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'GREENER' ORGANIC SYNTHESES AND TRANSFORMATIONS USING MICROWAVES UNDER SOLVENT-FREE CONDITIONS OR AQUEOUS MEDIA
Citation:
VARMA, R. S. 'GREENER' ORGANIC SYNTHESES AND TRANSFORMATIONS USING MICROWAVES UNDER SOLVENT-FREE CONDITIONS OR AQUEOUS MEDIA. Presented at Meeting at Edwards Air Force Base, Lancaster, CA, May 23 - 24, 2006.
Description:
A 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. MW-assisted solventless preparation of ionic liquids and their application in alkylation reaction and metal-catalyzed chemical transformations is described. An aqueous N-alkylation of amines by alkyl halides to produce tertiary amines is described that proceeds expeditiously in the presence of NaOH. N-azacycloalkanes, an important class of building blocks in natural products and pharmaceuticals, are synthesized via a simple, efficient and eco-friendly MW protocol that involves double N-alkylation of primary amines. This reaction utilized readily available aniline derivatives and alkyl dihalides or ditosylates to assemble two C-N bonds in a SN2-like heterocyclization sequence which cannot be fully realized under conventional heating conditions. The reaction is accelerated by exposure to microwaves in the presence of aqueous potassium carbonate and is applied to the synthesis 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. Overall, these MW-assisted 'greener' chemical transformations circumvent the need for multi-step processes that use expensive metal catalysts, afford excellent product yields, and accommodate reactive functional groups because of mild reaction conditions. Additional advantages include considerably reduced reaction times, and minimization or elimination of byproducts.