Promoted Zirconia Solid Acid Catalysts for Organic Synthesis

118 Current Organic Chemistry, 2008, 12, 118-140

1385-2728/08 $55.00+.00 © 2008 Bentham Science Publishers Ltd.

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 so-lid acid catalysts for various acid-catalyzed organic synthesis and transformation reactions in the liquid phase. These pro-moted zirconia catalysts exhibit superacidity which mainly depends on the preparation conditions. In particular, the sulfa-ted zirconia catalyst exhibits very strong solid acidity and excellent catalytic activity not only for simple acylation, con-densation 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. Mol-ybdate 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 pre-sent clear advantages with respect to the former. These ad-vantages include along with their handling requirements, simplicity and versatility of process engineering, catalyst regeneration, decreasing reactor and plant corrosion prob-lems, and environmentally safe disposal.

In 1979, Arata and co-investigators [1,2] reported that zirconia, upon proper treatment with sulfuric acid or ammo-nium 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 sul-fated 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 consid-ered as the strongest halide-free solid superacid [5,6]. Recent investigations reveal that sulfate-free ZrO2-based solid su-peracids could be synthesized by incorporating molybdate or tungstate promoters under certain preparation conditions [5,7]. The typical H0 values reported for SO4

2 /ZrO2 (650°C), WOx/ZrO2 (800°C) and MoOx/ZrO2 (800°C) catalysts cal-cined at different temperatures are 16.1, 14.6 and 13.3, respectively, which reveal the superacidic character [3,8]. *Address correspondence to this author at the Inorganic and Physical Chem-istry Division, Indian Institute of Chemical Technology, Hyderabad – 500 007, India; Fax: +91 40 2716 0921; E-mail: [email protected], [email protected]

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 gen-eration of wasteful byproducts [10-12]. In that connection we were interested in investigating various industrially im-portant organic reactions aimed at replacing toxic and corro-sive reagents, noxious or expensive solvents and multistep processes, with single-step solvent-free ones by using envi-ronmentally benign solid acid catalysts. Interestingly, sul-fate, molybdate and tungstate promoted zirconia catalysts exhibit excellent activity for a wide range of organic synthe-sis 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 sul-fated 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 hydro-lyzed with aqueous ammonia or urea [13-15]. Other precur-

Not For Distribution

Promoted Zirconia Solid Acid Catalysts for Organic Synthesis Current Organic Chemistry, 2008, Vol. 12, No. 2 119

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 550-650°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 tempera-ture 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 550-650°C. The typical sulfur content of SZ catalysts calcined in this range is 0.8-3 wt.%. The calcination temperature signifi-cantly affects the catalytic performance of the resulting SZ samples.

2.2. One-Step Method

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 zir-conium 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 stir-ring, 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 prepa-ration of SZ catalyst may also be performed in one step by thermal decomposition of Zr(SO4)2 as shown in the follow-ing equation [22,23]:

ZrO2 + 2 SO3Zr(SO4)2

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

3. CATARACTERIZATION AND CATALYST STRUCTURE

The bulk and surface properties of promoted ZrO2 cata-lysts have been extensively examined by using several tech-niques such as X-ray powder diffraction, X-ray photoelec-tron spectroscopy, infrared spectroscopy, Raman spectros-copy, BET surface area, temperature programmed desorption

of ammonia and other methods [14,24,25]. All characteriza-tion 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 cata-lyst exhibits enhanced acid strength and higher specific sur-face 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 sur-face 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 concen-tration, activation temperature and surface area of the precur-sor 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:

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-

Scheme 1.

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

ZrOCl2.8H2O + NH4OH.1.4H2O Zr(OH)4 + 2NH4Cl + 9.8H2O

H2SO4 or(NH4)2SO4

SO42-/Zr(OH)4

Calcination

550-650°C SO4

2-/ZrO2

O

Zr

O

Zr

O

#Zr

O

Zr Zr

O

#Zr

O

O

O O

O

O O

S

OO

S

O O* *H H

* Brönsted acid site; # Lewis acid site


120 Current Organic Chemistry, 2008, Vol. 12, No. 2 Reddy et al.

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 characteri-zation results the surface structure of tungstated zirconia (WZ) has been proposed as shown in Fig. (2) [29]. It is gen-erally believed that tungsten oxide could exist on the zirco-nia surface in the form of polyoxotungstate clusters as pre-sented in the figure.

4. CATALYTIC ACTIVITY OF SULFATED ZIRCO-NIA (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], pen-tane [32] and other hydrocarbons. More details of these va-pour-phase applications could be found in various articles published in the literature [33-35]. These details are not cov-ered in this review. The application of SZ catalyst for vari-ous liquid-phase organic synthesis and transformation reac-tions as reported in the literature are reviewed in the follow-ing sections.

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 ben-zyldiphenyl oxide in excellent yields employing SZ catalyst (Scheme 2). They also investigated the effect of various pa-rameters at 90°C in a batch reactor to establish the kinetics

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 resis-tance 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-Hinshelwood-Hougen-Watson (LHHW) model was tested which revealed that the reaction is intrinsically kinetically controlled.

