Organic Synthesis Using Clay and Clay-Supported Catalysts.Pdf

Applied Clay Science 53 (2011) 106–138

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Applied Clay Science

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Review Article Organic synthesis using clay and clay-supported catalysts

Gopalpur Nagendrappa ⁎,1

Department of Chemistry, Bangalore University, Bangalore 560 001, India article info abstract

Article history: Clays and modified clays are used to catalyze various types of organic reactions such as addition, Michael Received 20 May 2010 addition, carbene addition and insertion, hydrogenation, allylation, alkylation, acylation, pericyclic reactions, Received in revised form 17 October 2010 condensation reactions, aldol formation, imine synthesis, diazotization reactions, synthesis of heterocycles, Accepted 19 October 2010 esterification reactions, rearrangement/isomerization reactions, cyclization reactions, oxidation of alcohols, Available online 6 October 2010 dehydrogenation, epoxidation and several more. Clays function as Brønsted and/or Lewis acids, or as bases. Clays with combined acidic and basic properties have been developed by simple procedures of modification. Keywords: Clay mineral Such clays are employed to catalyze a sequence of acid and base-catalyzed reactions in one pot. Good Activated bentonite enantioselectivity and stereoselectivity are achieved using chiral organic compounds and chiral complexes Montmorillonite intercalated between clay layers. Examples from recent literature are described here. Saponite © 2010 Elsevier B.V. All rights reserved. Organic synthesis Heterogeneous catalyst

1. Introduction compatibility and cheapness, much effort is expended in discovering newer methods of using clays in their native and modified forms as Clays are widespread, easily available and low-cost chemical catalysts for diverse organic reactions. substances. Both in their native state and in numerous modified Clays have a long history of use as catalysts and as supports in organic forms, clays are versatile materials that catalyze a variety of chemical reactions (Vogels et al., 2005). Several excellent reviews on clay reactions. Just as they can be molded into any shape, their micro catalyzed organic reactions have appeared in the recent past (Varma, structure can be changed to suit chemists' needs to promote diverse 2002; Dasgupta and Török, 2008; Ranu and Chattopadhyay, 2009). Zhou chemical reactions. It is convincingly argued that clays initiated, (2010) has briefly summarized the emerging trends in synthetic clay- supported and sustained the process of formation of small molecules based materials. The present review attempts to report some of the on the earth millions of years ago, which gradually developed into developments that have taken place in the area of organic synthesis more complex molecules. In the course of time, there emerged from using clays and clay-supported catalysts during the past decade. the latter the self replicating assemblies that evolved into simple life Much of the work on clays focus on the use of “normal” smectites, forms and progressed to the present elaborate living world of plants mostly the commercially available K10 and KSF or native varieties with and animals (Saladino et al., 2004; Stern and Jedrzejas, 2008; Ciciriello Brønsted or Lewis acid sites and enhancing their catalytic performance et al., 2009). by pillaring techniques to manipulate the pore size, surface area and Clays are nanoparticles with layered structures. The layers possess stability or replace interlayer cations to alter acid-base properties (Singh net negative charge that is neutralized by cations such as Na+,K+,Ca2+, et al., 2007; Moronta et al., 2008). Clays have been intercalated with a etc., which occupy the interlamellar space. The amazing amenability of variety of inorganic and organic ions, metal complexes, and organic clays for modification lies in the fact that these interlamellar cations can compounds. These have brought about radical changes in the be very easily replaced by other cations or other molecules. Molecules performance of clays in terms of increasing the rates of reactions, yields, can be covalently anchored to layer atoms. All this can be done by very product selectivity, and stereoselectivity including enantioselectivity. simple procedures. This provides tremendous scope for altering the Clays have been modified to act as acid-base combination catalysts properties of clays like acidity, pore size, surface area, polarity and other which have been employed to carry out acid and base-catalyzed characteristics that govern their performance as catalysts. Because of reactions in a sequence in one pot (Motokura et al., 2005, 2009). The these wide ranging possibilities, in addition to their environmental possibilities seem to be limited only to the power of our imagination to modify clays for any reaction. The review describes seven types of organic reactions in as many ⁎ Permanent address: #13, Basappa Layout, Gavipuram Extension, Bangalore-560019, sections—Addition reactions, Condensation reactions, Diels–Alder and India. Tel.: +91 80 26670899. fi – E-mail address: [email protected]. related reactions, Esteri cation reactions, Friedel Crafts and related 1 Retired from the organization. reactions, Isomerization reactions, and Oxidation reactions. It should be

Scheme 1. Addition of allylsilanes to aromatic and aliphatic alkenes. noted that the literature covered is essentially from articles in interlamellar space of the catalyst and is helped by its Lewis acid mainstream journals published between 2000 and 2010, with a few character. This modiﬁed Hosomi–Sakurai reaction is environment exceptions; the patent literature is completely omitted. As a result the friendly and delivers protected homoallylic ethers due to six membered review is not exhaustive; some aspects and several types of reactions are pericyclic transition state of ene reaction (Scheme 3). left out for various reasons. The reaction with aliphatic aldehydes and ketones was successful in some cases, which are given below, with yields of the homoallylic 2. Addition reactions ether products mentioned in the parentheses.

In this section examples of addition reactions leading to carbon– CHO O O carbon and carbon–heteroatom bonds are considered. In the past ten CHO years several groups of workers have reported a variety of addition CHO CHO reactions efficiently facilitated by montmorillonite clays. They include (70%) (90%) (47%) (61%) (16%) (25%) addition of allylsilanes to C=C and C=O bonds, carbene addition, epoxidation, Michael addition, etc. Some of them are considered here. Motokura et al. (2010) have found excellent catalytic performance by proton-exchanged montmorillonite in the addition of allylsilanes Allylation of ketones and aldehydes has been carried out using to aromatic and aliphatic alkenes (Scheme 1). potassium salts of allyl- and crotyltrifluoroborates using borontrifluoride The mechanism has been studied in detail, a summary of which is etherate or montmorillonite K10 catalyst (Nowrouzi et al., 2009) presented in Scheme 2. (Scheme 4). The authors find that K10 clay catalyzed reactions are robust, Activated montmorillonite K10 clay was found to catalyze the straightforward and easy to work up, and scalable, which means the K10 reaction of allyl trimethylsilane with aromatic aldehydes to give catalyzed reaction is far superior to the Lewis acid-catalyzed one. homoallylic silyl ethers (Dintzner et al., 2009). The authors suggest Other supports like alumina, silica gel and charcoal proved to be inferior. that the reaction proceeds through a cyclic transition state in the The yields are generally excellent. In each case one stereoisomer is

Scheme 2. Mechanism of addition of allylsilanes to aromatic and aliphatic alkenes.

Scheme 3. Reaction of allyl trimethylsilane with aromatic aldehydes. 108 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 4. Allylation of ketones and aldehydes using potassium salts of allyl- and crotyltriﬂuoroborates.

Scheme 5. Addition of aniline derivatives to cinnamaldehyde. predominantly more than the other. The K10 catalyzed reactions are dehydration and oxidation in the ﬁnal step to deliver quinolines in good better stereoregulated with the diastereomeric ratio being greater than in to excellent yields (De Paolis et al., 2009)(Scheme 5). The reaction is the case of BF3.OEt2 catalyzed reactions. carried out under solvent-free condition and with the assistance of Montmorillonite K10 clay catalyzes the addition of aniline deriva- microwave radiation. tives to cinnamaldehyde in a Michael fashion as the ﬁrst step of a A three-component reaction of enaminones, β-ketoesters/1,3- domino process involving cyclization in the second step followed by diketones and ammonium acetate takes place under the catalytic

Scheme 6. Reaction of enaminones, β-ketoesters and ammonium acetate to form pyridines. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 109

Scheme 10. Addition of carbenes to C=N double bonds to produce aziridines.

