Photochemistry of α-oxo-oximes. Part 4. Spectral properties, conformations, and photoisomerization of α-oxo-oximes and their acetates

Abstract

The α-oxo-oximes (1a–j) and (3a) and some of their esters have been prepared by ground state chemistry. In all cases the (E)-isomers were formed, except for (3a)(which was a mixture of E and Z). The (Z)-isomers of (1b–e) were obtained by irradiation of the corresponding (E)-isomers. The configurational and conformational structure of the various compounds has been assigned on the basis of n.m.r. (¹H and a.s.i.s.), i.r., and u.v. data. (E)- and (Z)-(3) have a fixed OC–CN structure. The (E)-isomers of the acyclic compounds exist in the OC–CN s-trans conformation which is twisted (i.e. non-planar) for the compounds which contain a phenyl adjacent to the carbonyl group. The (E)-isomers of compounds with a phenyl adjacent to the iminyl carbon have a dihedral angle of ca. 40° between the phenyl and the nodal plane of the iminyl group as a result of repulsion between the phenyl and the oxime oxygen. The (Z)-isomers of the acyclic compounds also exist in the OC–CN s-trans conformation which is non-planar due to repulsion between the oxime oxygen and the group bound to the carbonyl carbon. However, the phenyl is now fully conjugated with the iminyl group. The cyclopropyl group of (1e) has the bisected orientation relative to the carbonyl group s-trans for (E)-(1e) and s-cis for (Z)-(1e). Irradiation of the α-oxo-oximes (1b–e), (1j), and (3a), and of the acetates (2b–e), (2j), and (3b) leads to E–Z isomerization with formation of a photostationary state. The (E)-isomers of the oximes (1a), (1f–i) and of the acetates (2a, f, and h) and (4a–c) did not photoisomerize.