Radical-Chain Addition of Benzenethiol to Allenes. Analysis of Steric Effects and Reversibility

Article abstract of DOI:10.1021/jo00327a001

The radical-chain addition of benzenethiol to the monoalkylallenes 1a-e and the 1,1-dialkylallenes 6 and 8 has been studied.Attack by the benzenethiyl radical occurs at C2 and C3 of 1b-e in a ratio of 83:17.Increased attack (25percent) at C3 occurs with tert-butylallene (1a).The 1,1-dialkylallenes undergo attack only at C2.Deuterium-labeling studies indicate that the thiyl radical addition step is not detectably reversible under the reaction conditions.The internal alkene adducts 2, 3, 9 and 10 are the kinetically favored products.Rapid and reversible addition of the thiyl radical to the adducts results in complete stereochemical equilibration during the course of the reaction.The hydrogen atom abstraction step is reversible under the reaction conditions with 1a and 8 but not with 6 or the other monoalkylallenes.Treatment of the adducts derived from 1a and 8 with a catalytic quantity of iodine results in isomerization to the more thermodynamically stable adducts 4 and 11.The kinetics and thermodynamics of these reactions are discussed, and steric strain energies for the CH3...SC6H5 and t-Bu...SC6H5 interactions have been estimated to be 0.8-0.9 and 3.9-4.0 kcal/mol, respectively.