Mesolytic Cleavage of the O-CO-Bond in Enol Acetate Cation Radicals with Direct Formation of α-Carbonyl Cations. Mechanistic and Synthetic Aspects

Article abstract of DOI:10.1021/jo00114a020

For the first time, enol ester cation radicals are reversibly monitored in a cyclic voltammetry experiment.Preparative one-electron oxidation of enol acetates A1-A4 leads to the formation of benzofurans B1-B4 through mesolytic O-CO bond fragmentation to α-carbonyl cations and the acetyl radical.With A3(.+), the kinetics of the O-CO bond cleavage was investigated by cyclic voltammetry, providing ΔH(excit.) = 17.0 kcal mol^-1 and ΔS(excit.) = 11 cal mol^-1 K^-1 in dichloromethane.The slioghtly increased rate of bond dissociation upon adition of acetonitrile is explained with charge localization in the transition state rather than with a solvent-assisted bond cleavage mechanism.The occurrence of curve crossing and isopotential points in the cyclic voltammograms of the model compounds a1-A4 at low scan rates can be rationalized by a multiparameter reaction scheme based on an ECCE_DISP mechanism, digital simulation of which confirmed the cleavage selectivety and allowed for the determination of the involved rate constants of the homogeneous chemical reaction steps.