of mannopyranosyl sulfoxides with several alcohols by employing Nafion-H and SZ catalysts for direct synthesis of both - and -mannopyranosides in high yields. Their study revealed that SZ (100 or 300 wt.%) in the presence of 5A molecular sieves (100 or 300 wt.%) in Et2O (also MeCN, CH2Cl2, PhMe) at 25°C gives stereoselectively mannopyranoside in the glycosidation of the -mannopyranosyl sulfoxides (Scheme 14). Some of the results reported are summarized in Table 7. 4.6.3. Glycosidation of pyranosyl -Fluorides

Manno-

and

2-Deoxygluco-

The - and -mannopyranosides appear in many naturally occurring bioactive substances such as asparaginelinked glycoproteins and certain antibiotics [50]. Therefore, the stereocontrolled formation of - and -mannopyranosides is of particular importance in the chemistry as well as biology. Toshima et al. [51] reported the stereocontrolled glycosidation of manno- and 2-deoxyglucopyranosyl fluorides with several alcohols using SZ catalyst for direct and effective syntheses of both - and -manno-and 2-



OBn

BnO

OBn

BnO

O

SZ

O +

Y

BnO BnO

HO

125

BnO BnO

Solvent 25°C, 3 h

O

X

8:X=S(O)Ph,Y=H 9:X=H,Y=S(O)Ph

2 O

OH

OBn O HO

n-C8H17-OH

O

BnO BnO

HO

OBn OH 3

N3

OMe

7

5

4

10

Scheme 14. Table 7. Glycosidations of 8 and 9 with Various Alcohols Using SZ Catalyst

Entry

Glycosyl Donor

Alcohol

SZ (wt.%)

Additive

1

8

2

100

-

2

8

2

100

-

3

9

2

100

-

4

9

2

100

5

9

2

100

6

9

2

100

7

9

2

300

5A MS (300)

8

8

3

300

5A MS (300)

9

8

4

300

5A MS (300)

10

8

5

300

11

8

7

12

8

10

F t o

N

BnO

O

BnO

t s i

OR

BnO

38

Et2O

02

Et2O

17

MeCN

52

57/43

39/61 32/68

64/36

04

PhMe

04

74/26

Et2O

99

19/81

Et2O

99

21/79

Et2O

95

20/80

5A MS (300)

Et2O

93

23/77

300

5A MS (300)

Et2O

70

38/62

300

5A MS (300)

Et2O

85

26/74

-

X

BnO

+

BnO

Et2O,25°C MS 5A(100wt%)

BnO

SZ, (5wt%)

O

75/25

X O

BnO

R-OH CH3CN,40°C

BnO

1: X= H 11:X = OBn F

n-C8H17-OH

OR

HO

HO

3

HO 2

OBn O

O

BnO BnO

BnO O

HO OH

OBn 8

5

4

O OH

Scheme 15.

-

MeCN

CH2Cl2

BnO

SZ, (100wt%)

/ Ratio

u rib

-

D or

X

n tio

Yield (%)

Solvent

OMe

N3 7

deoxyglucopyranosides (Scheme 15). In this study they investigated several heterogeneous solid acids, namely Montmorillonite K-10, Nafion-H and SZ. They all possess Brönsted acidity and work as protic acids. Their study revealed that SZ gives better yields for this reaction. Furthermore, it

BnO 10

OBn OMe

provides maximum -selectivity when MeCN is employed as solvent at 40°C and -selectivity in Et2O as solvent at 25°C along with molecular sieve 5A (100 wt.%) (Scheme 15). Some of their significant results are summarized in Table 8 and 9.


Reddy et al.

4.7. Synthesis of 3,4-Dihydropyrimidin-2(1H)ones

4.8. Synthesis of 2,3-Dihydro-1H-1,5-Benzodiazepines

Dihydropyrimidinones (DHPMs) are an important class of compounds due to their diverse therapeutic and pharmacological applications [52]. The general method for the synthesis of DHPMs known as Biginelli reaction involves threecomponent condensation reaction between aldehyde, ketoester and urea under strong acidic conditions. Reddy et al. [13] reported a facile method for the synthesis of 3,4dihydropyrimidone-2(1H)–ones by a one-pot condensation reaction between an aldehyde, -ketoester and urea or thiourea under solvent-free conditions at 100°C catalyzed by SZ (Scheme 16). These reactions were found to proceed efficiently and various dihydropyrimidinones were produced in excellent yields (Table 10) in short reaction times (40-60 min).

