Conformations of Chiral α,β-Unsaturated Sulfoxides and Their Complexes with Lewis Acids. An ab Initio Study

Abstract

The rotational potential energy surface (PES) of methyl vinyl sulfoxide (1) was calculated at MP2/6-31+G*, B3LYP/6-311+G*, B3LYP/3-21G(*), MP2/3-21G(*), PM3, and AM1. All ab initio methods gave two conformations: one with the S−O bond (a) syncoplanar to the carbon−carbon double bond and one with the lone pair in plane (b); the first one is preferred by 1.7 kcal mol-1. The energy difference is strongly basis set dependent: B3LYP/6-311+G* offers the best compromise. Semiempirical methods give a qualitatively different rotational PES. Whereas the effect methyl group in the E-position is small, Z-substitution leads to destabilization of conformation a through sterical interaction, so that conformation b is preferred by 0.4 kcal mol-1. Electron-withdrawing substituents such as ester or keto groups in the α-position destabilize conformations c and d where the lone pair is syncoplanar to the CC double bond, so that the syncoplanar orientation of the S−O bond is favored by ca. 5 kcal mol-1, depending on the sub...