Scheme 7. Synthesis of ethers by the addition of alcohols to olefins. chiral bis(oxazaline)-copper (Box–Cu(II)) complexes supported on influence of montmorillonite K10 clay in refluxing isopropyl alcohol to laponite clay-like solid and Nafion-like solid (Scheme 9). The produce tri-substituted pyridines in good yields (Reddy et al., 2005c) suitability of the supported catalyst system is influenced by a variety (Scheme 6). The reaction starts with the initial Michael addition of of factors. Better stereochemical control was possible on the laponite-

NH3 (from NH4OAc) to enaminone, followed by two condensation supported catalyst, compared to other supported catalysts studied. reactions, and elimination of NMe2 to the final product. However, supported catalysts were inferior to the free chiral catalyst Synthesis of ethers by the addition of alcohols to olefins is an in terms of yields and stereochemical control, though they could be important reaction catalyzed by Brønsted acids. The problem, however, reused. in such reactions is the possibility of extensive isomerization of the The enantiomeric excess depended on the alkyl group of the double bond through the intermediate carbocation. Wang and Guin diazoester, and it was 83% in the case of menthyl ester, while in the (2002) have found that sulphuric acid-treated montmorillonite clay is a case of n-butyl ester it was 74%. The phenyl substituted (R=Ph) Box highly active catalyst for bringing about addition of methanol to 2,3- was a better catalyst than the t-butyl substituted one. dimethyl-1-butene, and that it is far more selective to addition than Borkin et al. (2010) have prepared cis-aziridines in high diaster- sulphated zirconia, Nafion, Amberlyst-15 or bentonite clay, which cause eoselectivity (N99%) and excellent yields (82–91%) by reacting Schiff isomerization (Scheme 7). The activity of the catalyst depended on the bases with ethyl diazoacetate in the presence of montmorillonite K10 sulphuric acid content in the clay. as catalyst at room temperature for 2 h (Scheme 10). The K10 was the Dintzner et al. (2006) have suggested an undergraduate green best catalyst among the several other acid catalysts, such as experiment on the synthesis of a natural insecticide, methylenediox- H4W12SiO40,Nafion-H, Amberlist-15, and Nafion-H on silicon, in yprecocene (MDP), with anti-juvenile hormone activity. It is a reaction achieving the highest diastereoselectivity. However, reactions con- of sesamol with 3-methyl-2-butenol catalyzed by basic montmorillonite ducted using Nafion-H gave better yields of products (mixtures of cis- + K10–K (prepared by washing K10 with K2CO3 solution), and assisted and trans-aziridines). The efficiency of the catalyst remained the same by microwave irradiation under solvent-free condition. The mechanism even after it was reused three times. involves initial addition of the deprotonated sesamol to enal, the Box–Cu complexes (and Cu-complexes of three other ligands) dehydration of the intermediate and finally an intramolecular hetero- immobilized in laponite clay are able to efficiently catalyze the Diels–Alder reaction (Scheme 8). insertion of carbene formed from methyl phenyldiazoacetate into C–H Addition of carbenes to carbon–carbon double bonds to produce bond of THF at the 2-position with high enantioselectivity (up to 88% cyclopropanes is a well known reaction. Fraile et al. (2004) have ee). The immobilization not only allows recovery and reuse of the studied the scope and limitations of the stereochemical course of the chiral catalyst, but also provides an improvement in selectivity over addition of alkoxycarbonyl carbene generated from the corresponding the results obtained in solution, probably due to a confinement effect diazoacetic esters. The reactions were carried out in the presence of of the bidimentional support (Fraile et al., 2007)(Scheme 11).

Scheme 8. Synthesis of methylenedioxyprecocene (MDP), a natural insecticide.

Scheme 9. Addition of carbenes to C=C double bonds to produce cyclopropanes. 110 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 11. Insertion of carbene into C–H bond.

An Iranian natural bentonite modified by N-alkylated 1,4- proposed mechanism is a bit elaborate; the interested reader may diazabicyclo[2,2,2]octane quaternary salt was found to act as a good refer to the original paper). catalyst in triphase reactions consisting of the solid catalyst and the Dithiocarbamic acid A adds to arylideneoxazalones B to form the aqueous and organic phases (Ghiaci et al., 2005). When a mixture of Michael adducts C which undergo cyclization to 1,3-thiazinan cyclooctene, the catalyst, chloroform and an aqueous solution of derivatives D. The reactions are carried out by microwave irradiation sodium hydroxide was refluxed, dichlorocarbene was generated, of the reactants adsorbed on montmorillonite K10, basic and neutral which added to the cyclooctene present in the reaction mixture, alumina, and silica gel. The best yields (76–91%) were obtained in the forming dichlorobicyclo[6.1.0]nonane as the product in almost shortest reaction times from the reactions performed on K10 clay. quantitative yield (Scheme 12). Reactions carried out on other solid catalysts were inferior. Micro- The triphase systems (water-petroleum ether-catalyst) with water wave irradiation without using the adsorbent K10 clay was found to soluble nucleophiles work well for nucleophilic substitution reactions be ineffective, demonstrating the significant role of clay for the also (Scheme 13). success of the reactions (Siddiqui et al., 2010)(Scheme 15). Triazenes have been synthesized (Scheme 14) by adding p- aminobenzene-1-sulfonyl azide or amide to a cold mixture of sodium 3. Condensation reactions nitrite and acid-treated clay (K10, bentonite, or kaolin), followed by a cyclic secondary amine, which adds to the diazo intermediate formed Carbon–carbon bond forming reactions are of primary importance in in the previous step (Dabbagh et al., 2007). The yields are moderate in organic synthesis. Among the numerous procedures developed for this all the cases and are similar to the yields obtained using HZSM-5 and purpose aldolization/aldol condensation occupies an important posi- sulfated zirconia. The mechanism does not involve formation of the tion. The aldol reaction is catalyzed by acids as well as bases. A base is the conventional diazonium salt intermediate. Instead it consists of initial preferred catalyst to obtain aldol. Because of the importance of these interaction of nitrite with protonated silicate of clay followed by a reactions attention is being paid to develop environment friendly series of nucleophilic addition and elimination processes. (The procedures using heterogeneous clay catalysts. Hydrotalcites (HT) as solid base catalysts have been successfully used in bringing about the aldol reactions. For example, Roelofs et al. (2000) have reported the self condensation of acetone (1) to give aldol 2 and cross condensation of acetone with citral (3) on modified hydrotalcite catalysts at 0 °C (Scheme 16). The catalyst shows high activity and a small amount (5%) of it is enough to effect the aldol formation. The cross aldol (4) is formed with very high selectivity. These reactions are 100% atom economic and are examples for good “green” procedures. Scheme 12. Addition of dichlorocarbene in triphase system. Acetaldehyde (5) condenses with heptanal (6) in the presence of hydrotalcite-type catalysts to give cross condensation product none- nal (7) in ethanol at 100 °C (Tichit et al., 2003)(Scheme 17). If the Brønsted base strength is increased due to a higher proportion of MgO or hydrated sites, the reaction takes a different course through the intermediate α-anion of heptanal (10) to give 8 and 9. In an interesting study an acid-layered clay was combined with a basic layered clay to bring about sequential acid and base-catalyzed reactions in one pot. Motokura et al. (2005) mixed Ti4+ intercalated Scheme 13. Nucleophilic substitution in triphase system. montmorillonite with surface tunable basic hydrotalcites. The acid-

Scheme 14. Triazenes from p-aminobenzene-1-sulfonyl azide. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 111

Scheme 15. 1,3-Thiazinans from dithiocarbamic acid and arylideneoxazalones.

the desired enantiomers by employing suitable chiral catalysts. Such catalysts immobilized in clays through intercalation have shown excellent results. Srivastava et al. (2009) report using montmorillonite clay in which the sodium ions have been exchanged with prolium (protonated proline) by treating the clay with prolium chloride in methanol. The immobilized prolium acts as a chiral molecular catalyst to induce chirality in the newly formed asymmetric carbon of the aldol product 15 (Scheme 19). They found that intercalated hydroxyproline behaves in a similar manner. They further report that pillaring the prolium embedded (Pro-Mont) clay with trimethylbutylammonium bistriflimide leads to significantly higher yields and enantiomeric excesses, and that the solvents too have influence on the yields and Scheme 16. Aldol reaction of acetone with itself and citral. enantiomeric excesses. Montmorillonite K10 has proved to be an effective catalyst in bringing base catalyst combination acts sequentially in multistep reactions about Mukaiyama crossed aldol condensation of silyl enol ethers with that require both acid and base catalysis. The authors report the various aldehydes (Loh and Li, 1999). If the clay is intercalated with a following multistep reactions, (i) deacetalization–aldol condensation, chiral catalyst an excess of one enantiomer of the aldol is obtained. For Michael addition–acetalization, (ii) esterification–aldol condensation– example, Fabra et al. (2008) describe the use of the chiral diphenylbis epoxidation, and (iii) esterification–aldol condensation–Michael addi- (oxazoline)-Cu2+ complex 16 immobilized on laponite clay to bring tion. Each one of these combined three-reaction sequences has been about the Mukaiyama aldol reaction in a stereo-controlled manner performed in one pot resulting in excellent yields. The aldol conden- (Scheme 20). They found that 2-(trimethylsilyloxy)furan (17)addsto sation reactions are initiated by deacetalization effected by Ti4+-mont, the α-ketoesters 18 to give the aldols 19a,b with the anti-isomers as followed by aldol condensation catalyzed by basic hydrotalcite. The major products (up to 90% ee; dr, 86:14). reaction sequences are depicted in Scheme 18. However, after surveying several such chiral Cu2+-complex-clay The next four reactions start with Michael addition catalyzed by catalyzed reactions, Fraile et al. (2009) offer a note of caution that the HT, followed by acetal formation catalyzed by Ti4+-montmorillonite. application of these catalysts is limited despite excellent results in In the last reaction sequence, the first reaction is acid-catalyzed some cases. esterification, the second is acid-catalyzed deacetalization, followed Knoevenagel condensation of malononitrile with carbonyl com- immediately by base-catalyzed aldol condensation. The olefinic pounds has been found to be activated by ultrasound and catalyzed by product 11 is then epoxidized to 12 by hydrogen peroxide under alkaline-doped saponites. The dopant ions were Li+ and Cs+. The Cs+ basic catalysis. Or the olefin 11 can be hydrogenated to 13, which doped clay was far superior to the Li+ doped one in providing higher undergoes Michael addition to acrylonitrile in the same pot to give 14. yields of products (Martin-Aranda et al., 2005)(Scheme 21). Thermal None of the intermediate products in any reaction depicted in reactions with the same basic saponites as catalysts but without Scheme 18 was isolated; each step was performed one after the other ultrasound were inferior. in one pot. Montmorillonite K10 catalyzes the condensation of primary mono- Aldolization creates a chiral centre in aldol product. This provides an (20)anddiamines(21)withβ-hydroxy-β-bis(trifluoromethyl) ketones opportunity to design enantio-selective synthetic procedures to deliver (22), obtained by aldol reaction of methyl ketones with hexafluoro