4.2. Protection and Deprotection of Aromatic Aldehydes

The use of protecting groups is very important in organic synthesis, being often the key for the success of many syn-thetic enterprises [37]. Negrón et al. [16] employed the SZ catalyst obtained by the sol-gel method for the chemo-selective 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 sol-vent-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 unaf-fected under the reaction conditions investigated. The depro-tection of acylals to the corresponding aromatic aldehydes was carried out using CH3CN 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 re-acting with different nucleophiles and their ability to un-dergo 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. -

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

Scheme. 2.

O Cl SZ+

O

+

O

W

O

OO

O O

Zr Zr

W

O

OO

W W

OO O

ZrO2 Support ZrO2 Support

Isolated mono-tungstate on zirconia support

Poly-tungstate growth on monolayer coverage



aminothiocyanates are the precursors of thiazoles or ben-zothiazoles having pesticidal properties [39] and -amino-sulfides 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 correspond-ing -aminothiocyanates in high yields (Scheme 7). Reac-tions 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

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 im-portant intermediates in the pharmaceutical, agricultural and

Scheme 3.

Scheme 4.

Table 1. Conversion of Aromatic Aldehydes

R1 Time (h) Yield (%)

H 6 97

o-CH3 5 99

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

Scheme 5.

Scheme 6.

Table 2. Conversion of Hetero-Aromatic Aldehydes

R1 Time (h) Yield (%)

H 5 85

CH3 8 98

NO2 8 95

Scheme 7.

SO

ZrO

OO

ClHH

SO

ZrO-

OO

Cl+

CH2

O

O

HH

CH

+

+

SO

ZrO-

OO

Cl

OSO

ZrOH

OO

Cl+

SO

ZrO

OO

ClHH

-HCl

CHO

R1

SZ ,Ac2O

0°CR1

OAc

OAc

O

O

CHOSZ , Ac2O

0°C, 9 h

O

O

OAC

OAc

Yield=75.3%

OCHOR1 OR1

OAc

OAc

SZ

Ac2O, 0 °C

N

R

Ts + KSCNSZ, CH3CN

r.t., 2h SCNR

NHTs

+NHTsR

SCN

major major when R= Aryl when R= Alkyl



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]. Wolf-son et al. [12] investigated the synthesis of aromatic gem-

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 self-solvent 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.

4.5. Esterification and Transesterification

Esters have a fruity odor and are prepared in large quanti-ties for various purposes such as artificial fruit essences, fla-vorings and components of perfumes. Sejidov et al. [44a] investigated the esterification of phthalic anhydride by using various solid acid catalysts including natural zeolite, syn-thetic 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 Ardiz-zone et al. [44b] by employing SZ catalysts. Excellent prod-uct 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

Table 3. Reaction of Aziridines with KSCN

R Yield (%)a

C6H5 91(5)

o-Me-C6H4 89(7)

p-Cl-C6H4 84(9)

p-OMe-C6H4 86(6)

p-CH3CO-C6H4 82(8)

n-C4H9 85(9)

n-C9H19 83(10)

a Yields in parenthesis are for the other isomer.

Table 4. Reaction of Aziridines with R1SH

R R1 Yield

(%)a

C6H5 p-Cl-C6H4 90(5)

p-Me-C6H4 C6H5 92(7)

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)

a Yields in parentheses are for the other isomer.

Scheme 8.

Scheme 9.

Scheme 10.

( ) nN Ts + KSCN

SZ, CH3CN

r.t., 2h ( ) n

NHTs

SCN

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

( ) nN Ts + R1SH

SZ, CH3CN

r.t., 2h ( ) n

NHTs

SR1

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

( 1) ( 2)

H

O

Cl

O

SZ

100°C, 1h

H

ClCl

2+ + (C6H5-CO)2O

major major when R= Aryl when R= Alkyl

N

R

Ts + R1SHSZ, CH3CN

r.t., 2h SR1R

NHTs

+NHTsR

SR1



(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 trans-esterification 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

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

4.6. Stereocontrolled Glycosidation

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 -mannopyrano-side 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.

Table 5. Esterification Using SZ Catalyst

Acid Alcohol Temperature

(°C)

Time

(Min.)

Conversion

(%)

Sebacic acid 2-Et-Hexanol 110–170 90 96.4

Trans-(2-hexenyl)succinic anhydride

2-Et-Hexanol 110–180 120 97.3

Adipic acid 2-Et-Hexanol 110–160 120 95.6

Caproic acid Diethyleneglycol 110–160 90 97.7

Caproic acid Pentaerythritol 110–190 150 96.5

Scheme 12.

