Catalytic Organic Reactions on ZnO.: Ingenta Connect

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Catalytic Organic Reactions on ZnO Mona Hosseini-Sarvari* Department of Chemistry, Faculty of Science, Shiraz University, Shiraz 71454, I.R. Iran Abstract: The use of ZnO is a topic of continuing interest due to numerous applications in many areas of the chemical industry. In addition, with the development of industrialization, organic chemists have been confronted with a new challenge of finding novel methods in organic synthesis that can reduce and finally eliminate the impact of volatile organic solvents and hazardous toxic chemicals in the environment. Due to this, great efforts have been made by different research groups to achieve the goal of employing catalytic amounts of ZnO in organic transformations. Interesting results have been achieved, such as the use of catalytic amounts of ZnO alone or ZnO in combination with another metal ormetal oxide. In fact, ZnO as a heterogeneous catalyst can be easily separated from the reaction mixture and reused; it is generally not corrosive and does not produce problematic side products. Different classes of organic transformations have been studied and utilized using ZnO. Owing to the great interest in ZnO, reviews can be found on the properties and applications of ZnO, but to the best of our knowledge there is not a reported review on the application of ZnO as a catalyst in organic syntheses and transformations on which the author’s group has also contributed significantly. Hence, the present review focuses on fine and specialty chemicals synthesis catalyzed by ZnO. It is out of the scope of this review to cover comprehensively all reactions that have ever been reported in the presence of mixed metals or metal oxides with ZnO.

Keywords: Catalytic reaction of ZnO, heterogeneous catalyst, metal oxides, organic synthesis, organic transformation, zinc oxide. 1. INTRODUCTION Zinc oxide (ZnO) is a reagent that has been used in chemistry for many centuries. Around 4500 B.C., known as the copper age, ZnO was found as a by-product of copper smelting. In that process, the zinc proportion of copper ores was reduced to zinc vapor, which oxidized and deposited in the furnace flues as an impure powder known as calamine. In ancient Egypt, zinc oxide was referred to as calamine, which is thenatural mineral Smithsonite with ZnCO3 as its main compound. Also, in China ZnO is known as “Lu-Gan-Stone,” belonging to the category of metal and stone. The Lu-Gan-Stone is converted to zinc oxide after processing and has been used in the treatment of skin diseases, eyes and tooth problems. From the 12th to the 16th century, zinc and zinc oxide were recognized and produced in India. From India, zinc manufacturers moved to China in the 17th century. From 1750 to 1850, zinc oxide was formally used in medical therapy in several European countries, which ledto increasing study and research on zinc oxide [1]. The modern history of ZnO started around the middle of the 18th century. In 1871, ZnO was used in the paint trade due to its whiteness, fine texture and opacity. Soon after the first radio broadcasting stations started transmitting in the 1920s, ZnO crystals came into popular demand for their semiconductor properties. ZnO has been intensively studied since 1935,leading to research in discovering how semiconductors function.Moreover, its lattice parameter studies date back to 1935, its vibration properties to1970, and in terms of devices, many studies peaked during the period from1970 to1980 [1]. Zinc oxide is a II-VI compound semiconductor which crystallizes in three forms: hexagonal wurtzite, cubic zincblende, and the rarely observed rock salt. Under ambient conditions, the hexagonal wurtzite structure is thermodynamically stable and thus most com mon. The zinc blende form can be stabilized by growing ZnO on *Address correspondence to this author at the Department of Chemistry, Faculty of Science, Shiraz University, Shiraz 71454, I.R. Iran; Tel:+987112284822; Fax: +987116460788; E-mail: [email protected] 1875-6271/13 $58.00+.00

substrates with a cubic lattice structure. In both cases, the zinc and oxide centers are tetrahedral [1]. From past to present, the number of applications for ZnO are numerous, and the principal ones are rubber manufacturing [2-4], medical applications [5-11], concrete industry [10], cigarette filters [12], food additive [13], pigment [14, 15], coatings [16-18], corrosion prevention in nuclear reactors [19-21], etc [22]. Most applications exploit the reactivity of zinc oxide as a precursor to other zinc compounds. For material science applications, zinc oxide has a high refractive index, high thermal conductivity, binding, antibacterial, and UV-protection properties. Consequently, it is added into various materials and products, including plastics, ceramics, glass, cement, rubber, lubricants, paints, ointments, adhesive, sealants, pigments, foods, batteries, ferrites, and fire retardants, just to name a few. On the other hand, ZnO has been explored as a catalystfor various organic reactions, facilitating the synthesis of fine and specialty chemicals. The application of ZnO catalysisnot only is industrially important, but also has academic merit. The ZnO catalyst was exploited in industriallyimportant reactions such as the isomerization of butane [23-25], and interestingly,in the synthesis of MeOH [26, 27]. Many current industries and our quality of life are critically reliant upon ZnO. The present review covers the development of ZnO as a catalyst for a diverse range of organic transformations. Some of the important organic reactions reported in the literature employing ZnO as a catalyst are organized and outlined below. 2. HYDROGENATION 2.1. Hydrogenation of Alkenes Such as Ethylene, Propylene, and 1, 3-Dienes In 1940, Woodman and Taylor [28] discovered that zinc oxide is an effective catalyst for the hydrogenation of ethylene. Since this discovery, a number of reports have appeared dealing with this aspect of the chemistry of zinc oxide [29-35]. Dent and Kokes [33a] have shown that hydrogenation of ethylene over ZnO to form ethane occurs stepwise as outlined below: © 2013 Bentham Science Publishers

698 Current Organic Synthesis, 2013, Vol. 10, No. 5

H2(g) + H H Zn O

Zn O

H H Zn O

+

H Zn O

O

H Zn O

(eq. 1)

+

H O

(eq. 2)

H Zn O

H Zn O

+

(eq. 3)

O

Zn O

+

H2C

CH2

H Zn O

H2C CH2 +

H2C

Mona Hosseini-Sarvari

H O

(eq. 4)

H Zn O

CH2

(eq. 5)

CH3 CH2 Zn O

H Zn O

(eq. 6)

cals, medicine and food additives [38]. As a beneficial and chloride-free process, the hydrogenation of methyl benzoate or benzoic acid over metal oxide catalysts has been studied for the synthesis of benzaldehyde. Since Al2O3[39] and copper chromites [40] were reported as the catalysts for the hydrogenation of methyl benzoate and/or benzoic acid to benzaldehyde in 1975, various metal oxides have been proposed as hydrogenation catalysts. In the hydrogenation of benzoic acid to benzaldehyde, the activity of ZnO was higher than that of other metal oxides [41-50]. Holderich et al. [43, 44] reported that high yields of aromatic aldehydes were obtained by direct hydrogenation of aromatic carboxylic acids over different ZnO catalysts with different specific surface properties. In their study, ZnO with weakly acidic surface properties provided benzaldehydes in high selectivity; however, ZnO with strong acidic sites caused a consecutive hydrogenation of benzaldehyde to toluene, and more strongly acidic sites led to coke formation. Yokoyama [41] and Lu [48] proposed the reaction mechanism of catalytic hydrogenation of benzoic methyl ester (1) to benzaldehyde (3) as shown in (Scheme 1). In this mechanism, a surface carboxylate 2 is formed as an intermediate prior to the formation of the benzaldehyde (3). 3. DEHYDRATION AND DEHYDROGENATION

CH3 CH2 O Zn

+

H O

Zn O

C2H6 +

In the above equations,

Zn O

O

+

(eq. 7)

represents the active siteand

O represents the oxide ions which form a wall separating theactive sites. (This is an oversimplification in the sense that the zinc of the active site is likely to haveseveral equivalent oxide nearest neighbors). The first equation represents the initial chemisorption of hydrogen; the second equation provides for the site-to-site migration of hydrogen. Equations 3 and 4 provide a mechanism whereby hydrogen is attached to zinc,which can lead to anexchange with hydrogen attached to an oxide ion. The adsorption of ethylene on zinc oxide is outlined in equation 5. Infrared studies have revealed that no bonds could be attributed to opening of double bond with formation of a saturated species. Therefore, chemisorption occurs by interaction of a bond with the surface. Finally, the formation of ethane results from the processesrevealed in equations 6 and 7.

Alcohols undergo catalytic dehydrogenation to aldehydes or ketones and dehydration to the corresponding alkenes. The catalytic decomposition of alcohols employingoxide catalysts via dehydrogenation and dehydration is considered a simple route for the production of valuable chemical compounds such as propylene. Therefore these conversions have been widely studied [51]. The hydrogenation and dehydrogenation of various alcohols have been studied with ZnO [52-58]. Different reaction mechanisms, such as E1, E2, or E1cB have been proposed to explain the product distribution. Recently, Saad and Raid [58] have shown that the dehydration of isobutanol on the prepared metal oxide catalysts can proceed through the two-elimination mechanisms, ElcB and E2. However, in the presence of the ZnO catalyst, dehydration is more likely to proceed through the E1cB mechanism (Scheme 2). The ElcB pathway involves a surface alkoxy intermediate on both a strongly basic site and a weak Lewis acid site (Zn2+O2 pairs). Formation of the olefin 4 takes place by -H elimination from the carbanion intermediate. Me Me

Another novel catalytic property of ZnO is its high activity and selectivity for the hydrogenation (deuteriogenation) of 1,3-dienes [36, 37].Preferential formation of cis-2-butene containing 2D atoms was observed. During the course of the hydrogenation reaction, the molecular identity of hydrogen (or D2) is maintained, i.e., either H (or D) atoms in a H2 (or D2) molecule are incorporated into one hydrogenated molecule. 2.2. Hydrogenation of Benzoic Acid to Benzaldehyde Benzaldehyde is an important intermediate in the organic chemical industry and is widely used in the production of chemi-

O

Zn

O

Zn O

H2

O

Zn

O

Zn

O O

O

Zn

H Zn

O Zn O

Zn

Me Me C CH H O

H

H O Zn O

4

H2O Zn

Scheme 2. E1cB mechanism of isobutene formation on ZnO.

Me O

O Zn

3

O Zn O

H H

Me Me C CH2 H O H

CH CH2

PhCO2Me O 1

H

H

H -CH3OH

O Zn

Zn

Ph

Ph O

O

O

Zn

Zn 2

+ PhCHO 3

Scheme 1. The cleavage of methanol and the formation of benzaldehyde over ZnO.

Catalytic Organic Reactions on ZnO

Current Organic Synthesis, 2013, Vol. 10, No. 5

4. FRIEDEL-CRAFTS ACYLATION REACTIONS

. Ar-H + RCOCl 5 6

Aromatic ketones are valuable intermediates in the production of various fine chemicals and are also used in the production of pharmaceuticals, cosmetics, agrochemicals, dyes, and specialty chemicals. Aromatic ketones are synthesized mainly by FriedelCrafts acylation of areneswith acid chlorides or carboxylic anhydrides [59]. Traditionally, these reactions have been carried out using stoichiometric amounts of liquid Brønsted acids or Lewis acids. Nowadays, the restrictions imposed by the wasteminimization laws and economic considerations have ledto the development of new catalytic technologies. Modern processes for acylation of aromatic compounds employing ZnO as a catalyst are present in the literature [60-64].

