Stereochemical studies. Part XLVI. The 2-alkylcyclohexyl tosylate solvolysis problem: the solvolysis of the 2-methyl-4-t-butylcyclohexyl tosylates

Abstract

Hypotheses formerly suggested to explain the solvolytic behaviour of the 2-alkylcyclohexyl tosylates are briefly reviewed. A study is reported of the acetolysis and ethanolysis of the four stereoisomeric 2-methyl-4-t-butylcyclohexyl tosylates. A comparison with analogous data for the cis- and trans-4-t-butylcyclohexyl tosylates clarifies the effects of a vicinal methyl group, in all four positions relative to the tosyloxy-group, on rate and product composition. The ethanolysis rate of (axial)cis-4-t-butylcyclohexyl tosylate is reduced by an axial 2-methyl group by a factor of 1·15 but accelerated by a factor of 9·5 by an equatorial 2-methyl group; the reaction of (equatorial)trans-4-t-butylcyclohexyl tosylate is slowed by a factor of about 3 by an equatorial 2-methyl but accelerated by factor of 30 by an axial 2-methyl group. The rate relationships for the acetolysis are quite similar. Olefins form 73–87% of the total product in acetolysis of all the four 2-methyl-4-t-butyltoluenesulphonates. The two isomers in which the toluenesulphonyloxy- and the methyl group are cis to each other afford a significantly higher proportion of rearranged products than do the others in which the tosyloxy- and the methyl groups are trans to each other. Further, the percentage of inverted unrearranged substitution products is notably lower in the ‘cis’- than in the ‘trans’-isomers. These results substantiate and extend former findings on conformationally less well-defined 2-alkylcyclohexyl tosylates (e.g., the methyl tosylate series) and are best rationalised (a) by postulating rate acceleration through hydrogen participation in the case of both‘cis’-isomers and (b) by assuming that the isomers with an equatorial tosyloxy-group react mainly, or exclusively, by way of a non-chair transition state.