Scheme 17. Aldol condensation of heptanal. 112 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 18. Sequential multiple reactions using combined acid and basic layered clay.

acetone hexahydrate, to give mono- (23) and di-imines (24). The reaction is conducted in chloroform at 70 °C for 4–5days.Anumberof di-imines have been synthesized which find use in the preparation of fluorinated chelate complexes for vapour deposition in microelectronics (Marquet et al., 2008)(Scheme 22). Motokura et al. (2009) have succeeded in preparing catalysts with coexisting acidic and basic sites in the montmorillonite clay interlayer by treating the clay first with hydrochloric acid and then with

triethoxysilylalkyl amines. Silicon of –Si(OEt)3 covalently binds with – SiO– in clay by displacing an EtO group, and thus the alkyl amine is immobilized. The acid sites and basic sites act independently to bring about both the acid-catalyzed deacetalyzation and the base-catalyzed Knoevenagal condensation reactions in tandem in one pot (Scheme 23). A three-step reaction of addition of phenylhydrazines to dihydro- 4-H-pyranone derivatives, followed by ring opening and intramolec- Scheme 19. Enantio-selective synthesis of aldols. ular condensation to deliver pyrazines in good yields is promoted by G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 113

Scheme 20. Mukaiyama reaction of silyl enol ethers with aldehydes.

catalyst to give tripyrrane (27)inN90% yield (Okujima et al., 2004) (Scheme 26). The Biginelli reaction, which involves a one-pot condensation of β- ketoesters (30)/β-dicarbonyl compounds (31) with aldehydes (32) and ureas (33) to give dihydropyrimidones (34), is conventionally conducted in ethanol with strong protic acid or Lewis acid as catalyst. Salmon et al. (2001) have shown that the reaction takes place on commercially available bentonite clay TAFF as catalyst under infrared irradiation and in the absence of solvent (Scheme 27). Earlier Li and Bao (2003) had demonstrated that the three- Scheme 21. Knoevenagel condensation of malononitrile with carbonyl compounds. component Biginelli reaction can be performed efﬁciently using samarium trichloride supported on montmorillonite clay (clay:

SmCl3 =10:1) as catalyst with microwave irradiation under solvent- montmorillonite KSF clay in ethanol solvent. The stereochemistry of free condition (Scheme 28). The catalyst was found to be reusable. the substituents is retained in the products (Yadav et al., 2004b) Hydrotalcite-like materials have been used by Zhu et al. (2009a) as (Scheme 24). catalysts for the condensation of aromatic as well as aliphatic primary A microwave-assisted montmorillonite KSF catalyzed, solventless amines with aromatic and aliphatic aldehydes, and cyclohexanone to aldol condensation of aromatic aldehydes with aryl methyl ketones obtain Schiff bases (35). The reaction is carried out under solvent-free has been developed by Chtourou et al. (2010) (Scheme 25). The condition by stirring together the reactants and the catalyst at room reaction takes just about 1 h for completion. It is highly selective (87– temperature (Scheme 29). The catalyst is recyclable. The yields in 98% trans chalcones) and the yields are good to excellent (80–95%). most cases are excellent. Aldehydes reacted faster than ketones. The authors maintain that their method is far more efficient than Malonic acid undergoes Knoevenagel condensation with salicylic other known methods which use a number of acid-treated supports, aldehydes in the presence of montmorillonite KSF clay on refluxing in alkalies and other catalysts in various solvent media. water for 24 h. The condensation products initially formed cyclize to In a series of reactions leading to the synthesis of conjugation- give coumarin-3-carboxylic acids, which are isolated by filtering off expanded carba- and azuliporphyrins (28 and 29) an intermediate the catalyst, evaporation of the solvent and finally recrystallization. step of condensation of bicyclo[2.2.2]octadiene-fused pyrrole (25) The selectivity was 95% and the yields were N90% in most cases. with bicyclo[2.2.2]octadiene-fused 2-acetoxymethyl-4-t-butoxycar- However, if diethyl malonate was used in place of malonic acid the bonylpyrrole (26) was carried out using montmorillonite K10 as yields of the coumarin-3-carboxylate esters were only 30–48% (Bigi

Scheme 22. Condensation of primary mono- and diamines with ketones. 114 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 23. Reaction using coexisting acidic and basic sites in tandem.

commonly prepared by the condensation of 1,2-diketones with o- phenylene diamines. A variety of catalysts are used, particularly acidic or oxidizing reagents. In most cases an organic solvent is also used. Huang et al. (2008) have overcome the eco-disadvantages associated with using such catalysts and organic solvents by making use of montmorillonite K10

Scheme 24. Pyrazines from dihydropyranones and phenylhydrazines. as catalyst in water at room temperature. The reaction takes about 3 h and gives excellent yields (90–100%; in one case, it was 70%, because the

diamine has an electron withdrawing NO2 substituent) (Scheme 31). The catalyst can be reused without much loss in its activity. et al., 1999)(Scheme 30). Montmorillonite K10 clay was much less The authors propose a mechanism suggesting an initial proton- effective than the KSF as catalyst. Since the Knoevenagel reaction ation of the diketone by the Brønsted acidic K10 clay. The nucleophilic requires basic conditions and the subsequent cyclization needs diamine then adds to the protonated diketone intermediate followed protons for its initiation, the authors presume that the clay catalyst by dehydration to give the quinoxaline products. must be ditopic and hence should contain both acid and basic sites. Isobezofuran-1(3H)-one derivatives have been prepared by The other important feature of the catalyst was that it retained its microwave irradiation of a mixture of phthalaldehydic acid and activity even after ﬁve reaction cycles. ketones well dispersed in montmorillonite K10 catalyst under Quinoxalines, which ﬁnd applications in the area of medicines, dyes, solventless conditions (Landge et al., 2008)(Scheme 32). The best electron luminescent materials, organic semiconductors and others, are results were obtained when the irradiation is carried out for 10–

Scheme 25. Microwave-assisted solventless aldol condensation.

Scheme 26. Synthesis of conjugation-expanded carba- and azuliporphyrins. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 115

reported by Kulkarni and Török (2010). The three-component system consists of an aromatic aldehyde, aniline or its derivative and phenyl acetylene or its derivative. Microwave irradiation of the reaction mixture combined with K10 at 100 °C for about 10 min was found to be the optimum condition for obtaining the best yields (Scheme 33). The reaction is a multistep domino process that starts with the formation of a Schiff base in the ﬁrst step. In the next step, phenyl acetylene adds to the base, then the adduct cyclizes followed by deprotonation to give the observed quinoline product. Microwave irradiation of a mixture of tryptamines and aromatic aldehydes dispersed in combined 10% Pd/C/K10 catalyst delivered Scheme 27. Solventless method for Biginelli reaction under IR irradiation. β-carbolines in good to excellent yields. The optimum reaction temperature was 130 °C. The selectivity for β-carbolines was 100% when 10% Pd/C with K10 was used (Kulkarni et al., 2009a) (Scheme 34). If Pd/C was less than 10%, the intermediate tetrahydro- β-carbolines were isolated along with β-carbolines, and if only K10 was used as catalyst, only the former compounds were formed as products. These results indicate that the cyclization process is catalyzed by K10, while the dehydrogenation is brought about by

Pd/C. Use of other acid catalysts, such as Al2O3 and acetic acid, gave very low yields. Tryptaphan as substrate in place of triptamine or Pt/C insteadofPd/Cgavethesameβ-carboline products.