Thakuria et al. [62] reported a comparative study of metal oxides such as CuO, NiO, CoO, Mn2O3, Cr2O3, and ZnO as catalysts for Friedel-Crafts acylation. They tried Friedel Crafts acylation under solvent-free conditions and showed that the best result employed half the amount of macroporous ZnO (0.25 mmol) compared to the other metal oxides that were used. Shan and co-workers [63] carried out the acylation of ferrocene with acylating agents using ZnO. The method has the advantages of low catalyst consumption, mild reaction condition, little acid substance generation, high yield, short production period, and low cost. They also investigated the mechanism of the reaction (Scheme 4). To understand the mechanism, they monitored the reaction via in situ IR and Es-MS technologies. They deduced that the C–H bond of ferrocene (8) was first activated by coordination of the cyclopentadienyl ring of ferrocene (8) to the zinc in the mixture of ferrocene and ZnO. When an acid chloride 6 was added to the system, the intermediateA was formed, which reacts further with 6to furnish directly the desired product 9 (route I). On the other hand, the intermediate A can also react with the 6 via transition state B,

O

Cl

R

Cl

H

Fe

6 O

R

Zn

O

8 -HCl

ZnOCOR Fe A I

RCOCl

RCOCl

II

6

6

Zn

ZnOCOR Fe

OCOR R

Fe

R

Cl

Cl

O

B

O C

-(RCO)2O RCOCl

-(ClZnOCOR)2

6 ZnCl COR

ArCOR + .HCl 7

Scheme 3. Friedel-Crafts acylation on ZnO.

O

Zn

rt, solvent-free 5-120 min, 50-98%

R= Alkyl and aryl

Hosseini-Sarvari and Sharghi [60] reported that ZnO is one of the best catalysts for the Friedel-Crafts acylation of various benzene derivatives 5 with acid chlorides 6 (Scheme 3). In comparing several metal oxides, namely, HgO, SiO2, Fe2O3, Ag2O, and ZnO, their study revealed that ZnO is the most effective, leading up to 95% selective formation of 4-methoxybenzophenone at room temperature under solvent-free conditions. Also, they found that inactivated benzenes such as chlorobenzene reacted smoothly in the presence of ZnO to afford the corresponding aromatic ketone in high yields. Furthermore, the recovered catalyst (ZnO) retained its catalytic activity.

Fe

ZnO (0.05 mmol)

699

H2O

Fe

Fe

R

Cl O

9 Scheme 4. Possible formation mechanism of acyl-ferrocene catalyzed by ZnO.

D



NH2

which further reacts with the acyl chloride in the system to give the desired product9 (route II). They proved that although there are two possible routes to accessto the acyl ferrocene 9 in this acylation reaction, route II (via B and D) was predominant. Finally, Chandrappa and co-workers [64] investigated the benzoylation of anisole with benzoyl chloride using nanostructured ZnO. These authors developed reproducible procedures for the synthesis and organization of nanostructured ZnO and used it for practical applications such as Friedel-Crafts acylation.

ZnO, rt, solvent-free

HO

NH2

HO

Friedel-Craft alkylation reactions are ubiquitous reactions in fine chemicals, intermediates, and petrochemical industries. The ZnO catalyst has been successfully utilized for various FriedelCrafts alkylation reactions.

ZnO

+

80oC, 120 min, 91% 10

11

12

Scheme 5. Friedel-Crafts benzylation of benzene over ZnO.

6. PROTECTION OF ALCOHOLS, PHENOLS, AMINES AND THIOLS The use of protecting groups is very important in organic synthesis, often being the key for the success of many synthetic enterprises. The acylation of alcohols, phenols, amines, and thiols is an important transformation in organic synthesis [67]. Acylation of such functional groups is often necessary during the course of various transformations in a synthetic sequence, especially in the construction of polyfunctional molecules such as nucleosides, carbohydrates, steroids and natural products. Hosseini-Sarvari and Sharghi [68] carried out the acylation reaction of alcohols, phenols and amines under solvent-free conditions employing ZnO as catalyst (Scheme 6). The catalyst was successfully applied for acylation of a diverse range of alcohols, phenols and amines. In the case of alcohols and phenols, an acid chloride was preferred over the corresponding acid anhydride. The reaction with acid anhydride was too slow to have practical applications. Primary, secondary, and tertiary alcohols acylated smoothly without formation of any side products. Also, the reactions of ROH or RNH2

R'COCl or (R'CO)2O ZnO, rt, solvent-free

13

14

8-240 min, 53-95%

R, R'= aliphtic and aromatic

Scheme 6. Protection of alcohols, phenols, and amines by ZnO.

16

HO

Ac2O

NHCOMe

20

10 min, 87%

NH2

HO

(PhCO)2O ZnO, rt, solvent-free

NHCOPh

HO

40 min, 64%

21

22

Scheme 7. Selectivity in the protection by ZnO.

amines with Ac2O were much faster in comparison to those of the aliphatic alcohols that the selective protection of an amino alcohol 21 appeared to be a distinct possibility. Also, the amino groups in aminophenols 17 and 19 were selectivity acylated (Scheme 7). These authors also compared the activity of several previously known catalysts in the acylation of alcohols and phenols. Their study revealed that ZnO exhibits dramatically higher activity. In addition, the ability to collect and reuse ZnO in the acylation reaction of aniline and phenol was demonstrated, showing thatthe yields of acetanilide and phenyl benzoate in the 2nd, 3rd, 4th and 5thuses of ZnO were almost as high as in the first use. Similarly, in a parallel study O-acylations of alcohols and phenols with acid chlorides were described using ZnO [69, 70]. Moreover, nanopowder ZnO for O-acylation of alcohols and phenols in solvent-free conditions was developed by Moghaddam et al. [71] (Scheme 8). They also proposed a mechanism as shown in (Scheme 8). In this mechanism ZnO is coordinated to the oxygen of the acyl chloride, resulting in the increased reactivity of acyl chloride. ZnO nanopowder is coordinated better than bulk ZnO, while the ZnO nanopowder has more surface atoms, participating at the reaction. ZnO Nano O

O ZnO nanopowder (10 mol%) R'

Cl + ROH 6

13

OR + HCl

R' rt, solvent-free 5 min, 92-96%

15

Scheme 8. Proposed mechanism for O-acylation of alcohols and phenols over nano ZnO.

Recently, Bandgar and coworkers [72] reported a convenient and efficient synthesis of thiol esters via the reaction of acyl chlorides with thiols using ZnO as a catalyst under solvent-free conditions at room temperature. The major advantages associated with this protocol include mild reaction conditions, short reaction times, excellent yields and the ability to recycle the catalyst. In addition, ZnO was reported as an economical and heterogeneous catalyst for the silylation of alcohols, phenols and naphthols.

R'CO2R or R'CONHR 15

18

ZnO, rt, solvent-free 19

Cl

HO

10 min, 90%

17

5. FRIEDEL-CRAFTS ALKYLATION REACTIONS

Arata and coworkers [65] reported that metal oxides such as ZnO, TiO2, and ZrO2 function as effective catalysts for the FriedelCrafts benzylation of toluene with benzyl chloride with high selectivity. They showed that ZnO was prepared by calcining its hydroxide in air at 400oC. The isomer distribution in the alkylation of benzyl toluene was 43% ortho-, 6% meta-, and 51% para-, respectively. Quite recently, Jadhav and Sawant [66] developed the Friedel-Crafts benzylation of benzene (10) with benzyl chloride (11) over mixed metal oxides that possess spinel structures, and the various mixed metal oxides’ activity was compared in the reaction to ZnO (Scheme 5). Their study also showed that ZnO catalyzed the Friedel-Crafts benzylation selectively and formed only diphenylmethane (12) in 91% yield after 120 min.

NHCOMe

Ac2O

ZnO (cat.) ROH + (Me3Si)2NH

.

Solvent-free, rt 23 6-135 min, 85-95% 13 R= Aryl, primary, secondary, and tertiary alkyl

R-OSiMe3 24

Scheme 9. Silylation of alcohols, phenols, and naphthols using ZnO.



ZnO

(Me3Si)2NH

ZnO + (Me3Si)2NH

701

23

ROH 13

NH3

ROSiMe3 24 Me3SiNH2

(Me3Si)2NH

ZnO

24 ZnO

NH3

ROH ROSiMe3

13

24 Scheme 10. Proposed mechanism for silylation of hydroxyl groups on ZnO.

Shaterian and coworkers [73] showed that a variety of alcohols, phenols and naphthols were effectively converted into their trimethylsilyl ether 24 with 1,1,1,3,3,3-hexamethyldisilazane (HMDS) (23) in the presence of ZnO (Scheme 9).

They also proposed a mechanism for the Knoevenagel condensation using ZnO (Scheme 12). According to this mechanism, the first step is proton abstraction from an aldehydes 25 and addition of the carbanion to the carbonyl group to give the intermediate oxyanion 30, which is complexedwith the cation of the ZnO.

They presented a proposed mechanism for the silylation of hydroxyl group by ZnO (Scheme 10).

In order to investigate this type of reaction, Hosseini-Sarvari et al. [78] recently reported that nanopowder ZnO could be used as a catalyst for the Knoevenagel condensation. This condensation was performed using various aliphatic, aromatic, and heterocyclic aldehydes under solvent-free conditions at room temperature (Scheme 13).

7. KNOEVENAGEL AND WITTIG REACTIONS The Knoevenagel [74] and Wittig [75] condensation reactions are twoof the most important C-C bond-forming reactions commonly used in the synthesis of fine chemical intermediates and products such as coumarin derivatives, cosmetics, perfumes, pharmaceuticals, calcium antagonists, and polymers [76]. These reactions were thoroughly investigated employing ZnO as catalyst. First, Moison et al. [77] found that ZnO could be used as a catalyst for Knoevenagel and Wittig reactions at room temperature (Scheme 11). Poor yields were obtained when dry ZnO was used because ZnO has a low surface area (5 m2/g) and contains very small amounts of water. They showed that the addition of water or a polar organic solvent to ZnO increased the yield of the desired product.

O

X nanoflake ZnO (5 mol%)

R(Ar)

H

NC

+

NC-CH2-X 26

25

R(Ar)

X = CN, CO2Et

95-98%

26

R(Ar)-CH=C(X)CN + H2O

20oC, DMSO

27

5 h, 100%

Knoevenagel

ZnO (4 gr)

' + R CH2PPh3Cl

25

28

R(Ar)-CH=CHR'+ OPPh3

-HCl 20oC, 24 h, 25-80%

29

R'= CN, CO2Et, ph

Wittig

Scheme 11. Knoevenagel and Wittig condensation over ZnO.

R

H O 25

CN HC H

H

R

X O Zn2+

Zn2+

CN C

26

O2-

Scheme 12. Proposedmechanism for Knoevenagel condensation using ZnO.

H 27

Scheme 13. Knoevenagel condensation catalyzed by nanocrystalline ZnO.