Benzodiazepines and their polycyclic derivatives are an important class of bioactive compounds. Many functionalized benzodiazepines are widely employed as anti-convulsant, anti-anxiety, analgesic, sedative, anti-depressive and hypnotic agents [53]. Reddy et al. [14,54] reported the synthesis of various 1,5-benzodiazepine derivatives (Scheme 17) by the condensation reaction of o-phenylenediamine with various ketones employing SZ catalyst under solventfree conditions. The o-phenylenediamine and ketone were taken in 1:2.5 molar ratio and the reaction was carried out at ambient conditions resulting in high product yields (Table 11). Activity comparison was also made between H-ZSM-5 and SZ catalysts for this reaction. Although H-ZSM-5 catalyst is active for condensation reactions, the yields obtained

Table 8. Glycosidations of 1 with Various Alcohols Using SZ Catalyst SZ, MS 5A,Et2O

n tio

SZ, CH 3CN Alcohol

/ Ratio

Yield (%)

/ Ratio

19:81

98

2

15:85

96

3

20:80

97

19:81

99

33:67

56

28:72

81

30:70

50

u rib 88:12

t s i D r 4 5 7 8

o F t o N

Yield (%)

10

84:16 83:17 82:18

98 92 92 97

80:20

80

86:14

82

88:12

53

Table 9. Glycosidations of 11 with Various Alcohols Using SZ Catalyst SZ, MS 5A,Et2O

/ Ratio

Yield (%)

17:83

99

20:80

SZ, CH 3CN

Alcohol

/ Ratio

Yield (%)

2

97:3

99

96

3

98:2

97

16:84

95

4

97:3

96

19:81

97

5

98:2

96

21:79

80

7

97:3

84

27:73

84

8

97:3

88

56:44

55

10

98:2

75

R1 O

H O +

R1

Scheme 16.

Me

O

X OR2

+

H2N

O

SZ NH2

( X= O,S)

100°C

NH

OR2 Me

N H

X



auxiliaries in asymmetric synthesis and in carbon-carbon bond forming reactions [57]. Reddy et al. [13] reported the synthesis of various diaryl sulfoxides by taking arenes and thionyl chloride (2:1 molar ratio) with catalytic amounts of SZ in a round-bottom flask and stirring for appropriate times under solvent-free conditions resulting in reasonably high product yields (Scheme 18 and Table 12). This is another good example for the versatility of the SZ catalyst.

Table 10. Results of the Synthesis of DHPMs Employing SZ Catalyst R1

R2

X

Yield (%)

H

Et

O

90

NO2

Et

O

88

H

Me

O

92

NO2

Me

O

90

H

Et

S

80

OH

Et

S

82

R1

+ R1

4.10. Synthesis of Bis(indolyl)methane Derivatives Indoles and their derivatives are important intermediates in organic synthesis and widely featured in a variety of pharmacologically active compounds [58]. During the past few years a large number of natural products containing

O

NH2

R1

H N

R1

N

R2 R3

SZ

R3

R2

n tio

R3

NH2

R2

Scheme 17.

u rib

Table 11. Reaction of o-Phenylenediamine with Ketones R1

R2

R3

H

CH3

CH3

H

CH3

C2H5

H

CH3

Ph

C2H5

C2H5

CH3

CH3

CH3

CH3

CH3

C2H5

R

+

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 [59]. Reddy et al. [13] reported the synthesis of bis(indolyl) methanes by electrophilic substitution reaction of indole with various aldehydes in the presence of SZ catalyst. The reaction was carried out by taking a mixture of aldehyde and indole (1:2.5 molar ratio) along with catalytic amounts of SZ in a round-bottomed flask and stirred for an appropriate time at room temperature (Scheme 19, 20 and 21). The reactions proceeded efficiently and the bis(indolyl)methanes were produced in excellent yields in short reaction times (Table 13).

Yield (%)

t s i D r 94

91

96

o F t o N H

Cl

84

94

91

O

O S

Cl

127

R

+

S

SZ R

R

Scheme 18.

Table 12. Results of Synthesis of Various Diaryl Sulfoxides Me

MeO

Arene Yield (%)

Me

Me Me

MeO

Ph

Br

Me

90

92

are not significant when compared to SZ catalysts, especially in the case of cyclic ketones. 4.9. Synthesis of Diaryl Sulfoxides Sulfoxides and sulfones are important intermediates for the synthesis of a large variety of organic sulfur compounds in the field of drugs and pharmaceuticals [55,56]. Recently, sulfoxides have received much attention as important chiral

85

83

88

80

A comparison of the activity results with various other catalysts reveals that the SZ catalyst is highly efficient in terms of product yields, reaction temperature and reaction times. 4.11. Tetrahydropyranylation of Alcohols and Phenols Tetrahydropyranylation is one of the most frequently used processes in organic synthesis for the protection of hy-


Reddy et al.