4. Diels–Alder and related reactions

Pericyclic reactions are one of the most important ways of constructing more complex organic molecules from the simpler ones. Among these the most celebrated are those that involve the 6-electron cyclic transition Scheme 28. Microwave-assisted Biginelli reaction without solvent. state, which include electrocyclic and cycloaddition reactions. Since they are thermally allowed processes, their energy demand can be easily met. Many of them, in fact, occur at room temperature and even below that in a number of cases. The Diels–Alder reaction is a cycloaddition process between a 4-electron and a 2-electron component which could be starting compounds or intermediates formed in a multistep reaction sequence. Many such reactions are assisted or initiated by acid catalysts. This has provided an opportunity to exploit clays, particularly montmorillonites, as Brønsted or Lewis acid catalysts for Diels–Alder reactions. In this section some recent examples of such catalyzed reactions are presented. A major problem in developing a suitable chiral organocatalyst entrapped in the montmorillonite interlayer is its instability due to leaching. This problem has been overcome by Mitsudome et al. (2008) by entrapping the cationic chiral organocatalyst (5S)-2,2,3-trimethyl- 5-phenylethyl-4-imidazolinone hydrochloride by replacing the interlayer cations in montmorillonite clay. The catalyst is very effective in carrying out asymmetric Diels–Alder reactions (Scheme 35). Several Scheme 29. Condensation of primary amines with aldehydes and ketones. other inorganic solids, such as silica, titania, zeolite, MCM-41, hydroxyapatite and γ-ZrP failed to function as support. The 30 min at 170 °C. The catalyst is reusable and its performance in the montmorillonite supported catalyst is stable and could be reused a fifth trial was as good as in the first one. few times. A microwave-assisted, montmorillonite K10 catalyzed three- Lopez et al. (2007) have made an interesting observation of component reaction for the preparation of quinolines has been montmorillonite K10 exerting considerable influence in terms of

Scheme 30. Coumarin-3-carboxylic acids from malonic acid and salicylic aldehydes. 116 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 31. Quinoxalines by condensing 1,2-diketones with o-phenylene diamines. stereochemical outcome and product yields on Diels–Alder reaction produced oligomeric and polymeric products rather than [4+2] addition carried out in ionic liquid, 1-hexyl-3-methylimidazolium (HMI) products. p-Chloro and p-nitrobenzaldehyde gave very poor yields of the tetrafluoroborate. The results with other supports like silica or corresponding dihydropyrans as compared to o-chloro- and o- alumina or without support in the same medium were not good. nitrobenzaldehyde. Microwave irradiation was also not as good as K10 clay. With K10 clay Chiba et al. (1999) have generated in situ the highly reactive o- as catalyst the yields were as high as 99% and endo:exo ratio was 93:7. quinomethanes from o-hydroxybenzyl alcohols at room temperature The reaction took just 30 min at room temperature (Scheme 36). The using wet monmorillonite K10 and lithium perchlorate in nitrometh- same medium after work up was reused four times without much loss ane. The presence of air is necessary. The quinomethanes formed add in yields. instantly to olefins to give benzodihydropyrans (Scheme 38). Several clay catalyzed hetero-Diels Alder reactions have been reported Methylene cyclopropanes function as dienophiles in their aza- in the last few years. Dintzner et al. (2007) found that the carbonyl group Diels–Alder addition reaction with ethyl (arylimino)acetates under of benzaldehyde and its derivatives adds to 2,3-dimethyl-1,3-butadiene the catalytic influence of montmorillonite K10 in dichloroethane at under the influence of montmorillonite K10 clay in carbon tetrachloride at room temperature to produce tetrahydroquinolines. The K10 clay 25 °C to form dihydropyrans. The clay was preheated to 250 °C which performs as good as triflic acid in catalyzing these reactions (Zhu et al., enabled the collapse of the interior structure of clay by extrusion of water 2009b)(Scheme 39). leading to a decrease in Brønsted acidity but an increase in Lewis acidity Two of the authors of this group, Shao and Shi (2003) had reported that is responsible for the catalytic activity of the clay, suggest the authors. similar aza-Diels–Alder reactions of methylene cyclopropanes with Based on the fact that benzaldehydes with ortho-substituents having lone Schiff bases. In this case they had employed monmorillonite KSF clay pair electrons give far better yields of dihydropyran products, the authors or scandium triflate as catalyst (Scheme 40). The Schiff bases were propose a clay metal ion coordinated transition state that favours the produced in situ by the reaction of aryl aldehydes and aryl amines reaction (Scheme 37). The less substituted dienes (e.g., isoprene) employed directly in the 3-phase reaction.

Scheme 32. Isobezofuranones from phthaldehydic acid and ketones without solvent. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 117

Scheme 33. Three-component reaction for the preparation of quinolines.

Aryl amines add to endocyclic enecarbonates resulting in the latter moderate to good yields varying from 42–81% depending on the undergoing ring opening to give aralkyl imines under the inﬂuence of substituents (Scheme 43). The hetero-Diels–Alder reaction proceeds montmorillonite KSF clay in THF at room temperature. The imines so by a multistep process in which the acetylene adds to the Schiff base formed are highly reactive and undergo rapid aza-Diels–Alder formed by the condensation of the amine and the aldehyde. Slightly reaction with a second molecule of the endocyclic enecarbonate better yields were obtained in oxygen atmosphere instead of open air. present to ﬁnally give hexahydro-1H-pyrrolo(3,2-c) quinoline deri- The research group also found that, among some twenty different vatives (Yadav et al., 2004a)(Scheme 41). solid supports, the HClO4 treated montmorillonite worked the best in The same group of workers had previously reported (Yadav et al., terms of yields, reaction time, work up procedure, etc. 2002) similar reaction of aryl amines with dihydrofuran and Cyclopentadiene and furan undergo Diels–Alder addition at room dihydropyran to produce furano- and pyrano-quinolines (Scheme 42). temperature with trans-2-methylene-1,3-dithiolane-1,3-dioxide in A similar mechanistic pathway is proposed for this reaction also. the presence of Fe3+-doped montmorillonite K10 combined with Guchhait et al. (2009) have reported a multicomponent Povarov 2,6-di-tert-butyl-4-methylphenol (butylated hydroxytoluene, BHT) to reaction involving aromatic amine, aromatic aldehyde and terminal give the product in a overall yield of 64% (Gültekin, 2004) acetylene. The three compounds were allowed to react in the (Scheme 44). presence of perchloric acid-treated montmorillonite clay at 70 °C in Aldimines generated in situ from aliphatic aldehydes and p- open air. Substituted quinolines were obtained as products in anisidine add to Danishefsky diene in the presence of montmorillonite

Scheme 34. Condensation of tryptamines and aldehydes to give β-carbolines. 118 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 35. Entrapped cationic chiral organocatalyst in asymmetric Diels–Alder reactions.

K10 in aqueous or aqueous acetonitrile medium to produce 2-alkyl- particularly as oils, fats and waxes. They are a major component of all 2,3-dihydro-4-pyridones in excellent yields (Akiyama et al., 2002) living organisms. They have numerous applications in industries and as (Scheme 45). foods and fuels, for which esters obtained from both natural as well as Montmorillonite K10, filtrol-24, bentonite and pyrophillite clays synthetic sources are employed. There are several ester forming were used as catalysts to bring about Diels–Alder addition of 1,4- reactions available, such as (i) reaction of alcohols or phenols with naphthoquinone and N-phenylmaleimide to 4,6-bis(4-methoxyphenyl)- carboxylic acids, carboxylic acid anhydrides or acyl halides, (ii) addition and 4-(4-methoxyphenyl)-6-methylpyran-2(H)-ones under a dry state of carboxylic acids to olefins, (iii) addition of alcohols to ketenes, adsorbed condition. Filtrol-24 performed the best under this condition. (iv) substitution of alkyl halides/tosylates with carboxylates, (v) Baeyer– Further, when montmorillonite K10 and bentonite were modified Villiger oxidation of ketones, etc. The most common and expedient by impregnating with AlCl3, ZnCl2 and FeCl3 using their aqueous or method is the acid-catalyzed reaction of a carboxylic acid with an nonaqueous solution (PhNO2 for AlCl3, MeCN for ZnCl2 and FeCl3), alcohol, called esterification, which is usually carried out in homoge- followed by washing with water and drying, the resulting modified K10 neous media. Though several Brønsted acids are used as catalysts for this, and bentonite catalysts performed as well as filtrol-24. Among the doped the most convenient and commonly used one is concentrated sulfuric catalysts FeCl3-containing ones worked the best (Kamath et al., 2000) acid. The disadvantages of using this acid, like corrosion problems, (Scheme 46). handling difficulties, waste disposal hassles, environmental hazards etc. are well known. This opens up a good opportunity for acid clays to 5. Esterification reactions replace the hazardous mineral acid in ester forming processes, and a substantial amount of research activity is going on in this area. Esters, which are normally understood to be alkyl or aryl carbox- Clays available commercially or occurring naturally, including ylates, are an important class of naturally occurring compounds, those found in the geographical region of the researchers, have been

Scheme 36. Diels–Alder reaction in ionic liquid. K10 is the best catalyst. Scheme 37. Hetero-Diels–Alder reaction of benzaldehyde with dimethylbutadiene. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 119

Scheme 38. o-Quinomethanes from o-hydroxybenzyl alcohols and their D–A addition.

found to work well with or without suitable modifications. The most 2 h reaction time). They also discovered that the supporting K10 clay is successful esterification catalysts have been the montmorillonites, essential for the reaction, since InCl3 alone was not effective as catalyst. normally those that possess high Brønsted acidity. Some selected The yield of benzyl benzoate was only 11% after 1 h of reaction at 50 °C examples from recent literature are described here. between benzyl alcohol and benzoyl chloride with InCl3 as catalyst, Kaolin, montmorillonite K10 and KSF supported with transition while with equivalent amount of InCl3/K10 as catalyst the ester yield metal chlorides, InCl3, GaCl3, FeCl3, and ZnCl2 were employed to was 96%. The authors further found that the InCl3/K10 catalyst was esterify tert-butanol with acetic anhydride to tert-butyl acetate with almost as effective in its fifth time reuse as in the beginning. more than 98% selectivity. The K10 clay was found to be the best Srinivas and Das (2003) have demonstrated that ferric chloride 3+ support and K10 supported InCl3was the best catalyst followed by supported on montmorillonite K10 clay (Fe /K10) is an efficient GaCl3/K10, FeCl3/K10 and ZnCl2/K10, in that order. A noteworthy esterification catalyst. The catalyst is highly selective in esterifying feature was the low activity of the catalysts for the dehydration of tert- both saturated and unsaturated aliphatic carboxylic acids, while the butanol below 50 °C (Choudhary et al., 2001a)(Scheme 47). aromatic acids are unreactive, as demonstrated by using mixtures of