X= CN, CO2Me, CONH2

R(Ar)CHO

CN

rt, solvent-free, 10 min-5 h

25

ZnO (4 gr) R(Ar)CHO

X

+

H

X

27 30

OH



R2 O R2

CHO

CH3CN/reflux

+ N

S

O

R1

S

32

N

31

R2

R1 33

ZnO/3 h 85-95%

C O

A B

N R1 =

ph, Et, Me R2 = H, Br, OMe, NO2

R1

S 34

Scheme 14. Knoevenagel-hetero-Diels-Alder condensation by ZnO.

8. KNOEVENAGEL-HETERO-DIELS-ALDER CONDENSATION A general way to improve synthetic efficiency and also to give access to a multitude of diversified molecules is the development of domino reactions, which allow the formation of complex compounds [79]. One example of this combination of reactions is the domino Knoevenagel-Hetero-Diels-Alderreaction, which has emerged as an important process for the synthesis of complex compounds such as natural products and also permits the preparation of highly diversified molecules [80]. Moghaddam et al. [81] recently described the ZnO mediated domino Knoevenagel-hetero-Diels– Alder reaction for the preparation of polycyclic compounds 34, which consist of an indole ring (A), a dihydrothiopyran ring (B) annulated to a dihydrochromene ring (C) (Scheme 14). They have optimized the reaction at different temperatures (rt to reflux temperature), different solvents (toluene, acetonitrile, and water), and different amounts of ZnO (from 5 to 100 mol %). Decreasing the ratio of ZnO from 100 to 10 mol% afforded the same result, but with 5 mol% of ZnO, the reaction time increased. As shown in (Scheme 14) the initial step is a Knoevenagel condensation between indolin-2-thiones 31 and aldehydes 32 to form intermediate 33 in which the aldehydes may be activated by ZnO to facilitate the condensation. Next, the triple bond is activated with ZnO through formation of a -complex for the subsequent intramolecular heteroDiels-Alder reaction.

9. KNOEVENAGEL/MICHAEL ADDITION Maghsoodlou et al. [82] examined ZnO and ZnO–acetyl chloride for the synthesis of 2,2'-arylmethylenebis(3-hydroxy-5,5dimethyl-2-cyclohexene-1-one) 36 and 1,8-dioxooctahydroxanthenes 37 via Knoevenagel/Michael addition (Scheme 15). Recently, Hekmatshoar et al. [83] reported the synthesis of 4amino-5-pyrimidinecarbonitriles 40 by a three-component reaction of malononitrile 26, aldehydes 25, and N-unsubstituted amidines 38, under aqueous conditions, using ZnO nanoparticles as thecatalyst in a Knoevenagel/Michael addition reaction (Scheme 16). The reaction occurs via initial formation of the cyano olefin 27 from the condensation of aryl aldehyde and malononitrile. The second step is followed by Michael addition, cycloaddition, isomerization, and aromatization to afford the 4-amino-5-pyrimidinecarbonitriles 40. Intermediate 39 was not stable and is easily oxidized by air to product 40. Major advantages associated with this protocol include reduced reaction times, greater yields, economic viability, and recyclingof the catalyst. 10. BIGINELLI REACTION The three-component condensation reaction between aldehyde,-keto ester, and urea under strong acidic conditions to furnish 3,4-dihydropyrimidin-2(1H)ones is known as the Biginelli reaction. This reaction was first reported by Biginelli in 1893 but suffers O

Ar

O

ZnO-CH3COCl O

CH3CN 2-6h, 82-96%

O

O

37

+ ArCHO

2 35

Ar

O

25

O

ZnO, CH3CN 6-15 h, 85-92% OH

OH 36

Scheme 15. Synthesis of 9-aryl-1,8-dioxooctahydroxanthenes and 2,2’-arylmethylenebis(3-hydroxy-2-cyclohexene-1-ones by ZnO.



703

NH R R'

CN nano ZnO

+

RCHO

R

CN

-H2O

CN

NH2

CN

25

CN

HN

38

NH

R'

27

N

26 R R N

-H2 R'

N

R'

CN

N

CN

N

NH2

NH2 40 39

Scheme 16. Proposed mechanism for Knoevenagel/Michael addition by nano ZnO.

O

RCHO

R

25 X + O 41

NH2 Y NH2

ZnO (12.5 mol%), neat 80oC, 6-35 min, 60-98%

42a: Y=O 42b:Y=S

X= OEt, OMe, Me

COX

HN Y

N H 43

Scheme 17. Solvent-free synthesis of dihydropyrimidinones catalyzed by ZnO.

from low product yields, especially with substituted aromatic aldehydes [84]. The Biginelli reaction has been carried out employing many reagents [85], each of which suffers from various drawbacks. Bahrami et al. [86] reported a simple, efficient and practical procedure for the Biginelli reaction using ZnO as a novel and reusable catalyst under solvent-free conditions. The reaction proceeds efficiently under these conditions, and the dihydropyrimidiones 43 were produced in high yields (Scheme 17). 11. HANTZSCH CONDENSATION Described more than one century ago by A. R. Hantzsch [87], 1,4-dihydropyridines (1,4-DHP) were synthesized by a multicomponent reaction between aldehydes, -ketoesters, andnitrogen donor molecules. 1,4-DHP derivatives possess a variety of biological activities such as vasodilator, bronchodilator, antiatherosclerotic, anti-tumor, geroprotective, hepatoprotective and anti-diabetic activity [88]. In addition, recent studies have suggested that 1,4-DHP derivatives also provide an antioxidant protective effect that may contribute to their pharmacological activities [89]. Oxidation of 1,4-DHP to pyridines has also been extensively studied [90]. Many homogeneous and heterogeneous catalysts have been reported for the preparation of 1,4-DHP via the Hantzsch condensation [91]. This condensation was thoroughly investigated employing ZnO catalyst [92-94]. Moghaddam et al. [92] reported that ZnO exhibitshighcatalytic activity in the Hantzsch condensation in a one-pot,four-component reaction for the synthesis of 1,4DHPs 44 and 45 (Scheme 18). This condensation was performed using various aromatic, and heterocyclic aldehydes using 10 mol% of ZnO in a one-step process. Most of the reactions investigated with ZnO catalyst were completed in 1 h to produce the corresponding 1,4-DHP 44 in good to high yields. In another study, Kassaee et al. [93] employed ZnO nanoparticles as an efficient and heterogeneous catalyst for the synthesis of polyhydroquinoline derivatives under solvent free conditions at room temperature. They showedthat ZnO nanoparticles reduced the reaction time (15-40 min) with higher yields (83-98%) in compari-

son with the same reaction catalyzed by the commercially available bulk ZnO [92]. Katkar et al. [94] also investigated the synthesis of polyhydroquinolines employing ZnO-beta zeolite, as an inexpensive and mild catalyst for the one-pot, four-component condensation of aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate in ethanol at room temperature. The remarkable advantages offered by this method are a green catalyst, mild reaction conditions, simple workup procedures, much faster reaction times, and excellent yield of products. Furthermore, the catalyst could be reused several times without loss of catalytic activity. 12. MICHAEL ADDITION REACTIONS The Michael reaction or Michael addition [95] is a conjugate addition of a carbanion or another nucleophile to an,unsaturated carbonyl compound. It is one of the most useful reactions in organic synthesis. Furthermore, it is a useful method for the mild formation of C-C, C-N, C-S and C-P bonds. 12.1. Aza-Michael Addition The aza-Michael addition is an organic chemistry reaction for the synthesis of C-N heterocyclic containing amino carbonyl functional group. Because of the importance of -amino carbonyl compounds, this reaction has attracted continued attention in organic synthesis. Campelo et al. [96] carried out the aza-Michael addition of nitromethane to 3-buten-2-one in the absence of solvent, using potassium fluoride supported on ZnO as the catalyst. They found that KF/ZnO easily performed the aza-Michael addition and thus, ZnO is a better support for the basic reagent than other metal oxides such as Al2O3, SnO2, and sepiolite, AlPO4, AlPO4–Al2O3 and AlPO4–ZnO. Recently, Zare et al. [97, 98] reported ZnO/tetra-butyl ammonium bromide (TBAB) as a catalyst system for aza-Michael addition reactions. They showed that this catalyst system was suitable for aza-Michael addition of phthalimide46, saccharin (48) [97], pyrimidines 50, purine nucleobases 52 [98], and sulfonamides



O

O

R

O

O 35 RCHO

+

ZnO (10 mol%)

O

O

X

EtOH, NH4OAc 80oC, 1 h, 81-92%

25 X

N H 44

41 O RCHO

O

O

R

O

ZnO (10 mol%)

+

2

X

25

41

X

X

EtOH, NH4OAc 80oC, 1 h, 79-87%

N H

X= OEt

45 Scheme 18. Hantzsch condensation over ZnO.

O

O

CO2R

CO2R

ZnO (20 mol%)/TBAB NH

N

MW, 300 W, 4-10 min, 90-96%

, 100oC, 0.5-3 h, 89-92%

O

46 O

CO2R

O

S

47

O O

O CO2R

S

ZnO (20 mol%)/TBAB NH

N MW, 300 W, 2 min,46-49%

, 100oC, 12-15 h, 57-60%

O

48 X

X

CO2R

O

NH

CO2R

ZnO (20 mol%)/TBAB HN

15-30 min, 41-91%

O

50

N

O MW, 200 and 300 W

HN

49

O

O

51

X=H, Me, F, Br Y

Y CO2R

N

N

ZnO (20 mol%)/TBAB N H

N 52

N

N

MW, 300 and 400 W,

N

N

15-35 min, 23-89% 53

Y=NH2, NHCH2Ph, OH, Cl

CO2R CO2R

O Ar

S O

O

ZnO (20 mol%)/[bmim]Br NH2 54

MW, 300 W 5-12 min, 70-86%

Ph

S O

CO2R

NH 55

Scheme 19. Aza-Michael addition over ZnO.

pyrimidines 50, purine nucleobases 52 [98], and sulfonamides 54 [99] to ,-unsaturated esters (Scheme 19). Recently, Ma Gee et al. [99a] reported ZnO nanoparticles as a recyclable heterogeneous catalyst for the synthesis of -amino car-

bonyl compounds via one-pot, three-component reaction of ketones, aldehydes and amines in water.More recently, Siddiqui demonstrated the ZnO nanoparticle catalyzed one-pot solvent-free synthesis of novel pyridine derivatives [99b].



705

CO2Me S Me

C

1. Acetone, -10oC, 15 min.

+ NH2

2. ZnO, 90oC, 1 h CO2Me

56

NH

CO2Me CO2Me

S 58

57

Scheme 20. Sulfa-Michael addition over ZnO.

