thiol mediated 5-(pi-endo)ortho vinyl radical cyclizations

The radical reaction of benzenethiol with alkynes 1a-o carried out at 154 degreesC affords a mixture of thiol/alkyne adduct 3 and benzothiophene 5, deriving from Vinyl radical intermediates 2 through hydrogen abstraction and 5-(pi -endo)orthocyclization onto the adjacent thiophenyl ring, respectively, This latter reaction occurs through the reversible formation of cyclohexadienyl radical intermediates 4 which ran evolve to 5 to an extent largely depending on the reaction conditions employed. The 5-ortho cyclization is governed both by stereoelectronic factors, which favor the cyclization of p-centered, linear alpha -arylvinyl radicals 2a-f, and polar factors, which favor the cyclization of alpha -EWG substituted vinyl radicals 2c,n,o.