Dynamic stereochemistry of imines and derivatives. Part V. Acid catalysis of E–Z imine interconversion

Abstract

The rate of E–Z isomerization in N-(α-phenyl-4-nitrobenzylidene)methylamine at 35° is shown to be markedly increased by trace amounts of benzoic acid. Dynamic n.m.r. experiments at higher temperature show that the isomerization rate is proportional to the acid concentration and enable activation parameters to be determined. CNDO/2 Calculations performed on the benzylideneammonium ion suggest that the C[graphic omitted] rotational barrier is ca.46 kcal mol^–1, and high temperature n.m.r. studies on a related immonium perchlorate also indicate a high rotational barrier. It is suggested that benzoic acid promotes imine isomerization by nucleophilic addition to the C[graphic omitted] bond in the conjugate acid. The relevance of the rate of imine isomerization to the mechanism of the imine–peroxyacid reaction is discussed.