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MICROWAVES: FROM MYTH, MYSTERY AND MAGIC TO A FAST-MOVING FRONTIER
Citation:
VARMA, R. S. MICROWAVES: FROM MYTH, MYSTERY AND MAGIC TO A FAST-MOVING FRONTIER. Presented at 4th International Microwaves in Chemistry Conference, Orlando, FL, March 08 - 11, 2006.
Impact/Purpose:
To inform the public.
Description:
A historical account of the utility of microwaves in a variety of chemical applications will be highlighted including solvent-free approach 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 is presented 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. 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 readily assembles two C-N bonds in a SN2-like heterocyclization sequence which cannot be fully realized under conventional heating conditions. The MW-assisted reaction proceeds in aqueous potassium carbonate and leads to the formation of a variety of five-membered heterocycles, e.g. isoindole, pyrazole and pyrazolidine etc. by condensation of amines or hydrazines with alkyl 1,3-dihalides or -ditosylates. Other MW-assisted transformations ranging from material science to drying of papers or books and glass-making is also described including the latest ground-breaking attributes of H- and E-field contributions that comprise the electromagnetic radiation.