12.2. Sulfa-Michael Addition Organ sulfur compounds have found general application in bioorganic chemistry and are present in many pharmaceuticals. Many studies have been reported on the synthesis of such compounds. Among these, Arjmandfard et al. [100] reported the sulfaMichael addition of thioacetamide 56 into dimethyl acetylene dicarboxylate 57 in the presence of ZnO in solvent-free conditions to afforddimethyl-(Z)-2-(ethanimidoylsulfanyl)-2-butenedioate 58 (Scheme 20). They also mentioned that other features of this process are under investigation. 12.3. Phospha-Michael Addition Similar to the Michaelis-Arbuzov [101] and the MichaelisBecker [102] reaction, the phospha-Michael addition, i.e., the addition of a phosphorus nucleophile to an acceptor-substituted alkene or alkyne, certainly represents one of the most versatile and powerful tools for the formation of P-C bonds,because many different electrophiles and phosphorusnucleophiles can be used in combination with one another. This offers the possibility to access many diverse,functionalized products. This reaction was investigated employing nano ZnO catalysis. Hosseini-Sarvari et al. [103] reported that nano flake ZnO was an efficient catalyst for the phospha-Michael addition of phosphorus nucleophile 59 to , unsaturated malonates 27 under solvent-free conditions at 50°C (Scheme 21). In this study, the activity clearly indicates that the particle size in the nano range helps to expedite the reaction. This is in agreement with their study on the activity of nano ZnO in comparison with bulk ZnO.

+ HP(O)(OR)2 Ar

O

nanoflake-ZnO

CN

solvent free, 50oC

X 27

(RO)2P

(5 mol %)

59

CN

Ar

0.5-10 h, 90-98%

X 60

X=CN, CO2Et R=Et Scheme 21. Phospha-Michael addition over nano ZnO.

13. TRANSESTERIFICATION AND ESTERIFICATION Nowadays, biodiesels have become very attractive as a biofuels because of their environmental benefits, including the emission of less air pollutants per net energy than diesel. Today, most biodiesels are produced by the transesterification of triglycerides of refined/edible type oils using methanol and an alkaline catalyst (NaOH, NaOMe) [104]. Various solid, liquid, acid and base catalysts have been investigated in the transesterification of triglycerides 61 with methanol (Scheme 22) [105]. An acidic catalyst such as sulfuric acid slowly catalyzes the transesterification of triglycCH2OCOR'

3 MeOH

CHOH

CH2OCOR

CH2OH

Scheme 22. The transesterification of triglyceride with alcohol.

R'COOR + H2O

R'COOH

+ ROH

R'COOH + NaOH

R'COONa

+ H2O

Scheme 23. Saponification of fatty acid alkyl ester.

To avoid the problem of product separation and isolation, replacement of the homogeneous catalysts by a heterogeneous catalyst has been proposed, suggesting that the production of alkyl esters will be simplified when heterogeneous catalysts are utilized. Among many heterogeneous catalysts that have been used for transesterification of triglycerides, ZnO was shown to be an effective catalyst for the transformation. Stern [106] and Xie [107] et al. reported ZnO and ZnO/KF as heterogeneous catalysts for the production of alkyl esters from vegetable oils or animal oils with alcohols. These ZnO catalyst systemsprovided good yields, as can be seen in (Tables 1 and 2), respectively. In another study, Jitputti et al. [108] reported the production of biodiesel using heterogeneous catalysts (ZrO2, ZnO, SO42/SnO2, SO42/ZrO2, KNO3/KL zeolites, and KNO3/ZrO2)for the transesterification of crude palm kernel oil (PKO) and crude coconut oil (CCO). They found that ZnO and SO42/ZrO2 exhibited the highest activity for the transesterification of both PKO and CCO. In the cases of ZnO, yields up to 86.1% of methyl ester from PKO and 77.5% from CCO were isolated. Qing Mei et al. [109] carried out experiments to deacidify highacid rice bran crude oil by esterification of the raw material of biodiesel using ZnO that had strong catalytic activity. The optimum conditions for esterification with ZnO were the theoretical amount of glycerol dosage (1.044 g), 0.1% (mol/mol) ZnO dosage, a reaction temperature of 200°C, and a reaction time of 6 h. The acid value of the rice bran oil after esterification decreased from 38.14 to 5.17 mg/g under the optimum conditions. This low-acid rice bran oil is suitable as a raw material for biodiesel. Mirajkar et al. [110] also investigated the transesterification of an alkyl ester of mycophenolic acid (Me ester) with 2-(4morphonlinyl) ethanol for the preparation of mycophenolate mofetil 62 in the presence of metallic zinc or ZnO (Scheme 24). Mycophenolate is derived from the fungus Penicillium Stoloniferum. Myco-

CH2OH

CHOCOR''

61

eride. Alkaline metal hydroxides (e.g., KOH and NaOH) are preferred as the basic catalysts. However, in the alkaline metal hydroxide-catalyzed transesterification, even if a water-free vegetable oil and alcohol are used, a certain amount of water is produced from the reaction of the hydroxide with alcohol. The presence of water leads to the hydrolysis of the esters, and as a result, soap is formed (Scheme 23). The formation of soap reduces the biodiesel yield, and causes significant difficulty in product separation (ester and glycerol).

+ RCO2Me + R'CO2Me + R''CO2Me


Table 1.


Transesterification of Rapeseed oil over ZnO as Catalyst

Oil (g)

Table 2.

Methanol (g)

Catalyst(g)

FAME Yield(%) 2h

FAME Yield (%)6h

120

120

--

227

57

75.6

120

120

1.2

225-230

88

92.7

120

120

1.2 (reused)

225-230

91.1

94.3

180

60

1.8

170-175

59.4

--

both blood cholesterol levels and triglycerides and can be used in pharmaceuticals and cosmetics. The highest yields of phytosterol esters are obtained in the presence of zinc oxide.

Transesterification of Soybean oil (1 mmol) with MeOH (10 mmol) Over Various ZnO Catalysts at Reflux Condition after 9h

Catalyst

Conversion of Soybean Oil (%)

KF/ZnO

87

KOH/ZnO

82

K2CO3/ZnO

74

OH

N O

Temp (oC)

Recently, Lai et al. [112] reported that the nontoxic plasticizer tri-n-butyl citrate 66 was synthesized by esterification of citric acid 65and n-butanol as raw materials and nanosized ZnO as the catalyst (Scheme 26). They showed that the best molar ratio of 65 to nbutanol was 1:45, and the amount of ZnO was 1.5%. Vallribera et al. [113] reported the selective transesterification of -ketoesters 67 using ZnO as a catalyst (Scheme 27). They emphasized the reaction of methyl 3-(3,4-dimethoxyphenyl)-3oxopropanoate with a series of alcohols with different structures. They also mentioned that the reaction appears to be specific for the transesterification of -ketoesters. Other ester derivatives, such as ketoesters as well as normal esters, failed to undergo the reaction. This difference in reactivity of these compounds was offered as possible way to perform the transesterification in a chemoselective fashion.

O O

O

O O 62

Finally, the esterification reaction of oleic acid with pentaery thritol/oleic acid with iso-octyl alcohol, and oleic acid with tris(hydroxymethyl)propane in the presence of ZnO as a non-acidic catalyst was reported [114-116].

Scheme 24. Mycophenolate mofetil.

phenolate mofetil is metabolized in the liver to the active moiety mycophenolic acid. It inhibits inosine monophosphate dehydrogenase, the enzyme that controls the rate of synthesis of guanine monophosphate in the de novo pathway of purine synthesis used in the proliferation of B and T lymphocytes.

14. SYNTHESIS OF ORGANIC CARBONATES Dialkyl carbonates are important chemicals in industry. The simplest and most important dialkyl carbonates are dimethyl carbonate (DMC) and diethyl carbonate (DEC) [117]. These materials are mainly used in alkylation and alkoxycarbonylation reactions as safe substitutes for dimethyl sulfate or methyl halides. They are also

Furthermore, ZnO showed catalytic activity in the synthesis of phytosterol esters 64 from natural sterol 63 and methyl esters (Scheme 25) [111]. Phytosterol esters 64 are effective in reducing .

. .

RCOOMe

HO

ZnO 240oC, 7 h, 76%

+

ROOC

63

64

phytosterol

phytosterol ester Scheme 25. Synthesis of phytosterol esters over ZnO.

O

O HO

.HO O

O 65 Scheme 26. Synthesis of tri-n-butyl citrate.

HO

n-BuO

OH

ZnO

OH

n-BuOH 110-140oC, 2.5 h, 99%

On-Bu On-Bu

O O 66

MeOH



O

707

O CO2Me

CO2R ZnO (cat.)

MeO

+ ROH OMe

MeO

toluene, reflux 1-72 h, 62-99%

OMe 68 13 67 ROH =iso-butyl alcohol, propargylic alcohol, t-butanol, (-)menthol Scheme 27. Transesterification of -ketoesters using ZnO.

O

O

O + MeOH

H2N

NH2

+ MeOH MeO

-NH3

42a

MeO

NH2 -NH3

69

quick step

OMe 70

rate limiting step

Scheme 28. Synthesis of dimethylcarbonate.

Step 1

O O NH2 +

H2N

OH

HO

O

71

42a

O 72

Step 2 O O

O O

+ 2MeOH

OMe +

MeO

71

70

72

OH

HO

Scheme 29. Synthesis of dimethyl carbonate from urea and ethylene glycol.

used as ‘green’ solvents. In addition, they are believed to be ideal fuel additives for the future because of their higher oxygen content and ability to blend with octane. Thus, the effective synthesis of DMC becomes more and more important. The current reported routesforthe synthesis of dialkyl carbonates includes phosgenation of alcohols, oxidative carbonylation of alcohols, ester exchange, esterification of carbon dioxide with alcohol, and alcoholysis of urea. Interestingly, ZnO catalysis exhibits a high catalytic efficiently and a high yield of products for the synthesis of DMC, DEC, and cyclic carbonates [118-136]. Urea methanolysis for producing DMC (70) is an attractive method because both urea and methanol are cheap and readily available. In urea methanolysis, DMC (70) is usually produced via a two-step reaction (Scheme 28) [137]. Typical catalysts for urea alcoholysis reported in the literature include homogeneous and heterogeneous bases [138], acids [139] and organotin derivatives [140]. ZnO catalysis has also been reported to have a higher catalytic activity than the typical bases, acids and organotin derivatives in urea methanolysis reactions. Bhalchandra et al. [120] investigated two-step synthesis of DMC (70) by using various solid catalysts. The first step involved reaction of urea 42a with ethylene glycol 71 to form ethylene carbonate 72, and in the second step transesterification of 72 combines with methanol to give 70 and 71, respectively (Scheme 29). The authors found that ZnO is highly active and selective for the two steps. Similar to 71, other glycols such as 1,2- and 1,3-propanediols can also be transformed to the corresponding cyclic carbonates. Furthermore, the reaction of urea with various diols to obtain selectively five-member cyclic carbonates has been reported by Li et al. [128]. They found that the high selectivity of five-membered

O

O R OH

HO 73

+

H2N

NH2

ZnO

O

O

83-99%

42a 74

R

Scheme 30. Synthesis of five-membered cyclic carbonates over ZnO.

cyclic carbonates could be obtained when ZnO was the catalyst (Scheme 30). In another study, Huang et al. [129] synthesized propylene carbonate from propylene glycol and CO2 over ZnO and modified ZnO in the presence of CH3CN in 18.6-26% yield. CH3CN in such reactionsactsnot only as a solvent but also as a dehydrating agent. The test for the ability to recycle the catalyst indicated that the modified catalysts had the highest stability. Another study on the catalytic activity of ZnO for the synthesis of dialkyl carbonates was reported by Zhao et al. [132, 133]. They showedthat ZnO was an ideal heterogeneous catalyst for the synthesis of dialkyl carbonates in comparison with other metal oxides such as PbO, CaO, and Zn(OAc)2. In principle, the catalysts active for the reaction of urea and methanol should be also active for the reaction of methyl carbamate and methanol, since the latter reaction is the rate-limiting step for the former reaction. They also observed that the yields of DMC and DEC withZnO were34.6 and 32.5%, respectively. A mechanism was proposed based on FT-IR and XRDcharacterization as shown in (Scheme 31). First, ZnO reacts with HNCO, which can be easily obtained by thermal decomposition of urea 42 to give rise to Zn(NCO)2 76. Then, 76 coordinates



(NH2)2CO

[HNCO]

42a

75

2HNCO + ZnO

.