In a later study employing the InCl3/K10 catalyst Choudhary et al. aromatic and aliphatic acids. The yields of esters are high and the (2004) reported the preparation of thirteen esters by the reaction of catalyst is reusable. The catalyst is shown to be useful also in the benzyl alcohol, phenol, 4-nitrophenol, 1-naphthol and 2-naphthol with preparation of amides of aliphatic carboxylic acids with aliphatic as benzoyl chloride, acetyl chloride and n-butyryl chloride. The reaction well as aromatic amines, but ineffective in the preparation of the gave high yields of esters (up to 98%) under mild conditions (50 °C, 0.2– amides of aromatic acids with aromatic amines (Scheme 48).

Scheme 39. Aza-Diels–Alder reaction of arylimino esters with methylene cyclopropanes.

Scheme 40. Aza-Diels–Alder reaction of Schiff bases with methylene cyclopropanes. 120 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 41. Hexahydropyrroloquinolines from aryl amines and enecarbonates.

Scheme 42. Furano-/pyrano-quinolines from aryl amines and dihydrofuran/dihydropyran.

Some Brazilian natural clays (smectite, atapulgite and vermicu- Vijayakumar et al. (2004, 2005a,b) have shown that acid activated lite), without pretreatment or activation, have been demonstrated by Indian bentonite is a good catalyst, in some cases better than zeolites, Silva et al. (2004) to act as good catalysts for transesterification of for the preparation of aryl and alkyl esters of fourteen different ethyl acetoacetate and ethyl bezoylacetate by six carbohydrate- aromatic and aliphatic carboxylic acids. Particularly, the yields of aryl acetonides. Refluxing a mixture of the reactants and the catalyst in esters of long chain fatty acids are very impressive, which were more toluene for ~48 h produced the acetonide esters in good yields by than 90%, are very impressive. Results of a part of their work are replacing the ethyl group of the β-keto esters. The reaction mixture on depicted in Scheme 50. cooling to room temperature was stirred (~24 h) with bezylamine to Earlier Kantam et al. (2002) observed that Fe3+-impregnated produce β-benzyl enamino esters (Scheme 49). montmorillonite clay catalyzed the esterification of various aliphatic

Scheme 43. Multicomponent Povarov reaction—synthesis of quinolines. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 121

Scheme 44. Diels–Alder reaction of 2-methylene-1,3-dithiolane-1,3-dioxide. Scheme 47. Esterification of tert-butanol with acetic anhydride. acids, including the long chain fatty acids, aromatic acids, and α,β- unsaturated mono- and dicarboxylic acids with alcohols, under mild reaction conditions. The yields were obtained in the range of good to excellent. In three reports Reddy et al. (2004, 2005a,b) describe their study on the esterification of monocarboxylic and dicarboxylic acids with phenols and alcohols including ethylene glycol. They found that Al3+ exchanged montmorillonite GK-129 was the best catalyst compared with the same clay exchanged with divalent and other trivalent metal cations. It is better than the similarly treated montmorillonite K10 or Indian bentonite. Its performance is comparable to the results Scheme 48. Ferric chloride supported on K10 for esterification. obtained using zeolite H-β or p-toluene sulfonic acid catalyst. They discuss various experimental conditions such as temperature, reaction time, solvents, catalyst preparation, etc., that give the best Methyl mandelate, used in flavouring and perfumery, has been results, as well as the catalyst's performance in its reuse. Their results prepared by esterification of mandelic acid with methyl alcohool in are consolidated in Scheme 51. Reddy et al. (2007) have carried out the presence of montmorillonite K10 supported dodecatungstopho- esterification of succinic acid with iso-butyl alcohol to di-iso-butyl sphoric acid (DTP/K10) and its cesium salt (Cs-DTP/K10) (Yadav and succinate in connection with the evaluation of surface activity of Mn+- Bhagat, 2005)(Scheme 52). The Cs-DTP/K10 catalyst was found to be n+ 3+ 3+ 3+ 2+ montmorillonite clay catalysts, where M =Al ,Fe ,Cr ,Zn , better than K10 and other solid catalysts like S-ZrO2. The catalyst was Ni2+,Cu2+ and H+. The reactions performed on Al3+-mont and H+- shown to be recyclable. Using the same catalyst (Cs-DTP/K10 clay) mont gave the best yields (96% and 97%), Fe3+-mont and Cr3+-mont Yadav and George (2008) have carried out the esterification of gave good yields (74% and 51%), while the other Mn+-mont catalysts benzoic acid with phenol to phenyl benzoate which then undergoes gave poor yields (23–27%). The results were rationalized based on Fries rearrangement to give 2- and 4-hydroxybenzophenones. The various properties of catalysts due to exchanged ions. selectivity of the products depended on a number of experimental As noted in the section on condensation reactions, Motokura et al. parameters. (2005) have esterified cyanoacetic acid with methanol to methyl The esterification strategy was used by Mittal (2007) to increase cyanoacetate over the combined Ti4+-mont, HT catalyst (Section 3, the basal plane spacing in clays in order to achieve shear induced Scheme 18). exfoliation. This was accomplished by modifying the clay platelets with

Scheme 45. Alkyldihydropyridones from Danishefsky diene and aldimines formed in situ.

Scheme 46. Diels–Alder addition of 1,4-naphthoquinone to N-phenylmaleimide. 122 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 49. Transesteriﬁcation of esters by carbohydrate-acetonides.

quaternary ammonium ions carrying hydroxyalkyl groups, which were catalytic activity of different batches of commercially available K10 then reacted with long chain fatty acids (Scheme 53). The paper reports montmorillonite clays. They have found that acid treatment improves various aspects of catalyst modification and resulting properties. the activity of the clay and suggest that esterification is suitable for Wallis et al. (2007) have employed the esterification of maleic determining the degree of clay delamination. They suggest that loss of anhydride with p-cresol, along with a transacetalization reaction, layer stacking and increase in available exchange sites for protonation diacetylation of bezaldehyde with acetic anhydride and tetrahydro- are responsible for clay activity enhancement. They compare this with pyranylation of ethanol (Scheme 54), to assess and improve the an earlier observation by Reddy et al. (2005a) who had observed

Scheme 50. Esteriﬁcation using Indian bentonite. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 123

Scheme 51. Preparation of diesters using Al3+ exchanged montmorillonite GK-129.

Scheme 52. K10 supported dodecatungstophosphoric acid-Cs salt for esterification. dramatic improvement in the catalytic activity of K10 montmorillon- p-Hydroxybenzoic acid esters, with the sobriquet parabens, find ite clay with much reduced specific surface area on acid treatment. application in cosmetic, pharmaceutical and food industries. Hazarika Thermally activated Nigerian Ukpor kaolinite clay and Udi clay et al. (2007) have prepared the methyl, ethyl and n-propyl p-hydroxy were shown to be good catalysts for the preparation of n-propyl benzoates by refluxing the mixtures of the aromatic acid and acetate (Igbokwe et al., 2008). Various experimental parameters were appropriate alcohol on montmorillonite K10 clay for 10–15 h, and measured to obtain the best yield of the ester. obtained the esters in 81–90% yields (Scheme 56). The acid-treated Esterification of long chain fatty acids, stearic, oleic and palmitic clay gave slightly better yields (~2% more). acid, with short chain alcohols, methanol, ethanol, 1-propanol, 1- Acetic acid reacts efficiently with 2-methoxyethanol in the butanol and 2-butanol has been carried out using a series of presence of clay catalyst treated with sulfuric acid and aluminium montmorillonite based clay catalysts, KSF/0, KP10, K10 (Neji et al., salts and calcined at 313–633 K, to give methoxyethyl acetate. Both 2009)(Scheme 55). The product esters are meant to be used as Lewis acid and Brønsted acid sites are active in catalyzing the biodiesel. A variety of reaction conditions were investigated. KSF/0, esterification process (Wang and Li, 2000)(Scheme 57). which had the lowest pH value, was found to be the best catalyst. The The waxy stearyl stearate ester was synthesized by reacting stearic yields in the case of primary alcohols were almost quantitative, but in acid with stearyl alcohol on montmorillonite clay under solvent-free the case of 2-butanol the ester was obtained in only 40% yield. The condition. The temperature was strictly maintained at 170 °C catalyst was recycled twice without significant loss in its activity. throughout the bulk of the reaction mixture in the pilot scale reactor by using microwave irradiation. The pure ester is obtained in 95% yield on filtering off the solid catalyst. The use of microwave radiation reduces the reaction time by a factor of 20–30 times compared to the time required by the reaction done in conventional reactor (Esveld et al., 2000)(Scheme 58).

