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GREENER SYNTHETIC TRANSFORMATIONS USING MICROWAVES
Citation:
Varma*, R S. GREENER SYNTHETIC TRANSFORMATIONS USING MICROWAVES. Presented at Bristol-Myers Squibb Pharmaceutical Research Institute, New Brunswick, NJ, 8/7/2001.
Description:
Microwave irradiation has been used for a variety of organic transformations wherein chemical reactions are expedited because of selective adsorption of microwave (MW) energy by polar molecules, non-polar molecules being inert to the MW dielectric loss. The MW application under solventless conditions enables rapid synthetic transformations at ambient pressure thus providing unique chemical processes with special attributes such as ease of manipulation, enhanced reaction rates and higher yields. Recent results from our laboratory on this MW-expedited approach will be described for the synthesis of a variety of enamines, nitroalkenes, oxidized sulfur compounds and ionic liquids. This solvent-free synthetic methodology involves microwave exposure of reactants in neat form or their rection 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(NO3)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 a-tosyloxyketones and enamines. The application of this solventless 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 distinct advantages of this strategy, that reduces or presents 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 using paraformaldehyde and solid state synthesis of heterocyclic hydrazones under catalyst-free conditions.