.

Zn(NCO)2 + H2O

75

76

Zn(NCO)2 + 2NH3

Zn(NCO)2(NH3)2

76

77

O H2N OCN

2 NH3

OEt

Zn

2 EtOH

NCO

NH2

EtO

O 79 NH3 2 NH2CO2Et

OCN

Zn

2 EtOCO2Et

NCO

80 78

77

NH3

Scheme 31. Possible reaction mechanism.

O O R 81 R=H, Me

KI/ZnO/K2CO3

O

O+

CO2, MeOH R

O

73

68.6%

71.9%

OH KI/ZnO/K2CO3

+

OH

R

MeOH

R= H, Me

O O

R

R

OH

+ R

70

74

O

81

O

28%

Side reaction

OH

O

82 0.14%

83 0.12%

Scheme 32. Syntheses of DMC.

with ammonia to form Zn(NH3)(NCO)2 77. Consequently, NH3 of 77 was substituted by ethylene carbonate 78 and to produce Zn(NH2CO2Et)2(NCO)279, and finally ethanol attacks this complex to yield DEC 80. Chang et al. [125] investigated the one-pot syntheses of DMC using supercritical CO2, ethylene oxide or propylene oxide, and methanol with different solid catalysts. The results indicate that KI supported on ZnO with K2CO3 was a very effective catalyst (Scheme 32). Very high conversions of epoxides 81 and high yields of DMC (70) and glycols 73 (ethylene glycol or propylene glycol) can be achieved, and the amount of byproducts 82, 83 wasvery small. Supercritical CO2 acts as both the reactant and the solvent. This process has advantagesover the two-step process, such as that it can eliminate the separation process after the first reaction step. Also, the reaction was conducted in homogeneous supercritical conditions where non-corrosive CO2 was the main component in the reaction mixture, which increases the lifespan of the catalysts. The catalyst was also reused up tofour times, and the yield and selectivity remained unchanged. The authors also showed that no leaching of the catalyst was observed. Ethyl methyl carbonate (EMC) can be used as a co-solvent in a non-aqueous electrolyte, which is able to improve the discharge

characteristics of the cells including the energy density, discharge capacity, etc. [141]. However, the price of EMC is relatively high. Several methods have been developed for the synthesis of EMC. The esterification of methyl chloroformate with ethanol in the presence of basic catalysts is an effective route. However, this route is not environmentally friendly because the toxicity of the reagent and because a stoichiometric strong base has to be used to neutralize the acid byproduct. Another route is the transesterification of DMC with ethanol. Jiang et al. [136] developed an effective and inexpensive catalyst for the transesterification of dimethyl carbonate and diethyl carbonate to give ethyl methyl carbonate (Scheme 33). They investigated the reaction by employing various metal oxide catalysts, namely, MgO, La2O3, ZnO and CeO2. A comparison of the activities of different catalysts revealed that the catalytic activity decreased in the order: MgO > ZnO > La2O3>CeO2, and it seemed that there was a relationship between the transesterification activity and the surface properties of the catalysts. They noted thatZnO also has moderately basic sites, and its activity is higher than that of La2O3 and CeO2. These results imply that the strength of the basic sites of the catalysts is an important property for this transesterification reaction. The lower activity of ZnO than that of MgO was ascribed to the low surface area of ZnO)31 m2/g) compared to MgO)98 m2/g). A possible mechanism for the transesterification



O

O

709

O ZnO

O

or

O

O

70

O

O 103 oC, 4 h 29.5 %

80

O 84

Scheme 33. Transesterification of DMC and DEC over ZnO.

O O

O

CH3O + COOCH3

70 Zn

O

O

O - C2H5O

+CH3O

. O

O

O 80

O

O

O

O

O -CH3O

+ C2H5O O

O

70

O

O 84

85

O O

O

O

O

O

84

85

Scheme 34. Possible mechanism for the synthesis of DMC and DEC over ZnO.

reaction between dimethyl carbonate (70) and diethyl carbonate (80) to produce EMC (84) using ZnO is shown in (Scheme 34). As shown in this mechanism, an acyl cation was produced on ZnO and then reacted with DEC (80) or DMC (70) to yield intermediate 85.

ZnO HCO2H RNHR'

70oC, solvent-free

14

15. N-SUBSTITUTION REACTIONS

R= aryl, alkyl R'=aryl, alkyl, H

15.1. N-Formylation of Amines

R

N

R'

85

Scheme 35. N-Formylation of amines over ZnO.

Formamides are a class of important intermediates in organic synthesis. They have been widely used in the synthesis of pharmaceutically important compounds [142]. Numerous methods have been reported for the formation of formamides [143]. However, several factors such as low yield, difficulties in workup procedure and use of expensive reagents limittheir applications. This transformation was thoroughly investigated employing ZnO as the catalyst. The reaction of amines with CO in the presence of ZnO first appeared in 1999 [144]. Ikoma [144] patented a process by which formamides were prepared by liquid phase reaction of amines with CO in the presence of KF, ZnO, and MeOH at 120oC. Recently, Hosseini-Sarvari et al. [145] investigated the reaction using ZnO as a useful and reusable catalyst for N-formylation of amines 14 with aqueous formic acid (85%) under solvent-free conditions. This reaction was performed using various aliphatic, aromatic, heterocyclic, primary, and secondary amines (Scheme 35).

Thakuria et al. [62] reported that synthesized ZnO nanoparticles, which were prepared via thermal decomposition method, provided an efficient heterogeneous eco-friendly catalyst for Nformylation of amines with formic acid as indicated in (Table 3). The authors did not discussthe effect of the amount of catalyst on the reaction yields. 15.2. N-Alkylation of Imidazoles Imidazole-4-carboxaldehydeand 4-cyanoimidazole 86 were Nbenzylated and N-methylated using benzyl chloride and MeI on ZnO under basic conditions without using any solvents (Scheme 36) [146]. The effect of bases and solids on the product distribution of 1,5- and 1,4-substituted compounds wasinvestigated. The combination of Et3N and ZnO favored the 1,5-product 87 as shown in (Scheme 36).

Comparison of N-formylation of Aniline (1 mmol) with Formic Acid (2.5 mmol) by using ZnO as Catalyst

Table 3.

Entry

a

10-180 min 60-99%

O

Isolated yield

Catalyst

Catalyst (mmol)

Time (min)

Yield (%)a

1

Commercial ZnO

0.50

10

99

2

ZnO (macroporous)

0.50

08

99

3

ZnO (macroporous)

0.25

120

55


N


N H

N

Bn-Cl

R

ZnO, NEt3,

R

N

25oC

87

R=-CHO R=-CN

R= -CHO, -CN

+

R

N

Bn 92

86

(R1=H, C1-20 aliphatic, allylic, aromatic hydrocarbon residue; R2=C2-6 hydrocarbon residue) or theprecursors of carboxylic acids 92/amine 93 salts in the presence of ZnO in the liquid phase. They weretreated EtCONHCH2CH2OH (prepared form EtCO2H and ethanolamine) with ZnO at 200-210°C under 150 mmHg for 3h to give the desired 2-ethyl-2-oxazoline in 94% yield. In another study, Ishikawa [156] patented the preparation of alkenyl-2-oxazolines using ZnO. In this study the reaction of N-(2hydroxyethyl)-hydroxypropionamide with NaOMe in sulfone in the presence of ZnO and phenothiazine at 220-225°C and 150mmHgprovided a75% yield of vinyl-2-oxazoline.

Bn

N

88

92:93 / 69:31 92:93 / 50:50

Scheme 36. N-Alkylationof imidazoles over ZnO.

Recently, Garcia-tellado et al. [157] investigated the synthesis of 4,4-disubstituted-2-oxazoline using ZnO in solvent-free microwave-assisted conditions. They showed that zinc oxide was found to be the most effective catalyst. It acts both as a solid support and as a soft Lewis acid catalyst. Other acidic solid supports such as montmorillonites KSF and K10, calcinated Al2O3, and SiO2 were unsuccessful. They proposed that ZnO seems to play a double role: it creates a polar environment for the microwave catalysis (polar solid support) and activates the carbonyl group for the condensation (Lewis acid catalyst) (Scheme 39). Dotani [158] also reported that ZnO was a good catalyst for the preparation of 2,4-oxazolidiones 97 by cyclocondensation of 2hydroxycarboxylic acid esters 96 with urea. This process gave the target products in high yield efficiently and easily (Scheme 40).

15.3. N-Arylation of Nucleobases N-Aryl nucleobases are used as antineoplastic, antiviral, anticanceragents, antimicrobial and antitumor agents [147]. Therefore, the synthesis of this class of compounds is important. The synthetic routes toward N-aryl nucleobases include N-arylation of nucleobases via nucleophilic aromatic substitution (SNAr) [148]. Zare et al. [149] investigated the N-arylation of nucleobases 50 and 52 using ZnO in ([bmim]Br) ionic liquid with microwave heating as well as umder hydrothermal conditions (Scheme 37). These authors compared hydrothermal and microwave conditions. Their study revealed that microwave heating exhibits dramatically improved results in comparison with hydrothermal heating. 16. SYNTHESIS OF HETEROCYCLIC COMPOUNDS 16.1. Synthesis of Oxazolines

R1

O

Oxazolines [150] have been used as protecting groups [151], and chiral auxiliaries [152] or ligands [153] in asymmetric synthesis. Various methods have been developed for the preparation of 2oxazolines 95 from carboxylic acids 92 (Scheme 38) [154].

-2H2O, 4-10 min 47-95%

93

92

F

HN NO2

HN

ZnO, [bmim]Br

O

N

+ O

MW, 380 W, 130oC, 7 min, 92%

N H

NO2

, 130oC, 80 min, 94 %

89 50

90 NH2 F

NH2 N

ZnO, [bmim]Br

+ N H

N

N

N

NO2

N

N

N

MW, 400 W, 6 min, 86%

NO2

, 130oC, 120 min, 83 %

89

52

91 Scheme 37. N-Arylation of nucleobases using ZnO.

R1

O OH +

R 92

R1

O R2

H2N 93

OH

R

N H

R1 R2

OH

R2

N R

O

94 95

Scheme 38. Preparation of 2-oxazolines.

R

O 95

Scheme 39. Synthesis of 4, 4-disubstituted-2-oxazoline using ZnO.

