Citation:

VARMA, R. S. "GREENER" CHEMICAL SYNTHETIC PROCESSES USING ENZYMATIC, MECHANOCHEMICAL MIXING, OR MICROWAVE AND ULTRASOUND IRRADIATION. DOI: 10.1080/1751825, J. Warner (ed.), Green Chemistry Letters and Reviews. Taylor & Francis Group, London, Uk, 1(1):37-45, (2007).

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Several newer strategies, such as solvent-free (dry media), solid-supported with and without microwave (MW) irradiation, and mechanochemical mixing (grinding); and the use of room temperature ionic liquids, supercritical carbon dioxide, and water as reaction media that can be combined with MW or ultrasonic irradiation for rapid synthesis, are described with examples from authors laboratory. A solvent-free approach that involves microwave (MW) exposure of neat reactants (undiluted) catalyzed by the surfaces of less-expensive and recyclable mineral supports such as alumina, silica, clay, or ‘doped’ surfaces is presented which is applicable to a wide range of cleavage, condensation, cyclization, rearrangement, oxidation and reduction reactions including rapid one-pot assembly of heterocyclic compounds from in situ generated reactive intermediates. The strategy is adaptable to multi-component reactions e.g. Ugi and Biginelli reactions for rapid assembly of a library of compounds. Synthesis of a wide variety of significant precursors and intermediates namely, enones, imines, enamines, nitroalkenes, and oxidized sulfur species is possible and their value in concise MW synthesis of 2-aroylbenzofurans, and thiazole derivatives is illustrated. Ultrasound- and MW-assisted solventless preparation of ionic liquids and their application in alkylation and metal-catalyzed multi-component reactions is described. Efficient reaction of epoxides with carbon dioxide, CO₂, provides ready access to cyclic carbonates using only a catalytic amount of recyclable indium-based ionic liquid. MW heating in aqueous reaction media enables expeditious N-alkylation reactions of amines and hydrazines to afford a series of heterocyclic ring systems such as N-azacycloalkanes, 4,5-dihydropyrazoles and pyrazolidines etc.

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