O

O

O

+ 2 ROHSolid acid

- H2O

OR

O

OR

O

H3C O

OO

O

CH3

O

CH3

O

+

3 CH3OH

H3C O

OOH

O

CH3

O

+ H3C OCH3

OCatalyst

Diacetin

Catalyst CH3OH

H3C O

OOH

OH

+ H3C OCH3

OCatalyst

CH3OHHO

OH

OH

+H3C OCH3

O

Stepwise reactions

Overall reaction

H3C O

OO

O

CH3

O

CH3

O

+ HO

OH

OH

+H3C OCH3

O

3Catalyst

Triacetin

CH3OH



4.6.1. Glycosidation of 2-Deoxyglucopyranosyl Fluorides

The direct stereocontrolled glycosidation of 2-deoxy sug-ars, 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 alco-hols 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 vari-ous conditions. They observed that glycosidation of 1 em-ploying 5 wt.% SZ in CH3CN at 25°C for 1 h gives 2-deoxyglucopyranoside in high yield with high -stereo-selectivity. On the other hand, the corresponding 2-deoxy-glucopyranoside was selectively ( -stereo-selectivity) ob-tained 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 summa-rized 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

both in chemistry and biology. Since - and - manno-pyranosides 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 stereo-controlled 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 pres-ence of 5A molecular sieves (100 or 300 wt.%) in Et2O (also MeCN, CH2Cl2, PhMe) at 25°C gives stereoselectively man-nopyranoside in the glycosidation of the -mannopyranosyl sulfoxides (Scheme 14). Some of the results reported are summarized in Table 7.

4.6.3. Glycosidation of Manno- and 2-Deoxygluco-pyranosyl -Fluorides

The - and -mannopyranosides appear in many natu-rally occurring bioactive substances such as asparagine-linked glycoproteins and certain antibiotics [50]. Therefore, the stereocontrolled formation of - and -mannopyrano-sides 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-

Scheme 13.

Table 6. Glycosidation of 1 with Alcohols Using SZ Catalyst

SZ, MS 5A, Et2O SZ, CH3CN

/ Ratio Yield (%)

Alcohol

/ Ratio Yield (%)

19:81 98 2 88:12 98

15:85 96 3 84:16 92

20:80 97 4 83:17 92

19:81 99 5 82:18 97

27:73 85 6 82:18 90

33:67 56 7 80:20 80

O

OBn

ORBnOBnO

SZ, MS 5A

Et2O, 0°C

O

OBn

BnOBnO

F

+ ROHSZ, 25°C

CH3CN

O

OBn

BnOBnO

OR1

HOHO

HOn-C8H17-OH

O

OH

MeOMeO

OMeOMe

O

OOBn

OH N3

2 3 4 5 6 7



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

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 Ta-ble 8 and 9.

Scheme 14.

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

Entry Glycosyl Donor Alcohol SZ

(wt.%) Additive Solvent

Yield

(%) / Ratio

1 8 2 100 - MeCN 38 57/43

2 8 2 100 - Et2O 02 39/61

3 9 2 100 - Et2O 17 32/68

4 9 2 100 - MeCN 52 64/36

5 9 2 100 - CH2Cl2 04 75/25

6 9 2 100 - PhMe 04 74/26

7 9 2 300 5A MS (300) Et2O 99 19/81

8 8 3 300 5A MS (300) Et2O 99 21/79

9 8 4 300 5A MS (300) Et2O 95 20/80

10 8 5 300 5A MS (300) Et2O 93 23/77

11 8 7 300 5A MS (300) Et2O 70 38/62

12 8 10 300 5A MS (300) Et2O 85 26/74

Scheme 15.

O

BnOBnO

X

Y

BnOOBn

HO

O

BnOBnO

BnOOBn

O

HO HO

O

BnOBnO

OBnOMe

OH

O

OBn

N3

O

OH

+SZ

Solvent25°C, 3 h

8:X=S(O)Ph,Y=H9:X=H,Y=S(O)Ph

2

n-C8H17-OH

3 4 5 107

OBnO

BnO

BnOX

F

OBnO

BnO

BnOX

OBnO

BnO

BnO X

OR

OR

OHO

BnO

OBnOMe

HO

O

OBn

N3

O

OH

O

BnOBnO

OBnOMe

OH

HOHO

+

SZ,(100wt%)

SZ,(5wt%)

1: X= H11:X = OBn

n-C8H17-OH

2

35

8

4

107

Et2O,25°CMS 5A(100wt%)

R-OHCH3CN,40°C

BnO



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

Dihydropyrimidinones (DHPMs) are an important class of compounds due to their diverse therapeutic and pharma-cological applications [52]. The general method for the syn-thesis of DHPMs known as Biginelli reaction involves three-component condensation reaction between aldehyde, -ketoester and urea under strong acidic conditions. Reddy et al. [13] reported a facile method for the synthesis of 3,4-dihydropyrimidone-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 effi-ciently and various dihydropyrimidinones were produced in excellent yields (Table 10) in short reaction times (40-60 min).

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

Benzodiazepines and their polycyclic derivatives are an important class of bioactive compounds. Many functional-ized benzodiazepines are widely employed as anti-con-vulsant, anti-anxiety, analgesic, sedative, anti-depressive and hypnotic agents [53]. Reddy et al. [14,54] reported the syn-thesis of various 1,5-benzodiazepine derivatives (Scheme 17) by the condensation reaction of o-phenylenediamine with various ketones employing SZ catalyst under solvent-free 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 cata-lyst is active for condensation reactions, the yields obtained

Table 8. Glycosidations of 1 with Various Alcohols Using SZ Catalyst

SZ, MS 5A,Et2O SZ, CH3CN

/ Ratio Yield (%)

Alcohol

/ Ratio Yield (%)

19:81 98 2 88:12 98

15:85 96 3 84:16 92

20:80 97 4 83:17 92

19:81 99 5 82:18 97

33:67 56 7 80:20 80

28:72 81 8 86:14 82

30:70 50 10 88:12 53

Table 9. Glycosidations of 11 with Various Alcohols Using SZ Catalyst

SZ, MS 5A,Et2O SZ, CH3CN

/ Ratio Yield (%)

Alcohol

/ Ratio Yield (%)

17:83 99 2 97:3 99

20:80 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

Scheme 16.

