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CHEMICAL SYNTHESIS & TRANSFORMATIONS USING MICROWAVES
Citation:
VARMA, R. S. CHEMICAL SYNTHESIS & TRANSFORMATIONS USING MICROWAVES. Presented at 39th Central Regional American Chemical Society Meeting, Covington, KY, May 20 - 23, 2007.
Impact/Purpose:
To inform the public.
Description:
A historical account of the utility of microwaves in a variety of chemical synthesis applications will be presented, including a solvent-free strategy that involves microwave (MW) exposure of neat reactants (undiluted) catalyzed by the surfaces of recyclable mineral supports such as alumina, silica, clay, or ‘doped’ surfaces. The approach 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. Aqueous N-alkylation of amines by alkyl halides to produce tertiary amines and synthesis of N-azacycloalkanes, an important class of building blocks in natural products and pharmaceuticals, can be achieved by a simple, efficient, and eco-friendly MW protocol. This double N-alkylation of primary amines proceeds in aqueous potassium carbonate medium and readily assembles two C-N bonds in a SN2-like heterocyclization sequence, leading to the formation of a variety of five-membered heterocycles, e.g. isoindole, pyrazole, and pyrazolidine, by condensation of amines or hydrazines with alkyl 1,3-dihalides or -ditosylates. Similarly, a practical and efficient MW synthesis of various azides, thiocyanates, and sulfones is possible in aqueous medium wherein nucleophilic substitution occurs using readily available materials such as halides or tosylates in reaction with azide, thiocyanate, or sulfinate salts; no phase transfer catalyst is required and the MW reaction tolerates functional groups such as carbonyl, hydroxyl, and esters. The salient features of using MW irradiation in conjunction with water as a reaction medium, which include considerable reduction in reaction times and minimization or elimination of byproduct formation, will be summarized.