Electroactive protecting groups and reaction units. Part 4. Mesolytic O–CO bond cleavage versus intramolecular cyclization reaction in enol trifluoroacetate cation radicals. A kinetic and mechanistic investigation

Abstract

The one-electron oxidation of two β,β-dimesityl enol trifluoroacetates with different substituents R at C_α was investigated. For the first time enol trifluoroacetate cation radicals could be reversibly monitored in cyclic voltammetry experiments allowing the determination of the rate constants of their follow-up reactions. While mesolytic cleavage of the O–CO bond (k_f= 1.4 s^–1) with direct formation of an α-carbonyl cation constitutes the primary reaction of the enol trifluoroacetate cation radical with R = Bu^t, the one with R = Ph exclusively cyclizes (k_f= 75 s^–1) to a phenanthrene derivative. This change in reactivity can be rationalized by the different nature of the electrophoric system as a function of the substituent R.