O H

Me

O O

OR2 H2N

X

NH2

SZ

100°C

R1

++

R1

NH

NHOR2

O

Me X( X= O,S)



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

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.

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

bis(indolyl)methanes and bis(indolyl)ethanes have been iso-lated 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 reac-tion 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).

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-

Scheme 17.

Table 11. Reaction of o-Phenylenediamine with Ketones

R1 R2 R3 Yield (%)

H CH3 CH3 94

H CH3 C2H5 91

H CH3 Ph 96

H C2H5 C2H5 84

CH3 CH3 CH3 94

CH3 CH3 C2H5 91

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

Scheme 18.

Table 12. Results of Synthesis of Various Diaryl Sulfoxides

Arene Me

Me

Me

Me

Me

MeO

MeO

Br Ph

Yield

(%)

90 92 85 83 88 80

R1

R1 NH2

NH2

R2

O

R3

N

HN

R2

R3

R2R1

R1

R3+

SZ

R + ClS

O

Cl

S

O

SZR RR+Not For Distribution


droxyl groups [60]. This is an interesting reaction due to high stability of the resulting THP ethers in a variety of reac-tion 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 reac-tion of various alcohols and phenols with 3,4-dihydro-2-H-pyran in the presence of catalytic amounts of SZ under sol-vent-free conditions at room temperature in short reaction times to offer various tetrahydropyranyl ethers in excellent yields (Scheme 22 and Table 14).

4.12. Alkylation of 4-Methoxyphenol with MTBE

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 di-alkylated products are used as anti-oxidants. These anti-oxidants are normally synthesized by using Friedel–Crafts alkylation. Yadav and Rahuman [64] investigated the alkyla-tion of 4-methoxyphenol with MTBE (Scheme 23) by em-ploying various solid acid catalysts. The order of catalytic

activity of various solid acid catalysts studied is as follows: Filtrol-24 > DTP/K-10 > Deloxane ASP resin > K-10 Mon-tmorillonite clay > SZ. Though the SZ catalyst exhibited the lowest activity as compared to other catalysts, it showed maximum selectivity to monoalkylated products. This reac-tion was carried out by taking 1:3 molar ratio of 4-methoxyphenol 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 com-pounds are obtained as camphene 12, tricyclene 13, -fenchene 14, bornylene 15, through a cyclic rearrangement, and on the other hand monocyclic compounds as -terpinene

Scheme 19.

Table 13. Reaction of Indole with Aromatic Aldehydes

Aldehyde

R1 R2

Yield (%)

H H 85

NO2 H 84

OMe H 78

OMe OMe 73

Scheme 20.

Scheme 21.

Scheme 22.

NH

NH

NH

R1

R2

CHOH

+SZ

R1

R2

NH

NH

NH

O2N

CHO H

+SZ

O2N

CH3

Me Me

Yield = 82%

NH N

HNH

H

+SZ

O CHO

O

Yield = 78%

R OH

O O OR

+SZ



16, limonene 17, -terpinene 18, terpinolene 19 and p-cymene 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 com-pounds. 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 fur-nace for 2 h at 250, 350 and 550°C and designated the cata-lysts 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. Fur-ther, 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 cam-phene 12 are 58.9, 67.4, 56.8, 59.3 and 12.0 %, respectively.

Table 14. Results of Tetrahydropyranylation of Alcohols and Phenols

Alcohol i-C4H9-OH

OH OH

OMe

OH OH

NO2OH

Yield (%)

94 96 82 90 92 94

Scheme 23.

Scheme 24.

H3C CH3CH3

OCH3 H+

CH3

CH2

H3C+ CH3OH

H+HO

CH3

OCH3

OH

BHA ( mono)

+

OCH3

OH

BHA ( di )

12 13 14 15

16 17 18 19 20



4.14. Nitration of Chlorobenzene

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 substi-tuted benzenes [66,67]. At about 68 w/w H2SO4, the nitration becomes extremely slow. Moreover, it is desirable to in-crease 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 pe-riod 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 ad-dition 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 for-mation 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

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 cyclo-hexene (Scheme 27) yields O- and C-alkylated products hav-ing commercial value. The O-alkylated product (cyclohexyl-2-methoxyphenyl ether) is a promising perfume. Alkylation of p-cresol with cyclohexene (Scheme 28) yields O- and C-alkylated products, 1-cyclohexyloxy-4-methylbenzene and 4-cyclohexyloxy-4-methylphenol respectively. Both products are also of commercial significance as perfume and insecti-cide respectively. Yadav et al. [69,70] investigated the alky-lation of p-cresol and guaiacol with cyclohexene by employ-ing several solid acids including the SZ. The alkylation of p-cresol 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% con-version and 82% O-alkylated product selectivity, which is the highest as compared to other catalysts studied. The alky-lation 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

Scheme 25.