R1 CHO

R2

H

SZ

+ R1

N H

R2

N H

N H

Scheme 19. Table 13. Reaction of Indole with Aromatic Aldehydes

4.12. Alkylation of 4-Methoxyphenol with MTBE Aldehyde

The most commonly used antioxidants to stabilize food against auto-oxidation are butylated hydroxy toluene (BHT), butylated hydroxy anisole (BHA) and n-propyl gallate [63]. The BHA is more in demand and both mono- and dialkylated products are used as anti-oxidants. These antioxidants are normally synthesized by using Friedel–Crafts alkylation. Yadav and Rahuman [64] investigated the alkylation of 4-methoxyphenol with MTBE (Scheme 23) by employing various solid acid catalysts. The order of catalytic

Yield (%) R1

R2

H

H

85

NO2

H

84

OMe

H

78

OMe

OMe

73

CH3

t s i

CHO +

N H

F t o

Scheme 20.

N

D or O2N

+

H

N Me H

N Me H

Yield = 82%

O

H

SZ

CHO

O

N H

SZ

n tio

u rib

O2N

N H

N H

Yield = 78%

Scheme 21.

O R

OH +

SZ

O

OR

Scheme 22.

droxyl groups [60]. This is an interesting reaction due to high stability of the resulting THP ethers in a variety of reaction conditions, such as reduction, oxidation, strongly acidic and basic media, as well as the ease in the deprotection of the formed THP ethers [61]. Reddy et al. [62] reported the reaction of various alcohols and phenols with 3,4-dihydro-2-Hpyran in the presence of catalytic amounts of SZ under solvent-free conditions at room temperature in short reaction times to offer various tetrahydropyranyl ethers in excellent yields (Scheme 22 and Table 14).

activity of various solid acid catalysts studied is as follows: Filtrol-24 > DTP/K-10 > Deloxane ASP resin > K-10 Montmorillonite clay > SZ. Though the SZ catalyst exhibited the lowest activity as compared to other catalysts, it showed maximum selectivity to monoalkylated products. This reaction was carried out by taking 1:3 molar ratio of 4methoxyphenol and MTBE at 150°C using 1,4-dioxane as the solvent. The SZ catalyst provided 22% conversion and 85% selectivity for the monoalkylated product. 4.13. -Pinene Isomerization to Camphene -Pinene isomerizes in the presence of acid catalysts by a mechanism in parallel where, on one hand bicyclic compounds are obtained as camphene 12, tricyclene 13, fenchene 14, bornylene 15, through a cyclic rearrangement, and on the other hand monocyclic compounds as -terpinene



129

Table 14. Results of Tetrahydropyranylation of Alcohols and Phenols OH

OH

OH

OH

Alcohol

OH

NO2

i-C4H9-OH

OMe

Yield (%)

94

96

OCH3 H3C

CH3

82

90

H+

92

94

CH2 +

CH3

H3C

CH3OH

CH3 HO

H+

n tio

CH3 OCH3 OCH3

u rib

+ OH

t s i D r BHA ( mono)

Scheme 23.

o F t o N

13

12

16

17

OH

BHA ( di )

15

14

18

19

20

Scheme 24.

16, limonene 17, -terpinene 18, terpinolene 19 and pcymene 20 are produced by means of the rupture of one of the rings (Scheme 24). The acidity of the catalyst is directly related to the activity as well as camphene yield. Camphene is used in the manufacture of camphor and its related compounds. Comelli et al. [65] investigated the isomerization of -pinene with SZ catalyst and compared it 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 and designated the catalysts as SZ250, SZ350 and SZ500. In a typical run, 5 ml of -pinene (98.7% purity) was placed in a reactor and heated

up to 120°C and then 75 mg of catalyst was added. In the case of ZrO2 catalyst that possesses only Lewis acid sites no activity was observed. On the other hand Brönsted acid H2SO4 exhibited barely any activity. Since SZ possesses both Brönsted and Lewis acid sites it showed good activity. Further, the treatment temperature of SZ also 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 selectivity for camphene 12 are 58.9, 67.4, 56.8, 59.3 and 12.0 %, respectively.


Reddy et al.

HNO3 + (MeCO)2O

MeCO2NO2 + MeCO2H

Cl

Cl

SZ +

NO2

MeCO2NO2

o- and p-nitro product

Scheme 25. O

O

S

O

O

O

S

CH3COONO2

O H3COOC Zr

Zr

O + O NO2 Cl

Ortho

n tio

Para

Cl

Cl

O2N H

+ O

O O H3COOC Zr

O

Cl

o F t o N Scheme 26.

