Electrochemistry of electron-transfer probes. The role of the leaving group in the cleavage of radical anions of alpha-aryloxyacetophenones

The formal reduction potential (E degrees) of alpha-phenoxyacetophenone has been determined from the voltammetric peak potential obtained by linear sweep voltammetry in combination with the rate constant for fragmentation of the radical anion which had been determined by laser flash photolysis. The E degrees values of a number of alpha-aryloxyacetophenones were then estimated from a correlation of the C-13 chemical shifts of the carbonyl carbon and a similar correlation (E degrees versus C-13 chemical shift) within a series of substituted alpha-anilinoacetophenones. Using these potentials (which vary by only 34 mV over a wide range of substituents) the rate constants for fragmentation of the alpha-aryloxyacetophenone radical anions were determined from digital simulation of the corresponding voltammetric waves. The fragmentation rate constants were shown to correlate with the pK(a) of the corresponding phenols. However, the kinetic range was too small and the experimental errors too large to allow a distinction between a linear and quadratic free energy dependence. A thermochemical analogy between the leaving group ability in reactions of radical anions and that in simple heterolysis of closed shell compounds is developed. The utility of these compounds as potential electron transfer probes is discussed.