O O

N

OH

OH + H2N

R

In 1997, Morimoto et al. [155] carried out the synthesis of 2substituted-2-oxazolines by dehydration of R1CONHR2OH 94

R1 ZnO, Mw, neat

R2

R2



O

O OH

OEt

+ H2N

ZnO

NH2

O

process: first, the lactic acid 100 is converted into oligolactic acid 101 by a polycondensation reaction; second, 101 are thermally depolymerized to form the cyclic lactide 102 via an unzipping mechanism (Scheme 43). Over the last two decades, considerable efforts have been made to improve the production process of lactide to reduce its production cost.

NH

reflux, 90.2%

O 42a

96

O

97

Zinc oxide powder proved to be one of the most effective catalysts for the preparation of D,L-lactide [164-172].

Scheme 40. Preparation of 2, 4-oxazolidinones using ZnO.

Niu et al. [167] developed the ZnO catalyzed synthesis of D,Llactide by polymerization and depolymerization in low vacuum level condition. They showed that the optimal conditions were as follows: ZnO 1.2%, dehydration temperature 120-160oC, and depolymerization temperature 220-250oC. Under this optimal condition, D,L-lactide was obtained in 40.2% yield. Shen et al. [168] presented a continuous process for the recycling ZnO as the catalyst in the synthesis of lactide from lactic acid. They reported that the heat-precipitation solution was used to recycle ZnO and dispose of wastes. DTA/TG and XRD were used to analyze the thermal decomposition process and to characterize the precursor and catalyst. The results showed that the recovery of catalyst can reach as high as 98%, and the process was easily operated with low-energy consumption. Gu et al. [169] synthesized optically active lactide catalyzed by ZnO, and Yang et al. [170] improved the preparation of D,L-lactide from D,L-lactic acid by using microwave irradiation. They also compared the microwave-assisted with the conventionally heated polycondensations. The results indicated that the polycondensation of D,L-lactic acid is significantly improved under microwave irradiation.

16.2. Synthesis of Tetrazoles Extensive work on the synthesis of tetrazoleshasbeen carried out in the field of material sciences, pharmaceuticals, explosives, and photography [159]. The synthesis of 5-substituted 1H-tetrazoles from nitriles has received much attention recently, and new preparative methods have appeared in the literature [160]. Kantam and co-workers [161] efficiently synthesized tetrazoles 99 by the reaction of nitriles 98 with NaN3 using nanocrystalline ZnO as the catalyst (Scheme 40). Nano ZnO was recovered quantitatively by simple centrifugation and reused for three cycles with minimal loss of activity. They also chose a variety of structurally divergent benzonitriles possessing a wide range of functional groups to understand the scope of the nano ZnO promoted [2+3]-cycloaddition reaction to form 5-substituted 1H-tetrazoles 99. HN

N N

CN R

+

N

nano ZnO

NaN3

DMF, 120-130oC R

5-14 min, 69-82%

98

99

16.4. Synthesis of Thiones 3H-1,2-Dithiole-3-thiones 103 have long been known [172] and are at present under intensive study because of a broad spectrum of their biological activity (Scheme 44).

Scheme 41. Nano ZnO catalyzed synthesis of 5-substituted 1H-tetrazoles.

More recently, Myznikov [162] investigated the reaction of 2cyanopyridine with NaN3 in DMF with microwave irradiationusing easily accessible ZnO as the catalyst. They were successful, and 5(2-pyridyl)tetrazole was obtained in 65% yield. Thus,using microwave irradiation in the synthesis of 5-substituted tetrazoles permitted successful application of commercial ZnO instead of employing the more difficult to acquire nanocrystalline substance.

S

O

O

O

H

O

O

103 R, R'= H, alkyl, aryl, etc. Scheme 44. Thione Structure.

A wide variety of alkyl and aryl derivatives have been synthesized and a great number of these compounds displayed biological activity and industrial applications. For instance, Oliptarez 104 has been clinically tested in China as a chemotherapeutic agent against liver cancer,and it has beendemonstrated toinhibit HIV-1 (AIDS) [173] (Scheme 45). Rossi et al. [174, 175] reported that alkyl esters of malonic acid 106 exposed to a mixture of P2S5/S8 in boiling xylene, in the presence of catalytic amounts of 2-mercaptobenzothiazol (MBT) and ZnO, led to 103 as the product. Employing this method it was possible to synthesize compounds with primary alkyl groups as thesubstituents, but it fails with secondary and tertiary alkyl groups and aryl esters (Scheme 46).

H

H O

O

O

H

O

H

O

O

Scheme 42. (R, R)-Lactide (left), (S,S)-Lactide (middle), meso-lactide (right).

O HO

CO2H

polymerization

decomposition

O

n O

100 Scheme 43. Mechanism for the synthesis of lactide.

S

S

Lactide is the cyclic di-ester of lactic acid (Scheme 42). Lactide can be polymerized to polylactic acid (polylactide, PLA) using suitable catalysts, with either syndiotactic or a heterotactic stereocontrol, to give materials with many useful properties. PLA is one of the most important biodegradable and biocompatible polyesters that are derived from annually renewable biomass such as corn and sugar beets [163]. Lactide 100 is usually prepared by a two-step

O

R'

R

16.3. Synthesis of Lactides

H

711

101

O

O 102

O



Goswami [178a] and Kumar [178b] carried out the synthesis of coumarin derivatives 110 using ZnO from phenols 109 and acetoacetyl esters 41 (Scheme 48). Coumarins were obtained in moderate-to-good yields.

MeO N N

S

S

S

S

16.6. Synthesis of Cyclic Urea

S

S

Cyclic ureas have been reported to constitute an entirely new class of potent and perspective nonpeptidic inhibitors of HIV protease [179]. Cyclic ureas have recently attracted much attention due to their applications as intermediates for biologically active molecules, such as fine chemicals, pharmaceuticals, cosmetics, and pesticides. Varma et al. [180] reported the facile synthesis of var ious cyclic ureas 111 and urethanes 112 from diamines or amino alcoholsand urea by using microwave irradiation in the presence of ZnO (Scheme 49). The reaction mechanism proposed is shown in (Scheme 50). Based on the proposed mechanism, ZnO played a role in facilitating the liberation of ammonia efficiently (path A), rather than allowing the reaction of 113 with isocyanate (HN=C=O) 75 (path B). These roles can be explained by the fact that the acidic proton in the NH(C=O)NH2 113 is abstracted more readily by moderately basic ZnO than other catalysts or catalyst-free system, thus avoiding the generation of byproducts 115-117.

105

104 Oliptarez

Trithioanethol

Scheme 45. Two kinds of thiones that display biological activities.

CO2R' R CO2R' 106

R

R'S

P2S5 / S8 ZnO MBT/ Xylene 1.5 h, 9-35%

.

S

S

S 103

R=H, bezyl R'= Ethyl, Butyl, Octyl Scheme 46. Synthesis of thione over ZnO.

In another study Rossi et al. [176] reported a method for synthesis of 103 starting from dithiol esters 107. This method was better than those previously published because higher yields were obtained and some derivatives that were not synthesized using the previous conditions were not successfully synthesized. Dithiolesters 107 with different alkyl and aryl groups were formed from malonyl dichloride and the corresponding thiols in anhydrous ether. Compounds 107 were sulfurized to form 108 by using P2S5/S8 or Lawson’s reagent (LR) and MBT and ZnO in a catalytic amount (Scheme 47). 16.5. Synthesis of Coumarins Coumarins are structural units of several natural products and are featured widely in pharmacologically and biologically active compounds; many exhibit a high level of biological activity. Besides functionalized coumarins, polycyclic coumarins such as calanolides, isolated from Calophyllum genus, and others have shown potent anti-HIV (NNRTI) activity [177].

16.7. Synthesis of Flavanones The flavanones are mainly distributed in citrus fruits and have attracted considerable attention because they possess antioxidant effects, potent inhibition of cancer cell proliferation, and cytotoxic activity [181]. The most widely used methodto prepare flavanones involves the isomerization of substituted 2-hydroxy chalcones, which was obtained by an aldol condensation reaction between a 2hydroxyacetophenone and an aldehyde. These cyclizations have been carried out under numerous conditions. Murugesan et al. [182] synthesized flavanone from 2'-hydroxyacetophenone 118 and benzaldehyde 25 over ZnO (Scheme 51). The first step is the Claisen– Schmidt condensation of 25 and 118 yielding 2'-hydroxychalcone119. Compound 119 undergoes intermolecular cyclization in the second step to yield flavanone 120. They also compared the effect of ZnO alone and supported ZnO. ZnO alone gave only 25.5% of falvanone, and the results showed that ZnO-supported MgO (10 wt %) was the best catalyst.

P2S5/S8 MBT/dioxane/ZnO O

O

RS

S

30 min, 17-65% SR

RS

107

RS

S

S

SR

S

108

S

103

LR/P8 MBT/dioxane/ZnO 30 min, 47-86% Scheme 47. Synthesis of 3H-1,2-dithiole-3-thiones catalyzed by ZnO.

OH

O

ZnO (nano)(5 mol%)

O

O

pyridine dicarboxylic acid +

R 109

R =OH, Me, OMe, etc. R' = Me, Ph X=OEt Scheme 48. Synthesis of coumarines by ZnO.

X

R' 41

3-6 h, 73-93% relux

R 110

R'

O



713

O O H2N

NH2

+

NH2

H2N

ZnO, DMF, MW 10 min

HN

NH

-2 NH3, 95% 111 O

O H2N

NH2 +

NH2

HO

ZnO, DMF, MW 10 min HN

O

-2 NH3, 100% 112

Scheme 49. Synthesis of imidazolidine-2-one by ZnO.

O HN

NH

N

H2N

C

O

114

111 (A)

- NH3 O H2N

NH2

NH2

H2N

- NH3

H2N

NHCO

42a

H N

NH2 O

75

113

O

C

NH

(B)

75 O H N

OCN 116

NH2

- NH3

H2N

HN

NH2 O

O

O

H N

N H 115

O N

NH2

117 Scheme 50. Proposed mechanism.

CHO

OH

OH

118

O 25

ZnO, 140oC, 3 h, 25.5% ZnO/MgO, 140oC, 3 h, 41.37%

ph

step 2

step 1

COMe

O

ZnO

ZnO

+

ph

119

O 120

Scheme 51. Synthesis of flavanone by ZnO.