6. Friedel–Crafts and related reactions

The Friedel–Crafts reaction is an important carbon–carbon bond forming process. It enjoys numerous applications in the synthesis of Scheme 53. Strategy to increase the basal plane spacing in clays by esteriﬁcation. bulk chemicals, ﬁne chemicals, pharmaceutical and perfumery 124 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 54. Esterification for determining the degree of clay delamination. chemicals, and several others. It is an electrophilic substitution reaction indeed catalysts, they are not really used in catalytic quantities, but in of usually aromatic and heteroaromatic compounds by alkyl or acyl much larger amounts. This obviously poses serious problems of groups catalyzed by a number of Lewis and Brønsted acids under handling, recovery and disposal of the waste products. In this respect, homogeneous or heterogeneous conditions. Alkyl halides, alcohols, Friedel–Crafts reactions are an antithesis of Green chemistry principles. sulfonates and olefins are used as alkylating agents, and acyl halides or This has provided challenging opportunity to explore alternative acid anhydrides are generally used for acylation. The most commonly catalytic procedures. Thus reusable, environmentally benign solid acid used catalysts are AlCl3,ZnCl2, SnCl4,BF3,FeCl3,SbCl5,H2SO4,H3PO4,etc., catalysts including clays are being explored for these reactions. Clays mostly under homogeneous conditions. Though these Lewis acids are and modified clays have been used successfully as catalysts for bringing about Friedel–Crafts alkylation and acylation. Cyclopentyl and cyclohexyl derivatives of benzene, toluene, o- and p-xylene, mesitylene and anisole have been prepared in 85–95% yield by refluxing a solution of the respective aromatic substrate with cyclopentanol or cyclohexanol in 1,2-dichloroethane on Fe3+- montmorillonite with 10 mol% of TsOH or MsOH as cocatalyst (Chaudary et al., 2002)(Scheme 59). The suggested mechanism involves the formation of sulfonate ester of the alcohol as intermediate which alkylates the arene selectively. In the absence of the cocatalyst TsOH or MsOH, the reaction with cyclopentanol produces only a minor amount of the expected alkylated product (~30%), while the major product is dicyclopentyl ether (~60%). The reaction does not take place 3+– Scheme 55. Esters for using as biodiesel. in the absence of Fe montmorillonite. An interesting case of Fe–Mg–hydrotalcite anionic clay being used for the Friedel–Crafts alkylation has been described by Choudhary et al. (2005a) (Scheme 60). They have benzylated anisole, mesitylene, p-xylene, toluene and naphthalene with high degree of conversions, using benzyl choride. They observed that calcining increases the activity of the catalyst, and that the higher the calcining temperature the more active the catalyst is. They attribute this to dehydration at 200 °C, formation of metal oxides on calcining at 500 °C and higher temperatures up to 800 °C. They also found that the used catalyst is Scheme 56. Preparation of p-hydroxybenzoic acid esters. more active than the one used for the initial reaction. This observation is explained as due to a possible formation of Lewis acid sites resulting from the reaction of HCl liberated in the benzylation process. In–Mg hydrotalcite anionic clay is found to function in a similar manner (Choudhary et al., 2005b). Ga–Mg–hydrotalcite anionic clay was used earlier for benzylation and benzoylation of benzene Scheme 57. Preparation of methoxyethyl acetate. (Choudhary et al., 2001b).

Scheme 58. Microwave-assisted esteriﬁcation without solvent. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 125

Scheme 59. Mesilates and tosylates in F–C alkylation.

Scheme 60. Anionic clay for the Friedel–Crafts alkylation.

3+ Benzene has been alkylated with propylene to cumene on Al - the presence of montmorillonite K10 clay-POCl3 under microwave exchanged synthetic Zn-saponite with 99% conversion (Scheme 61), irradiation. The effect of this catalyst was comparable to the silica compared to just 0.3% conversion when the same reaction was gel-POCl3 catalyzed reaction (Devi, 2006). performed using commercial solid phosphoric acid (SPA), (Vogels A number of other clay-based catalytic systems have been et al., 2005). developed by several groups of researchers for bezylation reactions. To introduce an isopropyl group on xylenes, isopropyl alcohol is The reactions are conducted in liquid phase or solvent-free conditions used by Yadav and Kamble (2009). They have obtained dimethylcu- with or without microwave irradiation. Studies were directed to find menes by reacting xylenes with isopropyl alcohol in an autoclave in the right experimental conditions to obtain the best results by the presence of cesium substituted dodecatungstophosphoric acid preparing the most active catalyst. supported on K10 as catalyst (Scheme 62). The Cs-DTP/K10 was the Ga/AlClx-grafted montmorillonite-K10 was found to be an efficient most active among the five catalysts studied, including K10, DTP/K10, catalyst for benzylation as well as benzoylation of benzene, substituted sulfated zirconia and filtrol-24. Various aspects of catalytic activity benzenes and naphthalene, using benzyl chloride and benzoyl chloride and kinetics of reactions are considered. Selectivity for mono- respectively. The catalyst is highly active, and as such benzoylation isopropyl products is very high, with the byproducts, diisopropylated occurs even if a strong electron withdrawing group like NO2 is present xylenes, diisopropyl ether and propylene being formed in small on benzene ring (Choudhary and Jha, 2008)(Scheme 65). amounts. Clay catalyists obtained from Pakistani clay minerals were found Cs-DTP/K10 has been used fruitfully in several other Friedel–Crafts to be useful in benzylation of toluene, naphthalene, anthracene, type reactions. For example, 1,3-dibenzyloxybenzene has been quinoline, 8-hydroxyquinoline and pyridine (Ehsan et al., 2006). acetylated to 3,5-dibenzyloxyacetophenone with acetic anhydride as shown in Scheme 63 (Yadav and Badure, 2008). Phenol has been benzoylated by benzoic acid to 4-hydroxybenzophenone via the formation of phenyl benzoate as intermediate which underwent Fries rearrangement, a variant of Friedel–Crafts acylation (Yadav and George, 2008)(Scheme 64). Cs-DTP/K10 has been found to be a good catalyst for benzoylation of p-xylene to 2,5-dimethylbezophenone (Yadav et al., 2003). Similar results were obtained when resorcinol was treated with phenylacetic acid in

Scheme 61. Al3+-exchanged synthetic Zn-saponite for alkylation of benzene. Scheme 62. Alkylation of xylenes using Cs-DTP/K10. 126 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

benzylation of toluene where toluene is ﬁrst brominated by bromine either in situ or separately to generate benzyl bromide, which then benzylates toluene to benzyl toluenes. Both the reactions are catalyzed by ion-exchanged bentonite and montmorillonite K10 (Scheme 66). Friedel–Crafts acylation of arenes has been carried out using chloroacetyl chloride on Fe3+-exchanged montmorillonite K10 in Scheme 63. Friedel–Crafts acetylation using Cs-DTP/K10. liquid phase (Paranjape et al., 2008)(Scheme 67). The yields are good only with polymethylated benzene derivatives, namely, durene, Durap et al. (2006) have used Fe3+,Cr3+ and Al3+ pillared bentonite mesitylene, p-xylene and m-xylene. for benzylation of benzene and toluene. The Fe3+-pillared clay was Liu et al. (2009) have prepared an efﬁcient montmorillonite K10 found to be the most active catalyst. Kurian and Sugunan (2005) have supported antimony trichloride catalyst for alkylation of nitrogen carried out benzylation of benzene on alumina pillared transition heterocycles, pyrrole, indole and indole derivatives, using epoxides as metal cation (Cr3+,Fe3+, etc.) exchanged clays. Mechanistic investi- alkylating agents. The reaction occurs at room temperature under gation indicated the formation of benzyl cation as intermediate. solvent-free conditions and takes less than an hour for completion in Ahmed and Dutta (2005) have prepared acid-treated Zn2+ and Cd2+ most cases (Scheme 68). The yields are good to excellent. ion-exchanged clay composites which act as very good catalysts for Acid-washed montmorillonite K10 was found to be a good catalyst benzylation of benzene. Barton et al. (2003) developed a process for for alkylation as well as acylation of aromatic and heterocyclic

Scheme 64. Benzoylation of aromatic compounds.

