A search for maximal neighboring group participation in Solvolyses of allylic substrates. Ethanolysis of 2-(ω-alkylthioalkyl)-3-methyl-2-cyclohexenyl p-nitrobenzoates

Abstract

The solvolysis of 2-(ω-methylthioalkyl)- and 2-(ω-benzylthioalkyl)-3-methyl-2-cyclohexenyl p-nitrobenzoates 5 and 6 in 80 vol % ethanol includes several competitive reactions. Path k 1 is a stepwise process (k H/k D = 1.18–1.20) which includes allylic cation 9 as a reaction intermediate, while path k 2 involves a neighboring sulfur participation (k H/k D = 1.01–1.03) and formation of an intermediate cyclic sulfonium cation 10. Esters 5 and 6 solvolyze in 80% EtOH with a greater neighboring group participation than in 97% TFE. It has been concluded that in solvolytic reactions of allylic substrates a pronounced neighboring group participation (k 2/k 1 = 900) may occur only with very strong internal nucleophiles, and in solvents which can not form strong hydrogen bonds with these nucleophiles.