Base‐Catalyzed Hydrogen‐Deuterium Exchange in Sulfonium Cations

The effects of various structural changes on the kinetic acidity of protons alpha to a sulfonium function are reported. These effects are consistent with a transition state closely resembling a carbanion. Strong rate enhancements are observed for the 4‐membered cyclic sulfonium cation but not for the other ring systems. In 5‐membered cyclic sulfonium cations the diastereotopic ring protons show high stereoselectivity towards abstraction by a base. Such stereoselectivity is diminished in going from D 2 O to dimethyl sulfoxide‐d 6 ‐D 2 O mixtures. By contrast no stereoselectivity is exhibited by the ring protons of the 6‐ and 7‐membered cyclic sulfonium cations. Although the rates for acyclic and cyclic sulfonium cations are closely similar, the activation parameters are characteristically different. It is suggested that solvation phenomena may be responsible for this behaviour.