Configuration of secondary amides and thioamides: spectroscopic and theoretical studies

Abstract

Infrared spectroscopy has been employed to study the cis–trans-equilibria and ΔH values in secondary amides and thioamides. Based on these results, several generalizations have been made regarding the structural factors affecting the configurations. Solvent effects on the cis–trans-equilibria and on the barrier height to rotation in secondary amides have been investigated by i.r. and n.m.r. spectroscopy; the barrier height increases with solvent polarity. Proton-donor or -acceptor solvents significantly affect the cis–trans equilibrium in amides. Solvent effect and temperature effect studies on the n.m.r. and i.r. spectra of 8-, 9- and 13-lactams and their N-methyl derivatives show that in 9-lactam and N-methyl-13-lactam both cis- and trans-isomers are present. Molecular orbital calculations satisfactorily predict relative stabilities of the different configurations of secondary amide derivatives; oxygen protonation of amides is favoured over nitrogen protonation. In the O-protonated amides, the barrier height to rotation is appreciably larger than in the parent amides.