Chemistry of sulphines. Part XXIV. A study of rotational isomerism in meta-disubstituted diphenyl sulphines in apolar and polar solvents by means of chemical shift differences

Abstract

The deshielding of the ortho-protons 2- and 6-H syn to the CSO system in 3,3′-disubstituted diphenyl sulphines (substituents Cl, NO₂, OMe, Me) is determined (Δ₂ and Δ₆) and discussed in terms of an equilibrium between a more and a less polar rotamer. This rotational equilibrium is strongly dependent on the polarity of the solvent. The n.m.r. spectrum of bis-3-chlorophenyl sulphine (Ia) in carbon tetrachloride shows that the deshielding of the two ortho-protons 2- and 6-H is about the same, which means that both rotamers are about equally populated. In acetone, however, only one proton (2-H) is shifted downfield, while the other proton (6-H) is incorporated in the multiplet of the other aromatic protons, indicating that the equilibrium is completely at the side of the more polar rotamer. The change in the equilibrium of bis-3-nitrophenyl sulphine (Ib) in going from deuteriochloroform to the polar solvent acetone is still more pronounced. While in deuteriochloroform the equilibrium is on the side of the less polar rotamer, the reverse is the case in acetone. The changes in the equilibrium of bis-3-methoxy-(Ic) and bis-3-methyl-phenyl sulphine (Id) are much smaller in going from an apolar to a polar solvent. The proportions of the rotamers are about the same in the apolar solvents carbon tetrachloride and hexadeuteriobenzene. The results are treated in a quantitative manner by applying the additivity rules of substituent constants and solvent effects of Diehl.