Thermal reactivity of 2-azido- and 3-azido-benzo[b]thiophene with alkenes

Abstract

Thermal decomposition of 2-azidobenzo[b]thiophene (2-BTA) in the presence of various (E)-and (Z)-alkenes, at room temperature, affords thiochroman-4-carbonitriles resulting from cycloaddition of an ortho-quinoidal enethione intermediate (2) to the olefin double bonds and/or 1-(2-benzothienyl)aziridines which generally occur in a non-stereospecific fashion. In one case, i.e. with diethyl fumarate, clear-cut spectroscopic and chemical evidence for the intermediacy of a triazoline adduct in the formation of the observed trans-and cis-aziridines has been obtained. In the presence of 1-pyrrolidinyl-cyclopentene or -cyclohexene the azide furnishes an isolated triazoline in quantitative yield, whereas with methyl (E)-3-(N-pyrrolidinylacrylate leads to methyl 1-(2-benzothienyl)triazole-4-carboxylate arising from an intermediate triazoline by readily occurring elimination of pyrrolidine. Results suggest that 2-BTA generally undergoes cycloaddition reactions to give triazoline adducts, from which aziridines can be eventually produced, in competition with unimolecular ring-cleavage fragmentation leading to the enethione (2) probably via concerted ring opening and nitrogen extrusion. Suitable support has been provided by the finding that 3-azidobenzo[b]thiophene (3-BTA) can exhibit analogous cycloaddition reactions with alkenes under the same reaction conditions. The present evidence contradicts our previous claim that a singlet nitrene should be an intermediate in the formation of aziridine and ring-cleavage products arising from decomposition of 2-BTA in the presence of alkenes.