Montmorillonite microwave irradiation

Keywords /V-allylaniline, 3-aza-Cope rearrangement, Zn2+ montmorillonite, microwave irradiation, 2-methylindoline... 

Keywords L-galactono-1,4-lactone, aldehyde, montmorillonite, microwave irradiation, acetals... 

Dimethyl-1,3-dioxolane, microwave irradiation. Montmorillonite KSF, 38-95% yield.""... 

The investigation on the use of K-10 montmorillonite under free solvent conditions was then extended to inner ring dienes such as furan and its 2,5-dimethyl derivative [9] (Table 4.3). The cycloadditions generally proceed slowly, and Zn(II)-doped clay and microwave irradiation were used to accelerate the reactions. The reaction with maleic anhydride preferentially affords the thermodynamically favored exo adduct. 

Metallocenes Mg(II)-catalyzed Microwave irradiation MO calculation Montmorillonite Moraceous plants Morphinadienes Mulberry tree Multifunctional catalysis Multi-step... 

The synthesis of imidazoles is another reaction where the assistance of microwaves has been intensely investigated. Apart from the first synthesis described since 1995 [40-42], recently a combinatorial synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles has been described on inorganic solid support imder solvent-free conditions [43]. Different aldehydes and 1,2 dicarbonyl compounds 42 (mainly benzil and analogues) were reacted in the presence of ammonium acetate to give the trisubstituted ring 43. When a primary amine was added to the mixture, the tetrasubstituted imidazoles were obtained (Scheme 13). The reaction was done by adsorption of the reagent on a solid support, such as silica gel, alumina, montmorillonite KIO, bentonite or alumina followed by microwave irradiation for 20 min in an open vial (multimode reactor). The authors observed that when a non-acid support was used, addition of acetic acid was necessary to obtain good yields of the products. 

Considering the formation of saturated five-membered heterocycles with two heteroatoms, it is worth to note the possibility to prepare 1,3-dioxolanes, dithiane, oxathianes 148 [93] and dioxolanones 149 [94] by condensation of the corresponding carbonyl compounds under microwave irradiation in acid medium (Scheme 52). The reaction, which is very useful for the protection of carbonyl compounds or for the preparation of useful synthetic intermediates, has also been carried out under batch conditions over Montmorillonite KIO clay in more than 150 g scale, using a 1 L quartz reactor [95]. 

For the synthesis of coumarins, the Pechmann reaction [145] is one of the most popular synthetic routes. As the reaction is conventionally carried out at high temperature, two microwave-assisted versions have been recently described. Besson and co-workers described the cyclocondensation of different m-amino phenols 226 with /1-ketoesters 227 on graphite/montmorillonite KIO support (Scheme 83). The use of graphite was crucial in the development of the reaction conditions. In fact, microwave irradiation of the reagents using different conditions gave poor results in terms of yields and purity. The optimized conditions, using a monomode microwave system, employed... 

A milder Clauson-Kaas pyrrole synthesis was reported that alleviated the need for acid or heat <06TL799>. The innovation involved the hydrolysis of 2,5-dimethoxytetrahydrofuran giving 2,5-dihydroxytetrahydrofuran. The latter was converted into pyrroles by treatment with primary amines in an acetate buffer. The Clauson-Kaas pyrrole synthesis was studied utilizing a K-10 montmorillonite acid catalyst and microwave irradiation <06OPP495>. Mild reaction conditions (cat. p-TsOH) allowed for the preparation of pyrrole-3-carboxaldehydes from 2,5-dimethoxytetrahydrofuran-3-carboxaldehydes <06S1494>. 

A new synthetic route for functionalized polyhydroxyalkyl-pyrimidines starting from unprotected aldoses and based on montmorillonite K-10 catalysis and solvent-free microwave irradiation conditions, has been reported by Yadav et al,m Thus, reaction of D-glucose and D-xylose with semicarbazide or thiosemicarbazide (186) in the presence of montmorillonite K-10, under microwave irradiation, proceeded via domino cycloisomerization, dehydrazination, and dehydration of the intermediate semi- or thiosemicarbazones (187) to afford l,3-oxazin-2-ones or l,3-oxazine-2-thiones (188) in one single step and in yields between 79% and 85% (Scheme 34). Other mineral catalysts tested, such as silica gel and basic alumina, were far less effective for this transformation and only silica gel was active at all, giving low yields (15-28%) of compounds 188a-d. The l,3-oxazin-2-ones(thiones) thus synthesized were subsequently converted into the target pyrimidines by reaction with aromatic... 

Imidazole has been condensed via a 1,4 Michael addition with ethyl acrylate by use of basic clays (Li+ and Cs+ montmorillonites) under solvent-free conditions with microwave irradiation [77] (Eq. 24). 

Cydocondensation of N-(trifluoroacetamido)-o-arylenediamines leads to a series of 2-trifluoromethylarylimidazoles with good yields on montmorillonite K10 in dry media under the action of microwave irradiation within 2 min. Using conventional heating under the same conditions, no reaction was observed [104] (Eq. (54) and Tab. 3.23). 

Carboxylic acids are regenerated from their corresponding substituted allyl esters on montmorillonite K10 using microwave irradiation under solvent-free conditions to afford enhanced yields and reduced reaction times when compared to thermal conditions [108] (Eq. 58). 

Loupy and colleagues have prepared acetals of 1-galactono-l,4-lactone in excellent yields [31] by adsorbing the lactone and the aldehyde on montmorillonite K 10 or KSF clay followed by exposing the reaction mixture to microwave irradiation (Scheme 6.1). 