Scheme 26.

HNO3 + (MeCO)2O MeCO2NO2 + MeCO2H

Cl

+ MeCO2NO2

SZCl

NO2

o- and p-nitro product

-CH3CO2H -CH3CO2H

OS

OZr

OOCH3COONO2

OS

OZr

OO

H3COOCNO2-

Cl

Ortho Para

Cl

O2N

H +

Cl

H NO2

+

OS

OZr

OO

H3COOC

+

OS

OZr

OO

Cl

O2N

Cl

NO2

+

-



is again the highest as compared to other catalysts investi-gated.

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 abun-dant sesqui-terpene hydrocarbons naturally occurring in Pinus Longifolia, P. roxburghii SARG and P. sylvestris. Its economical utilization involves transformation into the iso-meric product, iso-longifolene, 22, (2,2,7,7-tetramethyltri-cycloundec-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 iso-merization of 21 to 22 employing nano-crystalline SZ cata-lyst obtained by the sol-gel technique. In order to obtain maximum conversion the reaction was carried out at differ-

ent temperatures in the range of 120 - 200°C. It was ob-served 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 reac-tion time. The typical mechanism of the acid-catalyzed iso-merization of longifolene to iso-longifolene is also shown in Scheme 29.

4.17. Synthesis of Tetrahydroindolones from 1,4-

Dicarbonyl Compounds

The general procedure to prepare pyrrole derivatives in-volves the reaction of an enolizable 1,4-dicarbonyl com-pound 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 condi-tions, 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-

Scheme 27.

Scheme 28.

Scheme 29.

Scheme 30.

SZ

OH

OCH3

+

OH

OCH3

O

OCH3

+80°C

+SZ

80°C

OH

CH3

OH

CH3

+

O

CH3

O

OO

NO2

+

NH2

R

SZ, Toulene

Reflux, 3.5h

ONO2

N

R

H+

+

++

+++

21

22

H+ +



rahydroindolones employing SZ catalyst (Scheme 30 and Table 15). They carried out the reaction under reflux condi-tions 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 im-portant reaction.

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

Scheme 31.

Scheme 32.

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

Reaction Product(s) Acylating

Agent Aromatics T(°C) T (h)

Yield (%) Isomer(s)

Anisole 100 1.5 95 4-CH3O–BP (96),

2-CH3O-BP (4)

Mesitylene 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)

Benzoic anhydride

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)

Benzoyl chloride

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)

Mesitylene 100 3 42 2,4,6-(CH3)3–AP (100)

2-Chloroanisole 130 20 36 3-Cl–,4-CH3O–AP (100)

Acetic anhydride

m-Xylene 100 3 7 2,4-(CH3)2–AP (98),

2,6-(CH3)2–AP (2)

BP = benzophenone, AP = acetophenone.

R1

R2

R3

R4

X

O

O

O

O

SZ, -HX

SZ, -CH3CO2H

R1

R2

R3

R4

R1

R2

R3

R4

O

+ isomers

+ isomers

O

OHHO H2NSZ

-H2O+

HO

HN

NH2

-H2O

HN

NH



4.18. Condensation of Hydroquinone with Aniline and

Substituted Anilines

The condensation reaction of hydroquinone with aniline and substituted anilines is normally carried out for the pro-duction of N,N’-diphenylenediamines which are used as an-tioxidants in the rubber manufacturing [76]. The condensa-tion of aniline and hydroquinone leading to the formation of p-hydroxydiphenylamine and N,N"-di-o-toluyl-p-phenylene-diamine was reported by Kumbhar and Yadav [77] employ-ing various catalyst systems (Scheme 31). This reaction pro-ceeds via a combination of a series of parallel reactions. Their study revealed that the SZ catalyst exhibits comparable activity with that of commonly used PTSA (p-toluene-sulfonic acid) and much higher than that of pure zirconia. In terms of reusability the superacidic SZ catalyst performs much better than PTSA. The reactivity of the catalyst in-creased in the order p-toluidine > aniline > o- toluidine for the condensation reactions with hydroquinone.

4.19. Acylation of Aromatics

Aromatic ketones are intermediates or end-products em-ployed in the production of pharmaceuticals, cosmetics, ag-rochemicals, dyes, and specialty chemicals. The SZ catalyst was also profitably employed for acylation of various aro-matics as outlined below.

4.19.1. Acylation of Substituted Benzenes

Deutsch et al. [78] made a comparative study on the cata-lytic activity of SZ with other solid acid catalysts for the reactivity of anisole, halogen substituted anisoles, and meth-ylaromatics in their benzoylation and acetylation. The SZ catalyst exhibited the best performance for benzoylation of anisole at 50°C among various commercially available solid acids. The reactivity of the aromatics decreased in the fol-lowing order: anisole > mesitylene > m-xylene with both benzoic anhydride and benzoyl chloride. Further, there was no effect on the order of reactivity of aromatics when the reaction temperature was raised from 100 to 136°C. The

Scheme 33.

