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Mesolytic Cleavage of the O-CO-Bond in Enol Acetate Cation Radicals with Direct Formation of α-Carbonyl Cations. Mechanistic and Synthetic Aspects
Schmittel, Michael,Heinze, Juergen,Trenkle, Holger
, p. 2726 - 2733 (2007/10/02)
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 ECCEDISP 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.
