ELECTROACTIVE PROTECTING GROUPS AND REACTION UNITS .1. MESOLYTIC CLEAVAGE OF THE O-CO BOND IN ENOL ACETATE CATION RADICALS WITH DIRECT FORMATION OF ALPHA-CARBONYL CATIONS - MECHANISTIC AND SYNTHETIC ASPECTS

For the first time, enol ester cation radicals are reversibly monitore d in a cyclic voltammetry experiment. Preparative one-electron oxidati on of enol acetates A1-A4 leads to the formation of benzofurans B1-B4 through mesolytic O-CO bond fragmentation to alpha-carbonyl cations an d the acetyl radical. With A3(.+), the kinetics of the O-CO bond cleav age was investigated by cyclic voltammetry, providing Delta H double d agger = 17.0 kcal mol(-1) and Delta S double dagger = 11 cal mol(-1) K -1 in dichloromethane. The slightly increased rate of bond dissociatio n upon addition of acetonitrile is explained with charge localization in the transition state rather than with a solvent-assisted bond cleav age mechanism. The occurrence of curve crossings and isopotential poin ts in the cyclic voltammograms of the model compounds A1-A4 at low sca n rates can be rationalized by a multiparameter reaction scheme based on an ECCE(DISP) mechanism, digital simulation of which confirmed the cleavage selectivity and allowed for the determination of the involved rate constants of the homogeneous chemical reaction steps.