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

Aromatics Acylating Agent Yield (%)

R3 = C6H5, X = Cl < 85

R3 = C6H5, X= OCOC6H5 < 85

R1 = H, R2 = CH3O

R3= CH3, X = OCOCH3 < 85

R3 = C6H5, X = Cl 82

R3 = C6H5, X= OCOC6H5 ~100

R1 = CH3O, R2 = H

R3 = CH3, X = OCOCH3 ~100

R3 = C6H5, X = Cl 79

R3 = C6H5, X = OCOC6H5 93

R1 =CH3, R2 = CH3

R3 = CH3, X = OCOCH3 7

R3 = C6H5, X = Cl 17

R3 = C6H5, X = OCOC6H5 13

R1 = H, R2 = CH3

R3 = CH3, X= OCOCH3 No Reaction

R3 = C6H5, X =Cl 12

R3 = C6H5, X = OCOC6H5 7

R 1= CH3, R2 = H

R3 = CH3, X = OCOCH3 No Reaction

R3 = C6H5, X = Cl < 10

R3 = C6H5, X = OCOC6H5 <10

R1 = H,

R2 = H

R3 = CH3, X = OCOCH3 No Reaction

R1

R2

+R3

O

X

R1

R3O

SZ

-HX+ isomers

R2



aromatics reacted faster with benzoic anhydride than with benzoyl chloride. The only exception was m-xylene, which was acylated faster at 136°C with benzoyl chloride (initial ketone yields 12 and 7%, respectively). Also acylation of anisole, mesitylene, and m-xylene with acetic anhydride (Scheme 32) were carried out analogously to the benzoyla-tion experiments. Some of their results are summarized in Table 16. They have also carried out acylation of anisole and chlorobenzene with number of carboxylic anhydrides and acid chlorides [78b].

4.19.2. Acylation of Naphthalene and Anthracene

Deutsch et al. [79] also reported the acylation of methoxynaphthalenes, methylnaphthalenes, naphthalene, and anthracene with benzoic anhydride, benzoyl chloride, and acetic anhydride to synthesize aromatic ketones again em-ploying SZ catalyst. Their study showed that SZ exhibits a high catalytic activity for the benzoylation of 1-methoxy-naphthalene and a reaction temperature of 70°C was suffi-cient for the acetylation and benzoylation of more reactive aromatics. The typical yields of the acylations with naphtha-lenes (Scheme 33) and anthracene (Scheme 34) are summa-rized in Table 17 and 18, respectively. The rate of product formation with SZ catalyst was highly dependent on the sol-vent employed, on the nature of aromatic compound used, and on the ratio of aromatic to acylating agent.

There are some more examples of synthesis and trans-formation reactions catalyzed by SZ in the literature exhibit-ing moderate to good catalytic activity which include cycli-zation of 1-phenyl-2-propen-1-ones into 1-indanones [80],

Table 18. Acylation of Anthracene on SZ Catalyst at 70°C

Aromatics Acylating Agent Yield (%)

R3 = C6H5, X = Cl 67

R3 = C6H5, X = OCOC6H5 61

Anthracene

R3 = CH3, X = OCOCH3 12

Scheme 34.

Table 19. Acetylation of Alcohols and Phenols Catalyzed by WZ

Entry Alcohol Acetate Time (h) / Yield (%)

1 CH3OAc CH3OAc 4 / 95

2 Ph OH Ph OAc

2 / 97

3

CH2OH

NO2

CH2OAc

NO2

3 / 92

4

OH

CH3

OAc

CH3

3 / 90

5

OH

OH

OAc

OAc

3 / 92

6

HC CH-CH2-OH

HC CH-CH2-OAc

3 / 89

+R3

O

X

SZ

-HX

O R3

9-acylanthracene

WZ+ Ac2O R-OAc + AcOHR-OH



cyclization of citronellal to isopulegol [81], rearrangement of allyl-2,4-di-tert-butylphenyl ether to 6-allyl-2,4-di-tert-butylphenol [82], acetylation of benzo crown ethers [83], synthesis of dypnone [84], synthesis of -amino- , -unsaturated ketones and esters [85], synthesis of conjugated nitroalkenes [86], and cyclodehydration of some 1,n-diols [87]. Interested readers are advised to see these original pub-lications for more details.

5. REACTIONS OF TUNGSTATED ZIRCONIA (WZ)

In line with SZ catalysts WZ is also an excellent and en-vironmentally benign solid acid catalyst. The WZ catalyst has been mostly employed for various-vapour phase reac-tions namely, tert-butylation of p-cresol [88], production of biodiesel fuel [89] and isomerization [8]. This catalyst also exhibits excellent activity for various liquid-phase organic reactions as complied in the following paragraphs. This cata-lyst is highly stable and does not undergo deactivation like SZ where sulfate loss during the course of reaction is nor-mally expected under severe and reducing conditions.

5.1. Acetylation of Alcohols, Phenols and Amines

Acylation is a fundamental process in organic synthesis. This reaction is frequently used for derivatization and char-acterization of alcohols, phenols and also for further trans-formations. The direct acylation of alcohols can be carried out with mineral acids, sulfonic acids, and nucleofiles as well as with bases. Reddy and Sreekanth [7c] carried out acetylation of alcohols and phenols using WZ catalyst and acetic anhydride as the acylating agent (Table 19). Along with O-acylation, amines also underwent N-acetylation to give N-acylated products in good yields (Table 20). Sak-thivel et al. [7d] and Bordoloi et al. [7e] employed WZ cata-

lyst for acetylation of anisole with acetic anhydride to pro-duce 2- and 4-methoxyacetophenone and veratrole with ace-tic anhydride to produce 3,4-dimethoxy acetophenone in high yields, respectively. The main advantage of WZ catalyst is easy operation and simplicity in the work-up, which in-volves mere filtration of the catalyst, and its reusability.

