Heterogeneous catalysis and electrosorption phenomena in the cathodic reduction of acetylpyridines: Part II. Electrode kinetic investigations

Abstract

The mechanism of the cathodic reduction of three isomeric acetylpyridines has been evaluated by electrode kinetic and preparative measurements at the DME and rotating rod electrodes of different solid metal electrode materials. The influence of electrosorbed molecules, which may either act as proton donors and asymmetric inductors (strychnine, camphoric acid) or are chemically inert surfactants (tetraalkylammonium ions) on the electrode kinetics, has been studied. Up to pH 10, acetylpyridines are reduced from their N-protonated form. If the strongly pH-dependent reduction potential of the ketone is close to the E pzc of the electrode, electrosorbed ketyl radicals are formed in a 1 e −/1 H + EC reaction sequence and the pinacol is formed by heterogeneous dimerization of these adsorbed radicals. This holds for the 2- and 4-acetylpyridine reduction at low pH (1<pH<3.5). At higher pH (pH>4) and in the absence of electrosorbed proton donors, the alcohol formation proceeds by eventual irreversible protonation of the pyridyl-benzylic carbanion which is formed at more negative potentials in a fast and reversible (2 e −/H +)-reduction step. Reduction of 2- and 4-acetylpyridine in the presence of adsorbed proton donors (1<pH<7) is speeded up due to the participation of the adsorbed proton donor in the initial 2 e −/H + equilibrium. Heterogeneous protonation of the ketyl radical brought about by adsorbed chiral proton donors (inductor acid) is one of the key steps in cathodic asymmetric electro-organic synthesis. The ketyl radicals formed by a 1 e −/1 H + protonation reduction step from the three ketones possess relatively different basicities: for 2-acetylpyridine the radical is a 10 7 times stronger base, and for 4-acetylpyridine a 10 5 times stronger base than for 3-acetylpyridine. For the least basic ketyl radical of 3-acetylpyridine, only surfactants which are stronger acids, such as camphoric acid, show a decrease in lifetime and a speeding up of electrode kinetics which yields an optical induction.