Configurational and Conformational Preferences in Oximes and Oxime Carbanions. Ab Initio Study of the Syn Effect in Reactions of Oxyimine Enolate Equivalents
Geometries and relative energies of stationary structures of several conformers of geometrical isomers of NO s-trans-configured acetaldoxime are reported. The calculated energies and geometries agree well with comparable experimental data. Effects of the theoretical model on the NO bond lengths are discussed for formaldoxime. (E)-Acetaldoxime prefers the conformation in which the in-plane CH3 hydrogen and the CN double bond are cis; the staggered conformation is the transition-state structure for CH3 rotation (Ea = 1.9 kcal mol-1). In contrast, the rotation of the CH3 group is virtually free in the Z isomer. (Z)- and (E)-acetaldoxime are almost isoenergetic, but their anions show a significant syn preference. Planar carbanions are minima, and all of the conformers with pyramidal CH2 groups are transition-state structures for narcissistic rotation around the CC bond (Ea= 23.3-29.9 kcal mol-1). At RHF/6-31+G*//RHF/3-21+G and at RHF/6-31+G*// RHF/6-31+G* the syn anion of acetaldoxime is preferred by 2.6 kcal mol-1(2.0 kcal mol-1 including VZPEs), and CH3/H substitution at the hetero-carbonyl C atom increases the syn preference energy to 7.3 kcal mol-1. Electron density and analysis of the anions and the anti preference of the corresponding isomeric radicals (1.2 kcal mol-1 at PMP3/6-31G*//UHF/3-21G + VZPEs) show that 1,4-through-space conjugative stabilization of the syn anion is insignificant. Electrostatic effects appear to be more likely as the origin of the syn preference of the anions. Comparisons between the geometries of the free anions and those of the anions in the lithium and sodium ion pairs provide additional evidence for the ionic nature of the coordination of the metals. The theoretical results suggest that the regiochemistry of enolate equivalents of oxyimines in dissociating solvents is due to the thermodynamic syn preference of the anions. Syn/anti isomerization of the anions (Ea < 26 kcal mol-1) is rapid even at low temperatures. In contrast, the anti preference of the radicals of acetaldoxime indicates that the formation of the syn products in oxidative coupling reactions of the anions of oxime ethers is a kinetic effect.
R. Glaser and A. J. Streitwieser, "Configurational and Conformational Preferences in Oximes and Oxime Carbanions. Ab Initio Study of the Syn Effect in Reactions of Oxyimine Enolate Equivalents," Journal of the American Chemical Society, vol. 111, no. 19, pp. 7340-7348, American Chemical Society (ACS), Sep 1989.
The definitive version is available at https://doi.org/10.1021/ja00201a010
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