Scheme 65. Alkylation and benzoylation of aromatic compounds. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 127

Scheme 66. Benzylation of aromatic compounds. compounds under microwave irradiation and solvent-free condition Singh et al. (2002) synthesized three sesquiterpenes, elvirol, (Devi and Ganguly, 2008). The yields are good and comparable to or curcuphenol and sesquichemaenol by Friedel–Crafts alkylation of better than the yields obtained from reactions using similarly treated appropriate cresols using suitable alkylating agents. The reaction was silica gel catalyst (Scheme 69). brought about by heating the reaction mixtures in presence of montmorillonite K10 as catalyst. The targeted sesquiterpenes were formed in major amounts accompanied by minor quantities of isomeric compounds (Scheme 70). Zhang et al. (2008) have prepared a number of dihydrocoumarin derivatives by a microwave-assisted reaction of cinnamoyl chloride with phenol and its several derivatives using montmorillonite K10 as catalyst in chlorobenzene as solvent (Scheme 71). The reaction proceeds by initial formation of phenyl ester of cinnamic acid Scheme 67. Chloroacetylation of alkylbenzenes. followed by alkylation-cyclization of the ester to give the observed

Scheme 68. Alkylation of nitrogen heterocycles by epoxides. 128 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 71. Dihydrocoumarins by acylation–cyclization.

shape selectivity and di- and tri-substituted products are also obtained as well (Binitha and Sugunan, 2008)(Scheme 72). The catalyst was regenerated by heating and reused four times with little loss of activity. The authors propose that the nature of porosity in the pillared clay is responsible for selectivity and that methylation occurs by an Eley–Rideal type mechanism. A microwave-assisted montmorillonite K10 catalyzed alkylation of indoles by tert-butyl alcohol and hexane-2,5-diol has been reported by Kulkarni et al. (2009b). The alkylation occurs selectively to form 3- Scheme 69. Alkylation and acylation of aromatic and heterocyclic compounds. tert-butylindole. In the case of the diol, the alkylation is followed by cyclization to give finally carbazole derivatives (Scheme 73). Good results are obtained when the irradiation is carried out at 130 °C for 8– 15 min. Though the GC yields are reported to be good, the isolated products. The microwave irradiation reduces the reaction time from yields are moderate. several hours to just a few minutes. The yields vary from poor to excellent. 7. Isomerization reactions Methylation of toluene with methanol in the presence of chromia- pillared montmorillonite K10 gives xylenes selectively. Mixed pillar- Isomerization or rearrangement reactions are important in ing with chromia and titania or zirconia or alumina was found to producing a large number of bulk as well as fine chemicals. A majority produce more efficient catalysts than pillaring with a single metal of such transformations are brought about by acid-catalyzed process- oxide. In contrast the natural (unmodified) clay does not exhibit any es using conventional Brønsted and Lewis acids. A variety of acid clays

Scheme 70. Synthesis of sesquiterpenes by Friedel–Crafts alkylation. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 129

Epoxides are good substrates for isomerization under a variety of acid catalytic conditions. The epoxide ring opens under the influence of Brønsted as well as Lewis acid catalysis. The incipient cation may undergo skeletal rearrangement through bond migrations and/or experience nucleophilic attack. The products have diverse applications. The diastereomeric (R)-(+)-limonene diepoxides (36a,b) underwent isomerisation at room temperature on synthetic K10 clay calcined at 100 °C or natural ascanite-bentonite clay to give 37, 38, 39 and 40a,b in a ratio of ~4:3:7:2 (Salomatina et al., 2005)(Scheme 74). Il'ina et al. (2007) have reported an extensive study of the isomerization of allyl alcohols of the pinene series and their epoxides using askanite–bentonite clay (calcined at 110 °C) (Scheme 75). Treatment of (+)-trans-pinocarveol ((+)-41)withaskanite–bentonite at room temperature led to the formation of isomeric allyl alcohol (−)-myrtenol ((−)-42), and the dimeric ether (+)-43, selectively. In contrast, the allylic alcohol (+)-41 gives Wagner–Meerwein rearrangement product 44, under homogeneous acidic conditions, due to protonation of the exocyclic double bond. On the other hand, when (−)-42 was kept on clay for one hour at room temperature (+)-41 and the rearranged products (+)-45 and 46 were obtained. The authors suggest the formation of a common intermediate A from (+)- − Scheme 72. Methylation of toluene with methanol and its mechanism. 41 and ( )-42. To explain the formation of (+)-45 and 46, the authors propose that (−)-42 gets protonated at –OH to give the carbocation A as well as at the double bond to give B, while proposing protonation of only – OH group in the case of (+)-41. A similar treatment of the epoxides (+)-47 and (−)-48 of these two allyl alcohols on askanite–bentonite clay yielded slightly differing results, though the two epoxides were visualized to give the same intermediate C Scheme 76. The pinocarpeol epoxide (+)-47 yielded, among other unidentified products, the α,β-unsaturated ketone (+)- 49 and the monocyclic keto alcohol (+)-50. Myrtenol epoxide (−)-48 produced the aromatic alcohol 4-isopropylbenzyl alcohol 46 and the hydroxy aldehyde (+)-50. Interestingly, the epoxide (+)-47 did not produce 46, and myrtenol epoxide (−)-48 did not give the unsaturated ketone (+)-49 (Scheme 76). (+)-trans-Verbinol ((+)-51) and the epoxide (−)-53 of (−)-cis- verbinol ((−)-52) were similarly treated with askanite–bentonite clay. The results obtained are shown in Scheme 77. Scheme 73. Microwave-assisted alkylation of indoles by alcohols. Storing (−)-cis-verbenol ((−)-52) and its epoxide on askanite– bentonite clay in the presence of aldehydes produced heterocyclic compounds 57–65 as depicted in Scheme 78. also have served as good catalysts for functional group transformation Montmorillonite K10 treated with the heteropolyacid dodeca- and skeletal rearrangements. The clay catalyzed conversion of low tungstophosphoric acid has been found to be effective in opening 1,2- octane hydrocarbon fuels to high octane grade is an old well known epoxyoctane (66) to octanal (67), octenol (68) and other products reaction. Due to enormous progress achieved in the use of clay- (Yadav, 2005)(Scheme 79). supported catalysts in various areas of organic synthesis, much Isolongifolene (70) is a commercially important tricyclic sesqui- attention is being focused on the study of clay catalyzed isomeriza- terpene used extensively in the perfumery industry. It is obtained by tion/rearrangement reactions, in order to replace the hazardous acid-catalyzed isomerization of longifolene (69). A number of conventional Brønsted and Lewis acids. Many times the products homogeneous acid conditions are used. However, as isolongifolene formed from clay catalyzed reactions are quite different from those has the tendency to undergo further isomerization the conditions that are formed in homogenous mineral acid or Lewis acid-catalyzed have to be carefully maintained. Singh et al. (2007) have used alumina reactions. This aspect gives an opportunity to choose catalysts and and zirconia pillared clays and Ce3+ and La3+ modified montmoril- reaction conditions to prepare the desired compounds. lonite clays for the conversion of longifolene to isolongifolene. They

Scheme 74. Isomerization of diastereomeric (R)-(+)-limonene diepoxides. 130 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 75. Isomerization of allyl alcohols of pinenes. found that Al3+ pillared montmorillonite clay shows the highest pillared clay is the best catalyst for the conversion of 1-butene to trans- conversion rate and reasonable selectivity (Scheme 80). 2-butene and cis-2-butene accompanied by small amounts of cracking Hydroisomerization of n-heptane to isoheptane was carried out by products (Scheme 82). Vogels et al. (2005) on synthetic Co-, Mg- and Co/Mg-saponites. They Nucleophilic substitution at allylic position (SN1' or SN2' type) relate the isomerization to the Lewis acidity of the catalyst (Scheme 81). with isomerization of double bond has been observed by Shanmugam Moronta et al. (2008) have studied the isomerization of 1-butene and Vaithiyanathan (2008) on K10 clay under neat conditions in the over natural smectite clay (STx-1, USA) ion-exchanged with Al3+,Fe3+ case of the derivatives of oxindole 71 (Scheme 83). The products or pillared with Al and Fe polyoxocations. The results show that Al- (72a–c) were further used for construction of spirolactone ring (not

Scheme 76. Isomerization of epoxy alcohols of pinenes. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 131

Scheme 80. Longifolene to isolongifolene.

Scheme 77. Isomerization of trans-verbinol and cis-verbinol epoxide.

Scheme 81. Hydroisomerization of n-heptane to isoheptane.

phenylacetylene (73) carried out on Zn2+-exchanged montmorillonite K10 clay by the addition of aniline (74) to the triple bond, followed by isomerization of the product enamine (75)toimine(76) (Scheme 84). The authors suggest the activation of the triple bond by π-complexing with Zn2+ ions in K10, which enables the nucleophilic addition. The isomerization is due to protonation of the double bond followed by removal of proton on nitrogen. Ortega et al. (2006) have achieved success in enriching the cis- isomer 77a in an isomeric mixture of cis- and trans-lauthisan (77a and 77b) from an isomer ratio of cis:trans=1:1.7 to 17:1, on montmorillonite K10. It should be noted with much appreciation that many

other acid catalysts including BF3.OEt2,CF3COOH and TsOH had either failed to bring about any reaction or had led to decomposition of 77a, b. This K10 catalyzed enrichment procedure enabled the authors to develop a short route to the desired isomer (+)-cis-lauthisan in good yield (Scheme 85). Tomooka et al. (2000) have observed an interesting case of stereoregulated substitutive migration of a phenyl group from TBDPS ether 78 to give 79, thereby achieving an asymmetric synthesis of α- aryl and β-hydroxycyclic amines and silanols (Scheme 86). Two highly branched 25-carbon isoprenoid oleﬁns, a diene (80) and a triene (81), isolated from Haslea ostrearia were treated with montmorillonite K10 clay at room temperature. The diene was found to unergo isomerization of a disubstituted double bond to a tri- substituted double bond, whereas the triene underwent cyclization. The results were rationalized by assuming that K10 clay acts as Brønsted acid and hence protonates one of the double bonds to give tertiary cations 82 and 83, which ﬁnally gave the observed products 84, 85 and 86 (Belt et al., 2000)(Scheme 87). The authors claim that the work has implications in understanding the presence of the observed organic compounds in sedimentary materials. Dintzner et al. (2004) have investigated the migration of isoprenyl Scheme 78. Reactions of cis-verbinol and its epoxide with aldehydes. group in phenyl isoprenyl ether (87) using montmorillonite K10 and montmorillonite KSF clays. With K10 clay the isoprenyl group moves shown in the scheme) through Morita–Baylis–Hillman reaction mainly to ortho-position by a [1,3] migration to give ortho-prenylated followed by hydrolysis. phenol 88 (Scheme 88). o-Prenylated phenols show broad range of Isomerization of enamine 75 to imine 76 has been observed by pharmacological activity, and therefore the reaction has the potential as Shanbhag and Halligudi (2004). They report hydroamination of a ‘green chemical’ method of synthesis of o-prenyl phenolic derivatives.

