Stereoselective hydrogen transfer reactions of vinyl radicals: Cyclization of alkynyl iodides by unimolecular chain transfer from silicon hydrides

The cyclization of several substituted hexynyl and heptynyl iodides proceeds stereoselectively to give either E- or Z-exocyclic double bonds depending on the type of precursor and radical chain used. In UniMolecular Chain Transfer (UMCT) reactions, the intramolecular abstraction of hydrogen by the intermediate vinyl radical leads exclusively to the E-isomer while the traditional tin hydride method usually provides the Z-isomer with good selectivity. (C) 1997 Elsevier Science Ltd.