Thiol radical addition to alkynes. Sulfanyl radical addition and hydrogen atom abstraction relative reaction rates.

2-(Toluenesulfanyl)- 1 and 2-(benzenesulfanyl)-phenylacetylene 10 reacted with benzenethiol and toluenethiol respectively, in the presence of AIBN at 84 and 154 degrees C to give products deriving from vinyl radicals 2 which undergo hydrogen abstraction reaction and 5-ortho and 5-exo cyclization onto both the adjacent phenyl rings in competition with the beta-fragmentation. Definitive evidence has been obtained that alkanesulfanyl radical addition to the alkyne triple bond is a non-reversible process, whereas arenesulfanyl radicals add in a reversible mode. Competing experiments involving several alkynes towards benzenethiol and benzeneethanethiol radical addition have been performed in order to determine the relative rate constants of the sulfanyl radical addition to the alkyne triple bond (k(1)) and the hydrogen abstraction reaction by the resulting vinyl radicals (k(H)). The k(1) values are mainly determined by the vinyl radical stabilization provided by the C-(alpha) vinyl radical substituent whereas the kH values seem to be mainly determined by polarity factors. An unexpected different behavior between alpha-propyl and alpha-long-chain substituents is discussed in terms of different hybridization of the vinyl radical. (C) 1999 Elsevier Science Ltd. All rights reserved.