Scheme 79. 1,2-Epoxyoctane ring opening. 132 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 82. Isomerization and cracking of 1-butene.

Scheme 85. cis-Lauthisan to trans-lauthisan isomerization.

Scheme 83. Allylic nucleophilic substitution–isomerization.

The isoprenyl group also migrates to the para-position by a [1,5] migration to give a small amount of p-prenylated phenol 89. o-Prenyl Scheme 86. Stereoregulated substitutive migration of phenyl group. phenol 88 undergoes further protonation and cyclization to dihydropyran derivative 90. The authors observed that the mont.KSF clay also catalyzes the isomerisation but at a very slow rate. For example, with mont.K10, the reaction is complete in just 30 min, while with mont.KSF clay, microwave irradiation of a mixture of 91a and 92 delivers it takes 15 h to complete. The K10 catalyzed reaction was faster in intermediate prenyl ether (not shown in the scheme) and not 93. The

CH2Cl2, but in CCl4 the selectivity was higher. authors have not suggested any mechanism for the reaction, but it is Prenylated phenolic compounds, such as prenylated xanthones conceivable that initially prenyl ethers are formed which then have been prepared by microwave irradiation of hydroxyxanthones undergo [1,3] migration, followed by proton catalyzed cyclization of (91a–c) with prenyl bromide (92) in chloroform solution in the the intermediates to the observed products 93–98 (Scheme 89). presence of montmorillonite K10 clay. The reaction with 91a takes A [3,3]-sigmatropic shift following the addition of arylhydrazine just 20 minutes to give about 86% yield of the ﬁnal dihydropyran hydrochlorides (99) to cyclic enol ethers and enol lactones (100) takes product 93 (Castanheiro et al., 2009)(Scheme 89). The conventional place, in a manner exactly similar to the one in Fischer indole reaction, to heating at 100 °C takes 1 h and at room temperature the reaction give a variety of substituted indoles (101)(Scheme 90). The reaction is takes 5 days to give 63% yield of the product. When the reaction was catalyzed by several Brønsted acids. However, montmorillonite K10 carried out at 200 °C with ZnCl2 as catalyst, the yield of the product promoted reaction in 50% aqueous N,N-dimethylacetamide at 80 °C was only 22%. The other two hydroxyxanthones 91b and 91c give gave the best yields. Amberlyst-15 and amberlyst-120 gave reasonably mixtures of products because two ortho-positions are available for good results, but zeolite-HY and silica performed poorly (Singh et al., cyclization in the prenyl ether intermediates. In the absence of K10 2008).

Scheme 84. Addition–isomerization reaction. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 133

Scheme 87. Isomerization of isoprenoid oleﬁns.

8. Oxidation reactions clay mainly catalyzes the trimerisation of aldehydes to trialkyl 1,3,5- trioxanes, which is a temperature dependent equilibrium reaction In this section oxidation reactions that are brought about by a few with trimer-to-aldehyde ratio of 3.9:1 attained at −20 °C. However, different types of oxidizing reagents in the presence of clay catalysts are when KSF clay is used, oxidation to carboxylic acid takes place under reviewed. The reactions described include oxidation of aldehydes to aerobic conditions. Only aliphatic aldehydes undergo oxidation, but carboxylic acids, alcohols to carbonyl compounds, epoxidation, Baeyer– not the aromatic or the unsaturated aliphatic aldehydes. Villiger oxidation, oxidation and dehydrogenation of hydrocarbons. The common procedures for the oxidation of alcohols to ketones or An interesting difference between montmorillonite K10 and aldehydes make use of compounds of transition metals, such as montmorillonite KSF in their interaction with aldehydes has been chromium, manganese, vanadium, etc., which are toxic. It is desirable observed by Dintzner et al. (2010) (Scheme 91). For example, the K10 to reduce their use or replace them by oxidants that are less hazardous

Scheme 88. [1,3] Migration of prenyl group-cyclization of product. 134 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

Scheme 89. Preparation of prenylated xanthones and their cyclization. to health and environment. Many clay-based oxidizing agents that fulfill nitrate and acetoacetate added catalysts give higher yields of products. this Green chemistry condition have been developed. Eftekhari-Sis et al. The oxidation takes place by a free radical mechanism. (2007) reported the oxidation of alcohols to aldehydes and ketones Cyclohexanone undergoes Baeyer–Villiger oxidation to ε-caprolactam using hydrogen peroxide as oxidizing agent in the presence of lithium by hydrogen peroxide in the presence of Brazilian kaolinite interca- chloride supported on montmorillonite K10. The proposed mechanism lated with a porphyrin derivative, [meso-tetrakis(pentafluorophenyl) involves the formation of lithium hypochlorite, which is the active porphyrin]-iron(II), Fe(TPFPP). The porphyrin moiety is introduced oxidizing agent (Scheme 92). between the clay layers after expanding and functionalizing the Hydrogen peroxide or iodosylbenzene epoxidizes cyclooctene to interlayer space by pretreatment of the clay. The authors (Bizaia et al., cyclooctene epoxide and cyclohexane to cyclohexanone in the presence 2009) claim that this is the first example of a porphyrin-in-clay of modified natural saponite clay (Scheme 93). The modification catalyzed Baeyer–Villiger reaction. In the presence of the same catalyst procedure involves first, the intercalation of the clay with aluminium iodosylbenzene brings about the epoxidation of cyclooctene and polycation, followed by calcination at 500 °C to get alumina pillared clay oxidation of cylcohexane to cyclohexanone (Scheme 94). The catalyst and then impregnation with nickel nitrate or chloride or acetoacetate was reused up to five times without significant diminution in the (Mata et al., 2009). The amount of nickel loaded may be varied. The yields of products.

Scheme 90. Arylhydrazine addition to cyclic enol ethers and enol lactones to form indoles. G. Nagendrappa / Applied Clay Science 53 (2011) 106–138 135

Scheme 91. Difference between K10 and KSF action on aldehydes.

Scheme 94. Porphyrin-in-clay catalyzed oxidation reactions.

Scheme 95. V2O5-on-clay/H2O2-AcOH for hydroxylation of benzene to phenol.

Scheme 92. Oxidation of alcohols to aldehydes and ketones.

Gao and Xu (2006) have used vanadium oxide catalyst supported on clay (chlorite, illite, and atapulgite from Inner Mongolia) for the oxidation of benzene to phenol using hydrogen peroxide as co- oxidant. The presence of acetic acid enhances the hydrogen peroxide Scheme 96. Substrate-based oxidation or nitration using nitric acid or nitrate salts. reaction as it avoids phase separation problem. A 14% conversion with 94% selectivity was achieved (Scheme 95). Several other oxides of metals, such as copper, iron, manganese, chromium, molybdenum, dilute nitric acid in the presence of natural bentonite oxidizes benzylic tungsten, etc., were tried in place of vanadium, but were found to be alcohols to the corresponding carbonyl derivatives. In contrast, the same not as effective. However, the TS-1 was found to be equally good. reagent-catalyst system nitrates activated aromatic compounds regio- Nitric acid and metal nitrate salts are used as both oxidizing and selectively (Bahulayan et al., 2002)(Scheme 96). nitrating agents depending on the nature of the substrate and reaction Ethylbenzene has been dehydrogenated to styrene using a conditions. In most cases the reaction is slow, but if vigorous conditions processed Venezuelan smectite clay intercalated with trinuclear iron + are used to hasten the reaction the substrate may decompose. However, complex, [Fe3O(OAc)6.3H2O] . Ethylbenzene was heated over the reports show that such reactions can be accomplished under very mild catalyst for 1 h at 410 °C. There was 50% increase in styrene formation conditions, if the reagents are supported on smectite clays. For example, with the use of this catalyst (Huerta et al., 2003)(Scheme 97).

Scheme 93. Cyclooctene epoxidation and cyclohexane oxidation by H2O2 or C6H5IO. 136 G. Nagendrappa / Applied Clay Science 53 (2011) 106–138

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