4.14. Nitration of Chlorobenzene

O

S

t s i D r

-CH3CO2H

O2N

u rib +

+

Industrial aromatic nitrations are generally carried out with a mixture of nitric acid and sulfuric acid predominantly giving ortho- and para-substituted products of the substituted benzenes [66,67]. At about 68 w/w H2SO4, the nitration becomes extremely slow. Moreover, it is desirable to increase the para-selectivity from the economic point of view. Therefore, there is tremendous interest on this reaction in view of its commercial significance. Yadav and Nair [68] investigated the nitration of chlorobenzene using various catalysts including SZ (Scheme 25). In a typical experiment, 70% nitric acid (0.1 g/mol) was added drop-wise over a period of 90 min to a mixture of chlorobenzene (0.2 g/mol) and acetic anhydride (0.5 g/mol) containing the desired amount of catalyst (2.24 g) at 30°C in a fully baffled 100 ml glass reactor under constant stirring. In a short while after the addition of nitric acid was complete, conversion of nitric acid was calculated on the basis of chlorobenzene consumed, and found that the SZ catalyst provides maximum conversion (47%) with 91% para-selectivity [68]. Furthermore, the formation of meta product as well as dinitrated by-products were not observed. However, after 1.5 h of reaction time, the conversion reached 100% and the para-selectivity dropped to 77% [34]. The mechanism of the reaction envisaged is presented in Scheme 26. This is another good example on

S

O

O

O

Zr

H

NO2

-CH3CO2H

Cl

NO2

the utility of SZ catalysts for various commercially important reactions. 4.15. Chemoselective Alkylation of Guaiacol and p-Cresol with Cyclohexene

Alkylation of guaiacol (2-methoxyphenol) with cyclohexene (Scheme 27) yields O- and C-alkylated products having commercial value. The O-alkylated product (cyclohexyl2-methoxyphenyl ether) is a promising perfume. Alkylation of p-cresol with cyclohexene (Scheme 28) yields O- and Calkylated products, 1-cyclohexyloxy-4-methylbenzene and 4cyclohexyloxy-4-methylphenol respectively. Both products are also of commercial significance as perfume and insecticide respectively. Yadav et al. [69,70] investigated the alkylation of p-cresol and guaiacol with cyclohexene by employing several solid acids including the SZ. The alkylation of pcresol was carried out by taking 1:1 molar ratio of p-cresol and cyclohexene and 20 kg/m3 of catalyst at 80°C using toluene as the solvent. The SZ catalyst provided 47% conversion and 82% O-alkylated product selectivity, which is the highest as compared to other catalysts studied. The alkylation of guaiacol was carried out by taking 0.226: 0.045 molar ratio of guaiacol and cyclohexene and 0.05 gm/cm3 of catalyst at 80°C. Here too the SZ catalyst provided 74% conversion and 68% O-alkylated product selectivity, which



131

OH

OH

OCH3

OCH3

SZ +

+

O

80°C

OCH3

Scheme 27. OH

O

OH SZ +

+

80°C CH3

Scheme 28.

CH3

CH3

+

u rib

+ 21

t s i D r +

H+ +

o F t o N 22

Scheme 29.

O

O

n tio +

H+

NO2

+

NH2

+

NO2

O

SZ, Toulene

+

Reflux, 3.5h

N

O

R

Scheme 30.

is again the highest as compared to other catalysts investigated. 4.16. Solvent-Free Isomerization of Longifolene

Longifolene, 21, (decahydro-4,8,8-trimethyl-9-methylene -1-4-methanoazulene), a tricyclic sesquiterpene hydrocarbon is commercially important chemical used in the perfumery industry owing to its woody odor. It is one of the most abundant sesqui-terpene hydrocarbons naturally occurring in Pinus Longifolia, P. roxburghii SARG and P. sylvestris. Its economical utilization involves transformation into the isomeric product, iso-longifolene, 22, (2,2,7,7-tetramethyltricycloundec-5-ene). Iso-longifolene, 2, and its acid catalyzed and hydroformylated products are also extensively used in the perfumery and pharmaceutical industries due to their woody amber odor [71,72]. Tyagi et al. [73] reported isomerization of 21 to 22 employing nano-crystalline SZ catalyst obtained by the sol-gel technique. In order to obtain maximum conversion the reaction was carried out at differ-

R

ent 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. A maximum conversion of 93% with 100% selectivity was noted within 15 min of reaction time. The typical mechanism of the acid-catalyzed isomerization of longifolene to iso-longifolene is also shown in Scheme 29. 4.17. Synthesis of Tetrahydroindolones Dicarbonyl Compounds

from

1,4-

The general procedure to prepare pyrrole derivatives involves the reaction of an enolizable 1,4-dicarbonyl compound with a dehydrating agent (H2SO4, P2O5, ZnCl2, etc.) and ammonia or a primary amine, or an inorganic sulfide (Paal-Knorr reaction) [74]. However, this method suffers from various disadvantages such as severe reaction conditions, use of excess and dangerous reagents, and tedious work-up procedure. Nergron et al. [75] reported cyclization of 1,4-dicarbonyl compounds and substituted anilines to tet-


Reddy et al.