16.8. Synthesis of Quinolines Quinolines are well known for a wide range of medicinal properties, being used as antimalarial, antiasthmatic, antihypertensive, antibacterial and tyrosine kinase inhibiting agents [183]. There have been great efforts to develop new and efficient synthetic routes to quinoline derivatives. Among several routes, acid- or basecatalyzed condensation followed by a cyclodehydration between a 2-aminoarylketone and a carbonyl compound containing a reactive methylene group, namely the Friedlander reaction, is one of the

simplest and most straightforward methods [184]. Although this reaction has been known for more than a century, it is still the most useful method for the synthesis of quinoline derivatives. Most of the previously reported synthesis methods suffer from poor yields, long reaction times, harsh conditions, use of stoichiometric and/or relatively expensive reagents, difficulties in work-up, and use of toxic/polar solvents leading to complex isolation and recovery procedures. For these reasons, recently, Hosseini-Sarvari [185] investigated a useful method by using nano ZnO as a heterogeneous solid



R1

R1

R2 O

R2

nano-flake ZnO (10 mol%) +

no solvent, 100oC

R3

O

NH2

R3

N

2-10 h, 60-98%

121 Scheme 52. Synthesis of quinolines catalyzed by nano ZnO.

catalyst under solvent-free conditions for the preparation of a wide range of quinolines 121 (Scheme 52). 16.9. Synthesis of Benzimidazoles Alinezhad et al. [186] investigated the synthesis of benzimidazole derivatives 123 from benzenediamines 122 and formic acid in the presence of 2 mol% ZnO nanoparticles under solvent-free conditions (Scheme 53). This method has several advantages,including the ability to recycle the catalyst, mild reaction conditions, a simple work-up, and environmentally friendly reaction conditions. R

R

NH2 + HCO2H NH2

N

nano ZnO (2 mol%) no solvent, 70oC

N H

6-240 min, 92-98%

the cyano functional group, dehydration of aldoximes remains a convenient route [189]. In 1961 Popov and Shuikin [190] reported using ZnO as an effective catalyst for the conversion of iso-alkanols into nitriles at 300°C. In the presence of zinc oxide at this temperature, isobutyl alcohol 125 reacted with ammonia to form isobutyronitrile 126. Isopentyl alcohol 127 reaction to provide isovaleronitrile 128 under the same conditions (Scheme 55). Hosseini-Sarvari [191] employed the combination of ZnO with acetyl chloride as catalyst for the dehydration of aldoximes 129 into nitriles 98 (Scheme 56). No yields were obtained in the presence of zinc oxide or CH3COCl alone; however, a 95% yield of 4methoxybenzonitrile was obtained when 3 mmol of ZnO was combined with 3 mmol of CH3COCl. Also, ZnO was able to be recycledup to three times.

123

122

ZnO

R(Ar)CHO Scheme 53. Synthesis of Benzimidazoles over nano ZnO.

25

R(Ar)CH=NOH

NH2OH.HCl, 60oC

129

16.10. Synthesis of 2-Amino-4H-Chromenes 2-Amino-4H-chromenes have shown several different biological activities including antimicrobial, antiviral, cancer therapy, sex pheromone, mutagenicity and central nervous system activity [187]. Recently, several methods have been reported for the synthesis of such compounds. The addition of a catalytic amount of ZnO has been found to be active for the synthesis of 2-amino-4H-chromenes 124 in water. (Scheme 54) [188]. 17. SYNTHESIS OF NITRILES The cyano moiety is important not only because of its synthetic value as a precursor to other functional groups,but also because of its presence in a variety of natural products, pharmaceuticals and novel materials. Although many methods are known for access to

More recently, Pasha et al. [193] carried out the synthesis of nitriles from dehydration of aldehydes using ZnO under solvent-free microwave irradiation. To test the generality of reaction, they used various substituted aryl aldehydes having electron donating and electron withdrawing groups. In all cases the corresponding nitriles were obtained in excellent yields (90-98%).

CN

O Nano ZnO

Ar

H2O, 80oC 50-98% 0.5-10 h, 124

Scheme 54. Synthesis of 2-Amino-4H-Chromenes catalyzed by nano ZnO.

OH

+ NH3

125

-H2O 300oC, ZnO

NH2

- 2H2

127

-H2O 300oC, ZnO 71.4%

Scheme 55. ZnO catalyzed the conversion of isoalkanols into nitriles.

CN 126

76.6%

OH + NH 3

98

In another study, Zora et al. [192] reported a new, efficient, one-pot reaction for the synthesis of cyanoferrocene from ferrocenecarboxaldehyde through dehydration of in situ formed ferrocenecarboxaldoximes using a NH2OH/HCl/KI/ZnO/CH3CN system. This is a useful method for the preparation of cyanoferrocene because of its simplicity and high yield.

OH

CH2(CN)2

R(Ar)CN

Scheme 56. Synthesis of nitriles using ZnO.

NH2 ArCHO

ZnO 80oC 10-4- min 83-95%

- 2H2

CN

NH2 128



18. SYNTHESIS OF FATTY AMINES Fatty amines are nitrogen derivatives of fatty acids, olefins, or alcohols prepared from natural sources, fats and oils, or petrochemical raw materials. Commercially available fatty amines consist of either a mixture of carbon chains or a specific chain length from C8- C 22. In 2009, Cheng et al. [194] patented the synthesis of fatty primary amines from the reaction of fatty acids with ammonia in the presence of ZnO as catalyst at 300-340°C and 0.5-1.0 MPa for 2h. 19. SYNTHESIS OF -AMINOPHOSPHONATES The great importance of -aminophosphonates is based on their presence in a number of biologically active compounds, as well as in their application as substitutes for the corresponding -amino acids and versatile intermediates and catalysts in organic synthesis [195]. As a result, a variety of synthetic approaches have been developed for the synthesis of -aminophosphonates. However, because of their biological activities, the search for new catalysts leading to an efficient and practical method for the synthesis of these compounds is highly desired. Kassaee et al. [196] reported that zinc oxide nanoparticles were used as an effective catalyst in the solvent-free, three-component couplings of aldehydes 25, aromatic amines 14 and dialkyl phosphites 59 at room temperature to produce various-amino phosphonates 130 (Scheme 57). The major advantages with this protocol include short reaction times, mild reaction conditions, easy workup, and simplicity. More recently, Hosseini-Sarvari [197] utilized the nano ZnO catalyst for the synthesis of an -aminophosphonate bearing a ferrocene moiety. To expand the scope of this novel transformation, nano ZnO was used as a catalyst, and a range of new ferrocene aminophosphonates were synthesized. A proposed mechanism for the role of nano ZnO was also investigated. The author also compared the activities of several other nano and bulk metal oxides and catalysts with nano ZnO. This study showed that the yield of desired product in the presence of nano ZnO was comparably higher than other catalysts. 20. SYNTHESIS OF BIS(INDOLYL)METHANES Bis(indolyl)methanes 132 are the most active and highly recommended cruciferous substances for promoting beneficial estrogen metabolism and inducing apoptosis in human cancer cells. The electrophilic substitution reaction of indoles with aldehydes is one of the most simple and straightforward approaches for the synthesis of bis(indolyl)methanes. Hosseini-Sarvari [198] successfully synthesized bis(indolyl)methanes 132 by the reaction of indole 131 with various aldehydes 25 in the presence of ZnO catalyst in solvent-free conditions (Scheme 58). It was observed from this study that ZnO is an efficient catalyst for the synthesis of

bis(indolyl)methanes in terms of product yields, reaction temperature, and times. 21. SYNTHESIS OF -ACETAMIDO CARBONYL COMPOUNDS Multi-component reactions are becoming an increasingly important class of reactions because they allow the reaction of several simple starting materials in one flask to provide a complex product. Acetamido carbonyl compounds are useful as building blocks for a number of biologically and pharmaceutically important compounds. ZnO has been employed as the catalyst in the synthesis of acetamido ketones or esters 134 by a multi-component reaction of an aromatic aldehydes 25, an enolizable ketone or -ketoester 133, acetonitrile, and acetyl chloride (Scheme 59) [199, 200]. 22. SYNTHESIS OF SALICYLAMIDES Salicylamide is the common name for o-hydroxybenzamide. Salicylamide is a non-prescription drug with analgesic and antipyretic properties. Its medicinal uses are similar to those of aspirin. Salicylamide is used in combination with both aspirin and caffeine in over-the-counter pain remedies. The traditional method to produce salicylamide is mainly based on the amination reaction of salicylic acid or methyl salicylate with ammonia in the absence of catalyst, and the feedstock salicylic acid is prepared by amultistep process from phenol. However, such a synthetic route requires harsh reaction conditions. Moreover, a large amount of acid is generated, which is hazardous to the environment as well as the production system. Thus, the direct synthesis of salicylamide from phenol and urea has been explored as an environmentally friendlyalternative process. Peng et al. [201] synthesized salicylamide from phenol and urea over various solid catalysts. Among the catalysts tested, ZnO exhibited the best catalytic performance. During the reaction, salicylamide (135) was mainly produced with 4hydroxybenzamide (136) as a by-product, and then salicylamide was further converted to xanthone (137) (Scheme 60). 23. SYNTHESIS OFB-SELENO AMINES Narayanaperumal et al. [202] reported the synthesis of a series of -seleno amines 140 from N-protected -amino mesylates 138 mediated by Zn in ionic liquid catalyzed by ZnO nanoparticles, as shown in (Scheme 58). For this reaction, the standard condition reactions were: -amino mesylate (2.0 equiv), diphenyl diselenide (1.0 equiv) in the presence of 10 mol% of ZnO nanopowder and commercially available Zn dust (1.6 equiv) in ionic liquid (0.5 mL) for 2 h at room temperature (Scheme 61). 24. SYNTHESIS OF N-SULFONYLALDIMINES N-Sulfonylimines are becoming of increasing importance because they are one of the few types of electron-deficient imines that PO(OR)2

ZnO (20 mol%) neat, 25oC ArCHO

+ PhNH2

25

+

HP(O)(OR)2

Ar 14-17 h, 66-90%

59

14

NHPh 130

Scheme 57. ZnO nanoparticle catalyzed the synthesis of-amino phosphonates. Ar

O 10 mol% ZnO H

Ar

2 N H

25

131

Scheme 58. Synthesis of bis(indolyl)methanes by ZnO.

715

solvent free, 80oC 20-120 min 63-98%

HN

N H 132



Y

ZnO

Y

CHO

X

CH3COCl +

CH3CN

X

O

25

NH

O

2-6 h, 86-93%

133

134

O

X= H, Cl, NO2 Y = H, Cl, Me Scheme 59. Multi-component reaction by using ZnO.

O OH

NH2

O

O + PhOH

-NH3 H2N

NH2

NH2 +

HNCO

12h, 235oC

75

42a

OH 136 1.21%

135 34.04 % O

135

O

+ PhOH

-H2O

-NH3 OH

O 137

OH

Scheme 60. Proposed mechanism for the conversion of salicylamide to xanthone.

Zn

R . PG

X + R1SeSeR1

NH

139

138

nano ZnO BMIM-BF4 rt, 2h, 49-87%

O

R

PG

SeR1

Cl + H

R

NH

O

6

R' 143

ZnO R

R'

solvent-free, rt 10-180 min, 70-95%

H 144

140

X= OMs, OTs PG=Boc, Ts

Scheme 63. Addition of acid chlorides to terminal alkynes, catalyzed by ZnO.

