Concerted and Stepwise Solvolytic Elimination and Substitution Reactions:  Stereochemistry and Substituent Effects

Abstract

Solvolysis of the R,R and R,S isomers 2a-X and 2b-X, respectively, (X = I, Br, OBs) in 25 vol % acetonitrile in water gives the elimination products 4, 5a, and 5b along with the substitution products 2a-OH, 2b-OH, 2a-NHCOMe, and 2b-NHCOMe . The rates of elimination ( k E ) increase with increasing acidity of the β-hydrogen of the substrate as expressed by a Brønsted parameter of α = 0.08 and 0.07 for the iodides and the bromides, respectively. In contrast, the brosylates, which give much smaller fractions of the stable alkenes, exhibit a negative Brønsted parameter for the elimination, α = −0.10. This shows that the brosylates, in contrast to the halides, do not give elimination exclusively by a solvent-promoted E1cB reaction or an E2 reaction with a large component of hydron transfer in the transition state and indicates a large amount of reaction through carbocation intermediates to give stable alkenes.