Electrophilic Reactivity of the α,α-Dimethylbenzyl (Cumyl) Cation

Abstract

The cumyl cation was generated by laser flash photolysis of cumyl tris(4-chlorophenyl)phosphonium tetrafluoroborate in CH2Cl2 and identified by its UV spectrum. From the decay of its absorbance at λ = 335 nm in the presence of variable concentrations of several nucleophiles with CC double bonds, rate constants for the reactions of the cumyl cation with these π-nucleophiles were determined. The linear free energy relationship log k20°C = s(N + E) (eq 1) was used to calculate the electrophilicity parameter E = 5.74 of the cumyl cation from the rate constants determined in this work and the previously reported N and s parameters of the nucleophilic reaction partners. Substitution of E of the cumyl cation and of the previously reported N and s parameters of α-methylstyrene into eq 1 predicts the temperature-independent rate constant of the addition of the cumyl cation to α-methylstyrene (1.2 × 108 M−1 s−1), which is relevant for the cationic polymerization of α-methylstyrene.