Electron-Transfer Oxidation of Ketene Silyl Acetals and Other Organosilanes. The Mechanistic Insight into Lewis Acid Mediated Electron Transfer

Kinetic studies on photoinduced and thermal electron-transfer oxidation of a variety of organosilanes in acetonitrile at 298 K are reported in terms of the electron-transfer rate constants (k_et) with a series of oxidants having the known one-electron reduction potentials (E⁰_red). The Rehm-Weller Gibbs energy relationship is applied to determine the fundamental parameters for the electron-transfer oxidation, i.e., the one-electron oxidation potentials (E⁰_ox) and the intrinsic barrier for the electron-transfer oxidation (ΔG^╪_o). The E°m and ΔG^╪₀ values thus obtained are compared with the calculated values of the adiabatic ionization potentials (I_a) and the inner-sphere reorganization energies (λ_i) associated with the structural change upon electron-transfer oxidation by using the PM3 molecular orbital method. Ketene silyl acetals, especially hindered ones, are shown to act as unique and strong electron donors as compared to other organosilanes. On the other hand, Lewis acids such as SnCl₄, Ph₃SiClO₄, and Et₃SiClO₄, which catalyze the addition of hindered ketene acetals to α-enones, are shown to act as strong electron acceptors in the electron-transfer oxidation of ferrocene derivatives. The mechanistic insight to the electron-transfer oxidation of organosilanes, particularly in the case of hindered ketene silyl acetals which are employed in Lewis acid promoted carbon–carbon bond formation reactions, is discussed on the basis of the fundamental parameters for the electron-transfer oxidation.