General Preference for Alkyl/Phenyl Folded Conformations. Relevance of the CH/(pi) and CH/O Interactions to Stereochemistry as Evidenced by Ab Initio MO Calculations

Ab initio MO calculations were carried out, at the MP2/6-311G(d,p)//MP2/6-31G(d) level, to determine the conformational energy of alkyl 1-phenylethyl sulfoxides, CH3CH(C6H5)-S(O)-R (R = CH3, C2H5, i-C3H7, and t-C4H9). In every case, a geometry (rotamer a, R/C6H5 torsion angle (phi) ~ 60°) where the alkyl group (R) is gauche to the C6H5 (Ph) and anti to the benzylic methyl group (Me) has been found to be the most stable. Rotamer b ((phi) ~ 300°), where R is flanked by Me and Ph, has been shown the next most stable in every case except for isopropyl and t-butyl analogue with the threo configuration. The R/Ph anti conformation (rotamer c: (phi) ~ 180°) has been found the least stable. Interatomic distances (CH/(pi) as well as CH/O) at the stable geometries are found to be very short. These results are interpreted in the context of the attractive CH/(pi) and CH/O hydrogen bonds and the unfavorable electrostatic interaction between the S-O dipole and the quadrupole of the phenyl group. In view of the above result and of previous findings, the alkyl/phenyl congested folded conformation of aralkyl compounds has been suggested to be a general phenomenon.