‘Greener’ Organic Syntheses Under Non-Traditional Conditions Using Microwave and Ultrasound Irradiation and Mechanochemical Mixing

Abstract

Solvent-free mechanochemical methods that involve the use of hypervalent iodine reagents at room temperature are described for the synthesis of heterocyclic entities and conversion of ketones into β-keto sulfones in high yields. A solvent-free approach that involves microwave (MW) exposure of neat reactants (undiluted) catalyzed by the surfaces of lessexpensive and recyclable mineral supports such as alumina, silica, clay, or ‘doped’ surfaces is presented; it 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 multicomponent 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. Ultrasoundand 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. A general and expeditious MW-enhanced nucleophilic substitution approach uses easily accessible starting materials, such as halides or tosylates, in reaction with alkali azides, thiocyanates, or sulfinates in the absence of any phase transfer catalyst to produce azides, thiocyanates, and sulfones, respectively, wherein a variety of reactive functional groups are tolerated.