Microwave-Assisted Chemistry: a Rapid and Sustainable Route to Synthesis of Organics and Nanomaterials

The use of emerging microwave (MW)-assisted chemistry techniques in conjunction with benign reaction media is dramatically reducing chemical waste and reaction times in several organic syntheses and chemical transformations. The present review summarizes recent developments in MW-assisted synthesis, name reactions and organic transformations, and rapid generation of nanoparticles with uniform size distribution. Greener protocols have been developed for the synthesis of various bio-active heterocycles, namely 1,3,4-oxadiazoles, 1,3,4-thiadiazoles, 1,3-dioxanes, pyrazoles, hydrazones and 3,4-dihydropyrimidin-2(1H)-ones, which proceed under the influence of microwaves and using eco-friendly conditions. These high-yielding methods were catalyzed efficiently by solid-supported Nafion NR50 under solvent-free conditions and polystyrene sulfonic acid in aqueous media. The eco-friendly nucleophilic substitution chemistry in water to generate cyclic amines via double N-alkylation of primary amines or hydrazines by dihalides or tosylates enables the greener synthesis of a range of pharmaceutically active heterocycles. Similarly, efficient MW synthesis of various azides, thiocyanates, and sulfones in aqueous medium occurs wherein nucleophilic substitution reaction takes place in the absence of a phase-transfer catalyst. Bulk and shape-controlled synthesis of noble nanostructures via MW-assisted spontaneous reduction of noble metal salts using α-d-glucose, sucrose, and maltose is described. MW method also accomplishes the cross-linking reaction of poly(vinyl alcohol) with metallic systems such as Pt, Cu, and In; bimetallic systems, namely Pt–In, Ag–Pt, Pt–Fe, Cu–Pd, Pt–Pd, and Pd–Fe; and single-walled nanotubes, multi-walled nanotubes, and buckminsterfullerenes (C-60). The strategy is extended to the formation of biodegradable carboxymethyl cellulose (CMC) composite films with noble nanometals; such metal decoration and alignment of carbon nanotubes in CMC is possible using a MW approach that also enables the shape-controlled bulk synthesis of Ag and Fe nanorods in poly(ethylene glycol).