The alkyl/phenyl-folded conformation of alkyl 1-phenylethyl sulfides and sulfones as evidenced by ab initio MO calculations. Implication for the 1,2-asymmetric induction

Abstract

Ab initio MO calculations were carried out, at the MP2/6-311G(d,p)//MP2/6-31G(d) level, to investigate the conformational energy of alkyl 1-phenylethyl sulfides C₆H₅CHCH₃SR and sulfones C₆H₅CHCH₃SO₂R (R = CH₃, C₂H₅, i-C₃H₇, t-C₄H₉). In every case, a rotamer whereby the alkyl group (R) is synclinal to C₆H₅ (Ph) and antiperiplanar to the benzylic methyl group (Ph–C–S–R torsion angle ϕ ≅ 60°) has been found the most stable. Interatomic H/C_ipso distances at the stable geometries are found to be short. The results are interpreted in the context of the CH/π hydrogen bond occurring between CHs in R and the π-system of Ph. The implication of the above results to the stereochemical mechanism of the oxidation of sulfides to give diastereoisomeric sulfoxides was discussed. The mechanism of 1,2-asymmetric induction (Cram's open-chain model) was suggested to be explained on the basis of a simple premise that the geometries of the transition state resemble those of the ground state conformation of reactants.