Dynamics of the dimethyl sulfide exchange of (1,3‐diphenylallyl)dimethylsulfonium ions

Abstract

The dynamics of the allylic rearrangement of the (1,3‐diphenylallyl)dimethylsulfonium ion in CD2Cl2, which proceeds via intermediate 1,3‐diphenylallyl cations, has been investigated by variable temperature 1H NMR spectroscopy. At low temperature, the three allylic protons give rise to an AMX system, and the two diastereotopic S‐methyl groups resonate at different frequencies. At higher temperature, an AX2 system for the allylic protons and a single signal for the S‐methyl groups are observed. The resulting exchange rate constant of (364 ± 2) s–1 at 25°C, which corresponds to the rate of the heterolytic cleavage of the C–S bond, was used to explore the range of validity of the linear free energy relationship log khet(25°C) = sf (Nf + Ef), which describes the rates of heterolytic cleavages by the electrofugality parameter Ef and the solvent‐dependent nucleofuge‐specific parameters Nf and sf. The observed rate constant corroborates a previous conclusion that two different sets of Nf and sf parameters may exist for the same nucleofuge. Knowledge of whether the reverse bond‐forming reaction occurs under activation or under diffusion control is crucial for the choice of the appropriate set of nucleofugality parameters.