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SOLVENT-FREE APPROACH TO EXPEDITIOUS ORGANIC SYNTHESES USING MICROWAVE IRRADIATION
Citation:
Varma*, R S. SOLVENT-FREE APPROACH TO EXPEDITIOUS ORGANIC SYNTHESES USING MICROWAVE IRRADIATION. Presented at 53rd ACS Southeast Regional Meeting, Green Chemistry Symposium, Savannah, GA, 9/23-27/2001.
Description:
Microwave irradiation has been used for a variety of organic transformations wherein chemical reactions are accelerated because of selective adsorption of microwave (MW) energy by polar molecules, non-polar molecules being inert to the MW dielectric loss. The application of MW irradiation under solventless conditons enables expedited synthetic transformations at ambient pressure thus providing unique processing conditions with special attributes such as ease of manipulation and higher yhields. Results from our laboratory on this MW-accelerated approach will be described for the synthesis of a variety of industrially significant compounds and intermediates namely, imines, enamines, nitroalkenes, enones, oxidized sulfur compounds and ionic liquids. This solvent-free synthetic methdology involves microwave exposure of reactants in neat form or their reaction in presence of a catalyst or catalyzed by the surfaces of inexpensive and recyclable mineral supports namely silica, alumina, clay, or "doped" surfaces, such as Fe(NO)3-clay (clayfen), Cu(NO3)2-clay (claycop), NH2OH-clay, PhI(OAc)2-alumina, NaIO4-silica, and NaBH4-clay. A variety of condensation, cyclization, oxidation and reduction reactions will be presented including the convergent one-pot assembly of heterocyclic molecules from in situ generated intermediates such as enamines and a-tosyloxyketones. The application of this solvent-free MW approach to multi-component reactions will be highlighted which can be adapted for high-speed parallel synthesis of the library of biologically active molecules. The salient features of this strategy, that reduces or prevents pollution "at-source," will be exemplified by efficient functional group transformations involving non-metallic hypervalent iodine oxidants and substitution of conventional metal-based reducing agents as demonstrated in crossed Cannizzaro reaction on barium hydroxide surface.