Scheme 61. Synthesis of chiral -seleno amines catalyzed by ZnO nanopowder. O R

H

Cl

+ R'SO2NH2

25

ZnO 110oC

141

NSO2R'

2-8 h, 20-92%

R 142

are stable enough to be isolated but reactive enough to undergo addition reactions. Hosseini-Sarvari et al. [203] reported the ZnO mediated preparation of N-sulfonylimines 142 under solvent-free conditions by conventional heating (Scheme 62). The advantages of this method were as follows: i) there was no need of toxic and waste producing Lewis acids; ii) work-up was simple; iii) the reaction procedure does not require specialized equipment; iv) zinc oxide powder could be recycled; and v) the conditions are solvent-free. OF

R

81

R-NH2 14

ZnO (5 mol %) solvent free, 70oC

-CHLORO- , -UNSATURATED

A useful reaction for the synthesis of -chloro-,-unsaturated unsaturated ketones (as synthetic intermediates particularly for the synthesis of heterocyclic systems) involves the addition of acid chloride derivatives to terminal alkynes. However, the addition of acid chlorides to alkynes often proceeds with concomitant decarbonylation [204]. Recently, Hosseini-Sarvari et al. [205] successfully reported the addition of acid chlorides 6 to terminal alkynes

N H

1-36 h, 50-98%

R

146

H

Scheme 62. Preparations of N-sulfonylimines under solvent-free conditions by ZnO.

25. SYNTHESIS KETONES

OH O

Scheme 64. Synthesis of propargylic alcohols over ZnO.

143, catalyzed by ZnO, to afford (Z)-adducts 144 selectively without decarbonylation at room temperature under solvent-free conditions (Scheme 63). This protocol benefits from short reaction times, operational simplicity, neutral reaction conditions, the ability to reuse the catalyst, avoidance of solvents, reduced environmental and economic impacts, and chemo selectivity. No toxic reagent or byproducts were involved, and no laborious purifications were necessary. 26. SYNTHESIS OF PROPARGYLIC ALCOHOLS Hosseini-Sarvari and Mardaneh [206] developed an efficient, new catalytic alkynylation of aldehydes 25 using ZnO in solventand base-free conditions for the preparation of propargylic alcohol derivatives 145 (Scheme 64). This method was complementary to the previously reported methods, and applicable to various aromatic and aliphatic aldehydes and alkynes. The catalyst was commercially available and works well under solvent- and base-free conditions. 27. RING-OPENING OF EPOXIDES Amino alcohols are synthesized by acid catalyzed ringopening of epoxides. Hosseini-Sarvari [207] carried out the ring-



O

OCOR'

O

ZnO (10 mol%) + R'

R 81

Cl rt, 30 min 87-98%

6

Cl Cl

R

717

OCOR'

+ R

147 148

Scheme 65. Synthesis of mino alcohols catalyzed by ZnO.

ZnO, AcCl CH2Cl2, 80%

AcO

O

AcO

149

Cl OAc

150

Scheme 66. Ring opening of epoxides over ZnO.

opening of epoxides 81 such as cyclohexene oxide, phenoxy oxide, styrene oxide, and epichlorohydrin oxide with various aromatic amines toward the synthesis of amino alcohols 146 catalyzed by ZnO (Scheme 65). Moghaddam et al. [208] employed ZnO for the ring opening of epoxides 81 with acetyl/benzoyl chloride 6 and TMSCl. Encouraged by the results obtained with various epoxides, they turned their attention to -sitosterol, a natural product that has been isolated from Salvia Sahendica [209]. They found that ring opening of 5,6epoxysitosterol 149 with acetyl chloride was catalyzed by ZnO gave the corresponding chlorohydrin 150 in high yield (Scheme 66). 28. BECKMANN REARRANGEMENT The Beckmann rearrangement is a fundamental and useful reaction, long recognized as an extremely valuable and versatile method for the preparation of amides or lactams, and often employed even in industrial processes. The conventional Beckmann rearrangement usually requires the use of strong Brönsted or Lewis acids, i.e., concentrated sulfuric acid, phosphorus pentachloride in diethyl ether, or hydrogen chloride in acetic anhydride, causing large amounts of byproducts and serious corrosion problems. ZnO was shown to be an effective catalyst for the Beckmann rearrangement of various aldehydes and ketones in solvent-free conditions (Scheme 67) [210]. The authors also found that various types of aldehydes in the presence of ZnO were condensed cleanly, rapidly and selectively with hydroxylamine hydrochloride at 80 °C in 5-15 min to afford the corresponding Z-isomer of the oximes 129 (OH syn to aryl) in excellent yields. Onlya small amount of E-isomer (10-20%) was obtained (Scheme 68). O R

ZnO,

140-170oC,

Solvent-free,

R'

R'

H N

NH2OH.HCl

1-9 h, 60-95%

R O

151

R, R' = H, alkyl, aryl, heteroaryl Scheme 67. Beckmann rearrangement catalyzed by ZnO.

HO

O ZnO, (Ar)R

H

800C,

Solvent-free, NH2OH.HCl 5-15 min, 80-98%

Scheme 68. Synthesis of oximes catalyzed by ZnO.

N H

(Ar)R 129

29. DEACYLATION REACTION 1-Hydroxy sugars are important materials and valuable building blocks for the preparation of various glycosyl donors in the synthesis of oligosaccharides and glycol conjugates. The hydrolysis of acylglycosyl halides is a classical method for preparation of 1-OH peracylated glycopyranoses. Dong et al. [211] reported a new synthetic method for the preparation of 1-hydroxy sugars 153 via a regioselective deacylation of peracylated glycopyranoses 152 with a ZnO/ZnCl2catalytic system (Scheme 69). They showed that the ZnO/ZnCl2 system was used as the deacylating reagent. The reaction proceeded smoothly in THF-CH3OH (4:1) under reflux condition. In addition, ZnO is a well-investigated deacylating reagent for the synthesis of 4-oxo--tocopherol 155 from 6-O-acetylbromohydrin 154. ZnO was a very effective reagent as it caused simultaneous deacetylation, dehydroboration and tautomerization of the resulting enol intermediate. In this reaction benzofuran 156 was formed as the main byproduct (8%) by ring contraction according to an elimination-addition mechanism (Scheme 70) [212]. 30. OXIDATION REACTIONS Oxygen anions on metal oxide surfaces can act as Lewis as well as Brønsted bases. They may also oxidize adsorbed organic compounds. The most common examples of such reactions are nucleophilic oxidations of carbonyl compounds. Aldehydes are oxidized to the corresponding carboxylates on a number of oxide surfaces such as ZnO [213]. Long chain alcohols and aldehydes also form carboxylate intermediates on ZnO [213b-f]. Other related species such as esters exhibit similar chemistry (Scheme 71) [213b]. Recently, Gupta et al. [214] reported a rapid, economical, and environmentally friendly method for benzylic oxidations using inexpensive and non-toxic ZnO under microwave irradiation or oilbath heating. This method was a good alternative to well-known methods, because the oxidation proceeds expeditiously in high yields. Furthermore, ZnO can be re-used after simply washing with distilled water and diethyl ether (Scheme 72). Furthermore, a new synthetic method for the oxidation of sulfides 159 was reported by Sharma et al. [215] using ZnO/polyaniline (PANI) composite in the presence of H2O2 under solvent-free conditions (Scheme 73). The results reveal that PANI/ZnO composite has high activity and selectivity compared to the raw ZnO.



R3

R3 O

R 1O

ZnO/ZnCl2

OR1

THF/MeOH, Reflux

R1O

O

R1O

R2

3.5-48 h, 53-84%

OH R1O

152

R2 153

R1 = Ac, Bz R2 = OR1, NHOAc, R3 = Me, CH2OR1 Scheme 69. Deacylation of peracylated glycopyranoses with ZnO/ZnCl2 . OH HO OH

O

AcO

Finally, ZnO-DABCO was reported as a very simple and efficient catalyst for aerobic oxidation of benzoins 161 to benzyls 162. This method was successfully extended to enantioselective oxidation using a chiral zinc complex as a catalyst for the oxidative kinetic resolution (Scheme 74) [216].

Br

C16H33

Br ZnO O

155

74%

+

C16H33 HO

C15H31

154 O 8%

156

Scheme 70. ZnO as a deacetylating reagent.

H C H2C

O

O + ZnO

O

H

R C

O + ZnO

O

R

This review describes various organic reactions employing ZnO as a catalyst. During the past decades, numerous organic reactions have been developed using ZnO as a non-toxic metal oxide in response to the demand for more environmentally friendlyorganic transformations. This development promotes the use of ZnO because of its unique properties, as described in this review. ZnO appears to be attractive for conducting organic reactions under solvent-free conditions. Another important factor about employing ZnO is the ability to recycle the catalyst, especially when employing nano ZnO. When a new reaction is discovered, a devoted chemist can no longer ignore the possibility of catalyzing the reaction with ZnO. Thus, ZnO is a new, useful, and powerful catalyst for organic reactions. In addition, many efforts could be found in the literature to improve the activity and stability of the ZnO catalysts, including promotion of the catalyst with transition metals such as Pt, Pd, Fe, Mn, etc., and mixed metal oxides such as TiO2, ZrO2, CaO, CuO, etc., which were not mentioned in this review. ZnO and its promoted versions are much more promising for various organic reactions of practical significance and are expected to gain great interest in the coming years.

Zn H

31. CONCLUSION

O Zn

Scheme 71. Oxidation on ZnO surface.

CO2H MW, ZnO, DMF 3-15 min, 75-89% or ZnO, DMF 1-6 h, 65-82%

90oC,

R 157

CONFLICT OF INTEREST The author(s) confirm that this article content has no conflict of interest. R

ACKNOWLEDGEMENTS

158

The author thanks the Shiraz University Research Council for their financial support, and also Professor G. A. Molander and Steve Wisniewski are acknowledged for editing English of this paper.

Scheme 72. Oxidation of toluene over ZnO.

O R

S

1 mol% H2O2 R' + 0.18-1 mol% (ZnO/PANI)

R rt, solvent-free

159

160

20 min, 57-96%

R = aryl, alkyl R' = alkyl

S

R'

REFERENCES [1] [2]

Scheme 73. Selective oxidation of sulfide with ZnO.

OH Ar

Ar O 161

[3]

O

ZnO (5 mol%) ligand (10 mol%)

[4]

Ar

Ar K2CO3 (1 equiv.) PhMe, O2, 60oC 6-68 h, 83-97%

Scheme 74. Oxidation of benzoins to benzils.

O

[5]

162

[6] [7]

Morkoc, H.; Ozgur, H. Zinc Oxide: Fundamentals, Materials and Device Technology, Wiley-VCH, 2009. Porter, F. Zinc Handbook: Properties, Processing, and Use in Design, CRC Press, 1991. Mu, Q.; Feng, S.; Diao, G. Thermal conductivity of silicone rubber filled with ZnO. Polym. Composites 2007, 28(2), 125-130. Ellmer, K.; Rech, A. Transparent conductive zinc oxide: basics and applications in thin film solar cells, Berlin: Springer, 2008 (Springer Series in Materials Science) 104. Noort, R. van Introduction to Dental Materials, 2nd ed.; Elsevier Health Science, 2002. Nagarajan, P.; Rajagopalan, V. Enhanced bioactivity of ZnO nanoparticlesan antimicrobialstudy. Sci. Technol. Adv. Mater. 2008, 9, 1-7. Li, Q.; Chen, S.L.; Jiang, W.C. Durability of nano ZnO antibacterial cotton fabric to sweat. J. Appl. Poly. Sci. 2007, 103, 412-416.

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Revised: June 04, 2012

Accepted: June 27, 2012