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NON-TRADITIONAL 'GREENER' ALTERNATIVES TO CHEMICAL TRANSFORMATIONS
Citation:
Varma*, R S. NON-TRADITIONAL 'GREENER' ALTERNATIVES TO CHEMICAL TRANSFORMATIONS. Presented at 5th International Green Chemistry Conference, Barcelona, Spain, 11/19-11/2003.
Description:
Synthetic organic transformations performed under non-traditional conditions are becoming popular primarily to circumvent the growing environmental concerns. 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 described 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 for 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 are described and the methodology is exemplified by a concise synthesis of flavones, tetrahydroquinolones, 2-aroylbenzofurans, and thiazole derivatives. Ultrasound-assisted solventless protocol is presented that provides ready access to non-volatile solvents, ionic liquids with barely measurable vapor pressure. The utility of these techniques for rapid preparation of vanadium phosphate oxide (VPO) catalysts or benign epoxidation of alkenes using hydrotelecite catalysts will be presented. The case of supercritical (sc) CO2, an attractive 'Greener' media, is represented by selective hydrogenation of maleic anhydride to g-butyrolactone. The salient eco-friendly features of these processes namely selectivity, experimental ease of manipulation, and the enhanced reaction rates will be described.