Z–E isomerisation of N-sulphenylimines

Abstract

The free energies of Z–E isomerisation of a wide range of symmetrical N-sulphenylimines have been measured at the coalescence temperature by a ¹H n.m.r. procedure. In two cases the rate of isomerisation of unsymmetrical N-sulphenylimines at a series of temperatures has been measured and activation enthalpies and entropies determined. The effect of substituents on the isomerisation barrier ΔG* has been interpreted by the hyperconjugation of the nitrogen lone pair with substituents on imine carbon and on nitrogen in a linear transition state adopted for the inversion process. The results are discussed in orbital terms using an extended Hückel perturbation approach, which can be used to interpret qualitatively the effect of substituents on the inversion barrier of a wide range of imino compounds, including guanidines and imino carbonates.