Sodium cyanoborohydride [123], sodium triacetoxyborohydride [124] or NaBH4 coupled with sulfuric acid [125] are common agents used for the reductive amination of carbonyl compounds. These reagents either generate waste or involve the use of corrosive acids. The environmentally friendlier procedures developed by Varma and coworkers have been extended to a solvent-free reductive amination protocol for carbonyl compounds using moist montmorillonite K 10 day supported sodium borohydride that is facilitated by microwave irradiation (Scheme 6.39) [126]. 

A rapid one-pot synthesis of imidazo-[l,2-a]-pyridines, pyrazines and pyrimidines was described in 1999 by Varma et al. [50], who used recyclable montmorillonite clay Kio under solvent-free conditions and microwave irradiation (Scheme 8.32). 

The dispersion and solid-state ion exchange of ZnCl2 on to the surface of NaY zeolite by use of microwave irradiation [17] and modification of the surface of active carbon as catalyst support by means of microwave induced treatment have also been reported [18]. The ion-exchange reactions of both cationic (montmorillonites) and anionic clays (layered double hydroxides) were greatly accelerated under conditions of microwave heating compared with other techniques currently available [19.]... 

Microwave irradiation has also been applied to the preparation of Fe203/S04-superacid [20, 21] and high-surface aluminum pillared montmorillonites [20],... 

Acylhydrazides reacted with isothiocyanates in a one-pot procedure using microwave irradiation of the starting materials in the presence of either silica or montmorillonite K10 to give the 3-mercapto-l,2,4-triazoles 137a-f in high yields (Equation 42 and Table 26) <2006PS(181)1839>. 

An Iranian group described the synthesis of some [l,3,4]thiadiazolo[2,3-c][l,2,4]triazinones 88 <2002PS2399> and in the course of the synthetic pathway the dihydro derivative 87 was first obtained. These authors found that microwave irradiation of 87 on montmorillonite in the presence of nitrobenzene allowed to accomplish the final oxidative step and yielded the fully conjugated end-product in good yields (50-62%). The reaction as proceeding was interpreted by electron transfer to 89 caused by the microwave irradiation followed by the formation of the intermediate radical 90. 

Reaction of 218 with aromatic aldehydes to give the ring-closed product 220 takes place in boiling ethanol in excellent yield (Scheme 44) <2001PHA376>. The transformation obviously proceeds via formation of a dihydro thiadiazole 219, as also suggested for the transformation of 218 under microwave irradiation in the presence of montmorillonite <2002PS2399>. 

Acetals of o-GlcpNAc 28a (Z=NHAc) [99] and o-Glcp 28b [55] were prepared by condensation of the appropriate aldehyde or aldehyde dimethyl acetal with the unprotected carbohydrate. An improvement of the classical methods of acetalation (microwave irradiation of montmorillonite) made it possible to synthesize various 5,6-0- -alkylacetals of L-galactono-1,4-lactone [100]. 

The Lewis acid-catalyzed condensation of a,/3-unsaturated ketones 277 with amides 278 furnished 2,4,6-trisubsti-tuted-6//-l,3-oxazines 279. An environmentally benign solvent-free version of this process, based on the application of montmorillonite K-10 clay and a brief microwave irradiation, provided oxazines 278 in higher yields than in the conventional solution-phase method (Equation 26) <2004BCJ2265>. 

In terms of Michael additions at the indole C-3-position, montmorillonite KIO has been shown to be effective in promoting the conjugate addition of indoles to a,p-unsaturated carbonyl compounds <99MC929>. In what is also a Michael process, microwave irradiation has been shown to be effective in promoting the trimolecular condensation of substituted indoles 111 with... 

The Pechmann and Knoevenagel reactions have been widely used to synthesise coumarins and developments in both have been reported. Activated phenols react rapidly with ethyl acetoacetate, propenoic acid and propynoic acid under microwave irradiation using cation-exchange resins as catalyst <99SL608>. Similarly, salicylaldehydes are converted into coumarin-3-carboxylic acids when the reaction with malonic acid is catalysed by the montmorillonite KSF <99JOC1033>. In both cases the use of a solid catalyst has environmentally friendly benefits. Methyl 3-(3-coumarinyl)propenoate 44, prepared from dimethyl glutaconate and salicylaldehyde, is a stable electron deficient diene which reacts with enamines to form benzo[c]coumarins. An inverse electron demand Diels-Alder reaction is followed by elimination of a secondary amine and aromatisation (Scheme 26) <99SL477>. 

A facile one-pot synthesis of furopyrans takes place with aromatic aldehydes, cyclohexyl isocyanide, and 4-hydroxy-6-methyl-2-pyrone in the presence of a solid support such as Montmorillonite K-10 (Equation 62) <2005SC535>. The solvent-free reaction, which is enhanced by microwave irradiation, proceeds much more rapidly under these conditions than by conventional methods <2002TL2293>. The one-pot, three-component reaction is also reported to take place rapidly using water as solvent <2004M589>. 

Similar good results were obtained with Fe3+ montmorillonite and zeolite Beta with sulfonic acids, chlorides, or anhydrides as reagents.278 Iron(III) chloride is also very effective in this reaction under microwave irradiation.279 Sulfonylation by sulfonic acids catalyzed by Nafion-H allows the synthesis of both symmetric and unsymmetric sulfones.280... 

Bougrin, K., Loupy, A., Petit, A., Daou, B. and Soufiaoui, M., Novel route for synthesis of 2-trifluoromethyl-arylimidazoles oil K10 montmorillonite in a microwave-irradiated dry medium , Tetrahedron, 2001, 57, 163-168. 

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