5.2. Alkylation of Phenol with Long-Chain Olefins

Alkylation of phenol and alcohols with olefins is an in-dustrially important process for the production of a variety of products. Alkylation of phenol with 1-dodecene produces dodecylphenol, a dense viscous light yellow liquid with the characteristic smell. Dodecylphenol is used as raw material for the manufacture of lubricant additives, anionic deter-gents, antioxidants, and in phenol resins, adhesives, paints and accelerator for curing epoxy resins. Sarish et al. [90] reported the alkylation of phenol with 1-octene, 1-decene and 1-dodecene employing WZ catalyst. A series of catalysts with various WO3 loadings (5-30 wt.%) were prepared and calcined at 800°C. Among various catalysts investigated the 15 wt.% catalyst exhibited a maximum conversion of 67% at 70°C reaction temperature. The selectivity to phenyldodecyl ether was high at low reaction temperatures (51% at 70°C) and decreased with an increase in temperature and totally disappeared at 120°C. Data for the alkylation of phenol with various olefins are summarized in Table 21. Bordoloi et al. also reported alkylation of toluene with 1-dodecene using WZ catalyst [7e].

5.3. Esterification of Palmitic Acid with Methanol

Biodiesel is produced by transesterification of natural oil or fat of vegetable or animal origin. However, there are large

Table 20. Acetylation of Amines Catalyzed by WZ

Entry Amine Acetate Time (h) / Yield (%)

1

CH2NH2

CH2NHAc

1 / 95

2

NH2

NHAc

1 / 98

Table 21. Conversion and Selectivity of Alkylation of Phenol with Olefins Using WZ

Selectivity

Alkene Conversion

(wt.%) Monoalkyl

Phenols

Dialkyl

Phenols

Heavy

Products

1-Octene >99 79 21 0

1-Decene >99 93 0 7

1-Dodecene >99 90 0 10

Conditions: temperature = 120°C, total weight = 10 g, phenol/1-olefin molar ratio = 2; catalyst weight = 3 wt.%.

WZR-NH2 + Ac2O R-NHAc + AcOH



amounts of low-cost oils and fats such as non-traditional oils, restaurant waste oils and animal fats that could be converted into biodiesel. The problem with processing these low-cost oils and fats is that they often contain large amounts of free fatty acids (FFA) that cannot be converted into biodiesel using an alkaline catalyst. These free fatty acids need to be converted into their corresponding esters before transesterifi-cation. Thus, esterification assumes an essential step in the

production of biodiesel. Ramu et al. [91] reported the esteri-fication of palmitic acid with methanol using WZ catalysts. They investigated a series of WZ catalysts containing 2.5 to 25 wt.% WO3 and found that the catalyst with 5% loading shows maximum catalytic activity. This is a good example of the utility of WZ catalyst in the liquid-phase large-volume applications apart from its several vapour phase catalytic utilities.

Scheme 35.

Table 22. Synthesis of Coumarins Using WZ Catalyst

Entry Substrate Product Yield (%)

1

OH

OH

HO O O

CH3

80

2

OH

OHH3C

HO O O

CH3CH3

70

3

OH

OH

OH

HO O O

CH3

OH

65

4

OH

OHHO

HO O O

CH3OH

56

5

OH

OH

HO O O

CH3

CH3

67

6

OH

OHH3C

HO O O

CH3CH3

CH3

65

7

OH

OH

OH

HO O O

CH3

OH

CH3

60

8

OH

OHHO

HO O O

CH3OH

CH3

50

OH

OH

R

O O

OEt

X

O O

X

CH3

R

HO

WZ, Toulene

110°C, 6h+



5.4. Synthesis of Coumarins

Coumarins are structural units of several natural products and feature widely in pharmacologically and biologically active compounds and many exhibit high level of biological activity. Besides functionalized coumarins, polycyclic cou-marins such as calanolides, isolated from Calophyllum ge-nus, and others have shown potent anti-HIV (NNRTI) activ-ity. Reddy et al. [92] reported the synthesis of substituted coumarins from resorcinol and substituted resorcinol with ethyl acetoacetate and ethyl -methylacetoacetate employing WZ catalyst (Scheme 35). Some of these results are summa-rized in Table 22. For the entries 1-4, the reaction was car-ried out with ethyl acetoacetate; and for entries 5-8 ethyl -methylacetoacetate was used. Although several catalysts could be found in the literature including SZ [93], the eco-friendly WZ catalyst is worthy of pursuing further for large-volume product applications.

