RADICAL TRANSLOCATION REACTIONS OF VINYL RADICALS - SUBSTITUENT EFFECTS ON 1,5-HYDROGEN-TRANSFER REACTIONS

A systematic study of the radical translocation (1,5-hydrogen-transfer) reactions of vinyl radicals is described. The effects of monoalkoxy, dialkoxy, monoalkylthio, dialkylthio, thiohemiketal, phenyl, allyl, carboxylate ester, nitrile, and tertiary, secondary, and primary alkyl substituents on the transferring C-H bond were studied. The protocol was first to generate vinyl radicals which were next translocated to new carbon centers by 1,5-hydrogen atom transfer processes. Rapid radical cyclization followed translocation. Stork's catalytic tin hydride method, a standard stoichiometric tin hydride method, and a syringe pump method were used to run the radical reactions, and all gave comparable results under standard conditions. Most substituents gave similar rates of 1,5-hydrogen atom transfer (generally 50-100% of the vinyl radicals were translocated to new carbon centers) except for some strong C-H bonds which gave no hydrogen atom transfer (for example, epoxide and methyl). Generally, sulfur substituents activated the C-H bonds best; however, in some cases, S(H)2 reactions of vinyl radicals attacking the sulfur atoms or tin hydride desulfurizations interfered with the desired 1,5-hydrogen atom transfer processes. Rate constants for 1,5-hydrogen transfer are typically in the range of 10(6) s-1. Substituent effects on intra- and intermolecular hydrogen-transfer reactions are compared, and implications of the results are discussed.