Role of electron-transfer processes in reactions of diarylcarbenium ions and related quinone methides with nucleophiles

Second-harmonic alternating current voltammetry has been used to determine one-electron reduction potentials of 15 diarylcarbeniurn ions and 5 structurally analogous quinone methides, which have been employed as reference electrophiles for the development of nucleophilicity scales. A linear correlation (r(2) = 0.993) between the empirical electrophilicity parameters E and the reduction potentials in acetonitrile (E = 14.091 E-red(o) - 0.279) covering a range of 1.64 V (or 158 kJ mol(-1)) has been observed. For a large number of nucleophiles, it has been demonstrated that the observed activation free energies of the electrophile-nucleophile combinations are 61-195 kJ mol(-1) smaller than the free energy change of electron transfer from nucleophile to electrophile, which definitely excludes outer-sphere electron transfer occurring during these reactions.