Table15. Cyclization of 1,4-Dicarbonyl Compounds to Tetrahydroindolones R

H

4-CH3

4-OCH3

4-F

4-Cl

4-Br

4-I

4-NO2

3-CH3

3-Cl

3-Br

Yield (%)

80

60

63

63

55

55

50

50

55

65

50

H N

SZ HO

OH

+

H2N -H2O

HO

NH2 -H2O H N

Scheme 31.

N H

O R3

X

n tio

O

R2 R3

+ isomers

SZ, -HX

R2

R1

R4

O

u rib

R1

R4

R3

O

t s i D r R2

O

SZ, -CH3CO2H

Scheme 32.

R1

O

+ isomers

R4

Table 16. The SZ Catalyzed Acylation Using Benzoyl Chloride (X=Cl), Benzoic Anhydride (X = OCOC6H5), and Acetic Anhydride

o F t o N Acylating Agent

Aromatics

Benzoic anhydride

Anisole

T(°C)

T (h)

100

Mesitylene

Reaction Product(s) Yield (%)

Isomer(s)

1.5

95

4-CH3O–BP (96), 2-CH3O-BP (4)

120

5

95

2,4,6-(CH3)3–BP (100)

3-Chloroanisole

120

20

85

2-Cl–,4-CH3Ov–BP (84), 4-Cl–,2-CH3O-BP (9), 2-Cl–,6-CH3O–BP (7)

2-Chloroanisole

120

5

70

3-Cl–,4-CH3O–BP (100)

m-Xylene

120

5

88

2,4-(CH3)2–BP (86), 2,6-(CH3)2–BP (14)

Toluene

110

20

70

4-CH3–BP (69), 2-CH3–BP (27), 3-CH3–BP(4)

Anisole

100

1.5

91

4-CH3O–AP (98), 2-CH3O–AP (2)

Benzoyl chloride

Acetic anhydride

Mesitylene

100

3

42

2,4,6-(CH3)3–AP (100)

2-Chloroanisole

130

20

36

3-Cl–,4-CH3O–AP (100)

m-Xylene

100

3

7

2,4-(CH3)2–AP (98), 2,6-(CH3)2–AP (2)

BP = benzophenone, AP = acetophenone.

rahydroindolones employing SZ catalyst (Scheme 30 and Table 15). They carried out the reaction under reflux conditions in toluene solvent for 3.5 h with a tricarbonyl com-

pound to catalyst ratio of 1:1 (wt.). As shown in Table 16, the SZ catalyst exhibited good catalytic activity for this important reaction.



133

R1

R1 O + R2

SZ R3

X

-HX

+ isomers R2 O

R3

Scheme 33. Table 17. Acylation of Naphthalenes on SZ Catalyst at 70°C Aromatics

Acylating Agent

Yield (%)

R1 = H, R2 = CH 3O

R3 = C6H5, X = Cl

< 85

R3 = C6H5, X= OCOC6H5

< 85

R3= CH3 , X = OCOCH 3

< 85

R1 = CH 3O, R2 = H

n tio

R3 = C6H5, X = Cl

82

R3 = C6H5, X= OCOC6H5 R3 = CH 3, X = OCOCH3

~100

R1 =CH3 , R 2 = CH 3

R3 = C6H5, X = OCOC6H 5

t s i D r R3 = CH 3, X = OCOCH3

R1 = H, R2 = CH3

R3 = C6H5, X = Cl

R3 = C6H5, X = OCOC6H 5

o F t o N R 1= CH 3, R2 = H

R1 = H, R2 = H

u rib

R3 = C6H5, X = Cl

R3 = CH 3, X= OCOCH 3 R3 = C6H5, X =Cl

~100 79

93 7

17

13

No Reaction 12

R3 = C6H5, X = OCOC6H 5

7

R3 = CH 3, X = OCOCH3

No Reaction

R3 = C6H5, X = Cl

< 10

R3 = C6H5, X = OCOC6H 5