6. REACTIONS WITH MOLYBDATED ZIRCONIA

(MZ)

6.1. Synthesis of Diphenylureas

Diphenylureas have received tremendous interest re-cently because of their diverse applications, e.g., for use as tranquilizing and antidiabetic drugs, antioxidants in gasoline, corrosion inhibitors and herbicides. Reddy and Reddy [7a] reported the synthesis of substituted diphenylureas in the presence of MZ catalyst (Scheme 36). As can be noted from Table 23, various substituted diphenylureas were synthesized in good yields from substituted aniline and ethyl acetoacetate under reflux conditions at 180°C.

6.2. Esterification of Mono- and Dicarboxylic Acids

Many esters have a fruity odor and are prepared syntheti-cally in large quantities for commercial use as artificial fruit essences, flavorings and as components of perfumes. All

Scheme 36.

Table 23. Synthesis of Substituted Diphenylureas by Using MZ Catalyst

Entry Substrate Product Yield (%)

1

NH2

HN

HN

O

70

2

NH2

H3C

HN

HN

OH3C CH3

75

3

NH2

Cl

HN

HN

OCl Cl

70

4

NH2

HO

HN

HN

OHO OH

72

5

NH2

F

HN

HN

OF F

65

6

NH2

HN

HN

O

60

NH2

R

O

OMe

OHN

O

HN

R R

MZ

180°C, 6 h+



natural fats and oils (other than mineral oils) and most waxes are mixtures of esters. Manohar et al. [7b] reported esterifi-cation of mono- and dicarboxylic acids employing eco-friendly MZ catalyst under reflux condition for 1-4 hr. As summarized in Table 24, the MZ catalyst exhibited excellent product yields for various esterification reactions.

6.3.Transesterification of -Ketoesters

Transesterification is one of the established organic reac-tions that commands numerous laboratory uses and industrial applications. Ketoesters are important chemical intermedi-ates because of their easy transformation into chiral building blocks by chemical and enzymatic transformations. Reddy et

al. [94] carried out the esterification of -ketoester with various alcohols employing MZ catalyst. This reaction was carried out by using a variety of alcohols (aliphatic, unsatu-rated, aromatic, hetero-aromatic) at 110°C under reflux con-ditions with toluene as the solvent and excellent product yields were obtained (Table 25). After completion of the reaction, the reaction mixture was separated and the wet catalyst was used for recycling. No appreciable change in activity was found in several cycles.

CONCLUDING REMARKS

Despite the expected drawbacks of the SZ catalyst in terms of deactivation due to sulfate loss, crystalline-phase transformation from tetragonal to monoclinic, and coke for-

Table 24. Esterification of Mono- and Dicarboxylic Acids

Entry Carboxylic acid Alcohol Product Yield ( %)

1 CH3COOH Bu-OH CH3COOBu 95

2 CH3COOH

CH2OH

H3COOCH2C

85

3 PhCOOH Bu-OH PhCOOBu 57

4 CH3COOH CH2OH

CH2OH

CH2OCOCH3

CH2OCOCH3 63

5 CH3COOH PhCH2OH CH3COOCH2Ph 71

6

COOH

COOH

CH3OH COOCH3

COOCH3

95

7 COOH

COOH

CH3OH COOCH3

COOCH3 77

Table 25. Transesterification of -Ketoesters with MZ Catalyst

Entry Yield ( %) Yield ( %) Entry Yield ( %) Yield ( %)

1 CH3-(CH2)3-OH 98 7 CH2=CH-CH2-OH 98

2 CH3-CH(CH3)-OH 90

3 CH3-(CH2)4-OH 75 8 CH2-OH

86

4 CH3-(CH2)2-CH(CH3)-OH 59 9 O

OH

73

5 CH3-(CH2)7-OH 62

6 CH3-(CH2)5-CH(CH3)-OH 43 10 OH

97

O

OHR1

R2-OH

MZ

O

OR2R1; HOOC COOH

( ) n R-OH

MZ ROOC COOR( ) n

O

H3C OCH3

O

+ ROHMZ

Toulene, 110°C

O

RO CH3

O



mation, the SZ catalyst has received tremendous interest in recent times due to its simplicity, versatility, and superior performance for various organic synthesis and transforma-tion reactions as elaborated in this review. The so-called dis-advantages are encountered mainly in the processes such as cracking/isomerization and are pronounced in reactions such as hydroisomerization and hydrocracking in the presence of hydrogen. However, such problems in the liquid phase or-ganic processes are fewer, since most of the reactions are conducted at lower temperatures and in inert atmosphere, where the possibility for loss of sulfur, zirconia phase-transformation, and sintering are minimized. Hence the sul-fated zirconia catalysts have attracted core attention and con-tinue to generate high interest in organic synthesis for nu-merous applications. Furthermore, the less exploited molyb-date- and tungstate- promoted ZrO2 catalysts, which are free from these deactivation disadvantages, are much more prom-ising for various organic reactions of practical significance and are expected to gain great interest in the coming years.

ACKNOWLEDGEMENTS

We wish to specially acknowledge all the researchers whose work is described in this review for their contribution on promoted zirconia solid acid catalysts. MKP is the recipi-ent of junior research fellowship of CSIR, New Delhi. Fi-nancial support was received from Department of Science and Technology, New Delhi under SERC Scheme (SR/S1/ PC-31/2004).

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Promoted Zirconia Solid Acid Catalysts for Organic Synthesis