Tin Radical Addition to Alkynyl Sulfides: Reactivity of the Intermediate Thioalkyl-Substituted β-(Tributylstannyl)vinyl Radicals

Article abstract of DOI:10.1021/jo00091a025

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 β-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-containing radical, which afforded a selenophene almost exclusively. (Benzylthio)alkyl-substituted radicals gave products deriving from intramolecular S_H2 substitution at the sulfur atom, whereas propargyl sulfides yielded a stannylallene via a β-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.