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 19 a,b, ethers 22a,b, and selenide 31 reacted with tributyltin radicals t o 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 ph enyl ring. The resulting spirocyclohexadienyl radicals gave thiophenes and thiopyrans by carbon-sulfur bond scission or ring expansion on th e exocyclic double bond, respectively. Similar behavior was exhibited by the corresponding selenium-containnig radical, which afforded a sel enophene almost exclusively. (Benzylthio) alkyl-substituted radicals g ave products deriving from intramolecular S(H)2 substitution at the su lfur atom, whereas propargyl sulfides yielded a stannylallene via a be ta-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 v inyl radicals also gave the (E)-adducts exclusively. An interaction be tween the unpaired electron orbital and the empty low-energy orbitals of the heteroatom might explain why sulfur and selenium can undergo 5- exo cyclization.