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MICROWAVE-ACCELERATED CARBON-NITROGEN BOND FORMATION: A GREEN CHEMICAL APPROACH FOR THE SYNTHESIS OF AMINES AND HETEROCYCLES IN AQUEOUS MEDIA
Citation:
JU, Y. AND R. S. VARMA. MICROWAVE-ACCELERATED CARBON-NITROGEN BOND FORMATION: A GREEN CHEMICAL APPROACH FOR THE SYNTHESIS OF AMINES AND HETEROCYCLES IN AQUEOUS MEDIA. Presented at 7TH INTERNATIONAL SYPOSIUM ON GREEN CHEMISTRY, ZHUHAI, CHINA, May 24 - 26, 2005.
Impact/Purpose:
To inform the public.
Description:
The concept of "green chemistry" is widely adopted to meet the fundamental scientific challenges of protecting the human health and environment while simultaneously achieving commercial viability. One of the thrust areas for achieving this target is to explore alternative reaction conditions and reaction media to accomplish the desired chemical transformations with minimum by-products or waste generation as well as eliminating the use of conventional organic solvents. Carbon-nitrogen (C-N) bond formation is one of the important transformations in organic synthesis as the ensuing amines are widely used as precursors or intermediates in the preparation of fine chemicals, agrochemicals, and pharmaceuticals. Microwave (MW) irradiation has attracted considerable attention in the past two decades for rapid and efficient syntheses because of the selective absorption of MW energy by polar molecules. An aqueous N-alkylation of amines by alkyl halides that proceeds expeditiously in the presence of NaOH to deliver tertiary amines will be described; N-aryl azacycloalkanes, an important class of building blocks in natural products and pharmaceuticals, are synthesized via a simple, efficient and eco-friendly protocol that involves double N-alkylation of amines. This green chemical reaction utilizes 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 protocol can be extended to a variety of five- and even six-membered heterocyclic systems, e.g. 4,5-dihydropyrazole and pyrazolidine. These MW-assisted environmentally friendlier chemical transformations circumvent the need for multi-step processes that use expensive metal catalysts, utilize water as a cleaner reaction medium, afford excellent product yields, and accommodate reactive functional groups such as carbonyl, hydroxyl, and ester etc. Additional advantages of using microwave irradiation include considerably reduced reaction times, and minimization or elimination of byproduct formation.