Mechanistic and synthetic studies of the intramolecular [4+2] cycloaddition of conjugated enynes

Abstract

An examination of the mechanism and scope of the intramolecular [4+2] cycloaddition of conjugated enynes has shown the reaction likely proceeds by a concerted cycloaddition of the enyne to give a highly reactive cyclic allene intermediate which isomerizes to aromatic and dihydroaromatic compounds through proton transfer or hydrogen atom transfer mechanisms. Studies of the intramolecular cycloaddition of conjugated arylacetylenes (arenynes) indicated that these compounds may react by a stepwise annulation mechanism involving biradical intermediates. An investigation of the use of protic solvents has shown that alcohols, in particular 2,2,2-trifluoroethanol, can significantly improve the isolated yield of products. The application of strained cyclic acetylenes, specifically arynes generated by a modification of the Kobayashi protocol with tetrabutylammonium triphenyldifluorosilicate (TBAT), has been shown to efficiently provide cycloaddition products with substituted enynes, arenynes, and hetarenynes at room temperature.