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

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 C C double bond, so that the syncoplanar orientation of the S−O bond is favored by ca. 5 kcal mol - 1, depending on the substituent. The structures of the chelate complexes 12 of α-(methanesulfinyl)acrylic acid methyl ester ( 6 ) with Me 2 AlCl were also optimized with B3LYP/6-311+G*. The complex 12c with the aluminum bonded to the carboxylic and the sulfoxide oxygen was calculated to be the most stable. The solvation energies of the complexes were calculated using the SCIPCM model, which allowed an approximation of the upper limit of the energy needed for the formation of the complexes.