TIN RADICAL-ADDITION TO ALKYNYL SULFIDES - REACTIVITY OF THE INTERMEDIATE THIOALKYL-SUBSTITUTED BETA-(TRIBUTYLSTANNYL)VINYL RADICALS

Phenyl and benzyl alkynyl sulfides 1a-g and 3a, phenylalkynylamines 19a,b, ethers 22a,b, and selenide 31 reacted with tributyltin radicals to give intermediate beta-stannylvinyl radicals, whose fate depended on the nature of the side chain. 4-Phenylthio-substituted but-1-en-2-yl radicals underwent stereospecific 5-exo cyclization on the adjacent phenyl ring. The resulting spirocyclohexadienyl radicals gave thiophenes and thiopyrans by carbon-sulfur bond scission or ring expansion on the exocyclic double bond, respectively. Similar behavior was exhibited by the corresponding selenium-containnig radical, which afforded a selenophene almost exclusively. (Benzylthio) alkyl-substituted radicals gave products deriving from intramolecular S(H)2 substitution at the sulfur atom, whereas propargyl sulfides yielded a stannylallene via a beta-scission reaction. No 6(or more)-membered ring closure was observed with pentynyl and hexynyl phenyl sulfides 1b,c, which gave only the (E)-addition products of tin hydride; oxygen- and nitrogen-containing vinyl radicals also gave the (E)-adducts exclusively. An interaction between the unpaired electron orbital and the empty low-energy orbitals of the heteroatom might explain why sulfur and selenium can undergo 5-exo cyclization.