One-electron oxidation of closed-shell molecules. Part 3. Oxidative cleavage of 1,2,2,2-tetrakis-(p-methoxyphenyl)ethanone with dibenzoyl and bis-(3,5-dinitrobenzoyl) peroxides: mechanistic changeover of the peroxide function from radical to molecular oxidation

Abstract

1,2,2,2-Tetrakis-(p-methoxyphenyl)ethanone (anispinacolone)(1) is cleaved by dibenzoyl peroxide (2) or bis-(3,5-dinitrobenzoyl) peroxide (3), affording tris-(p-methoxyphenyl)methyl benzoate (or 3,5-dinitrobenzoate) and benzoic (or 3,5-dinitrobenzoic)p-methoxybenzoic anhydride as the principal cleavage products. ¹³C N.m.r. CIDNP studies by use of labelled anispinacolone (An₃*C·*CO·An; *C 90%¹³C) indicated that p-methoxybenzoyl radical is formed, presumably by way of the radical cation [anispinacolone]⁺˙ which is produced by a single-electron transfer (s.e.t.) mechanism. The formation of the p-methoxybenzoyl radical was also indicated by spin-trapping experiments. The decomposition rates of (2) at 50.0°C are unaltered on addition of (1) in nonpolar solvents such as dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, and benzene, whereas those of (3) are markedly accelerated. The cleavage of (1) by (2) is suppressed by added 3,4-dichlorostyrene by a factor of 6.7, whereas that of (1) by (3) is almost unaffected. These results suggest that in the case of dibenzoyl peroxide (2) the thermally produced benzoyloxyl radical works as a one-electron acceptor (or oxidant) upon (1), whereas when bis-(3,5-dinitrobenzoyl) peroxide (3) is used the peroxide molecular oxidizes (1), probably by way of an s.e.t. mechanism even in such nonpolar solvents. On the other hand, in polar solvents such as (CF₃)₂CHOH, tetramethylene sulphone, and acetonitrile the decomposition of (2) is accelerated by added anispinacolone, suggesting that the intermolecular s.e.t. reaction is partially involved in such polar solvents. Consequently, the oxidative cleavage of anispinacolone (1) by diaroyl peroxide provides the first example of dichotomy in the s.e.t. reaction of diaroyl peroxides, which can be considered a counterpart of the S_N1–S_N2 dichotomy in nucleophilic substitution, as far as the molecularity of the peroxide is concerned.