Thermodynamic and kinetic basicities of the carbon and oxygen ends of enolates.: Their role in the reductive cleavage electrochemistry of α-substituted acetophenones

The electrochemical reductive cleavage of alpha-substituted acetophenones may follow a mechanism in which electron transfer and bond breaking are concerted, as with alpha-chloro-acetophenone, or a mechanism where the two steps are successive, as with, e.g. alpha-benzoyloxy-acetophenone. In both cases, the resulting phenacyl radical is immediately reduced, giving rise to the phenacyl enolate, the protonation of which is expected eventually to yield acetophenonel However, in cyclic voltammetry, the acetophenone wave, present at low scan rates, vanishes upon raising the scan rate. The disappearance of the wave is observed at lower scan rates when an acid, such as phenol, is added to the solution. This surprising behavior is the result of the oxygen end of the enolate being a thermodynamically weaker but kinetically faster base than its carbon end. (C) 1999 Elsevier Science S.A. All rights reserved.