Thermal racemisation of N-unsubstituted and various N-substituted S-o-methoxyphenyl S-phenyl sulphimides by pyramidal inversion

Abstract

The rates of the thermal racemisation by pyramidal inversion of optically active N-unsubstituted and N-substituted S-o-methoxyphenyl S-phenyl sulphimides have been measured. In the case of N-arylsulphonylsulphimides [Ar¹S (:NSO₂Ar²)Ar³] and N-acylsulphimides [Ar¹S(:NCOR)Ar²], the electron-withdrawing group (Ar² or R) retarded the rate of pyramidal inversion. In keeping with the observed trend, the enormous rate differences among the sulphoxide, the sulphimide, and the sulphonium ylide, each having a semi-polar bond, are considered to be predominantly due to the differences of electronegativities of the atoms bonded to sulphur (O 3.5, N 3.0, C 2.5). Intramolecular hydrogen bonding between the oxygen atom of the o-methoxy-group and the hydrogen atom of the imino-group plays an important role in the pyramidal inversion of S-o-methoxyphenyl S-phenyl sulphimide. A correlation has been found between the pyramidal inversion and the i.r. stretching frequency (ν_S–N,) of the S^IV–N bond of sulphimides.