Osamu Murai

One-electron oxidation of anispinacolone with diaroyl peroxides: Mechanistic changeover of the peroxide function from radical to molecular oxidation

Abstract Oxidative cleavage of anispinacolone (An3C·CO·An, An=p-methoxyphenyl) with diaroyl peroxides proceeds by electron-transfer mechanism, which starts with rate-determining decomposition of the peroxide, unimolecular for dibenzoyl and bimolecular for bis(3,5-dinitrobenzoyl) peroxide.

One-electron oxidation of closed-shell molecules. Part 4. Acid-induced oxidative cleavage of substituted 1,2,2,2-tetraphenylethanones (benzpinacolones) with diaroyl peroxides

The acid-induced oxidative cleavage of anispinacolone [1,2,2,2-tetrakis-(p-methoxyphenyl)ethanone] with diaroyl peroxides in 1,2-dichloroethane–trifluoroacetic acid (TFA) has been investigated. The principal two products after work-up are tris-(p-methoxyphenyl)methanol and p-methoxybenzoic acid; the latter was found as anhydrides in the reaction mixture. Free-radical formation in the course of the cleavage was verified by polymerization of added acrylonitrile in the oxidation by bis-(3,5-dinitrobenzoyl) peroxide. When the oxidation of [1,2-13C2]anispinacolone (90%13C) by dibenzoyl peroxide was carried out in a 13C n.m.r. probe, an emission peak, assigned to p-methoxybenzoic trifluoroacetic anhydride, was observed. The logarithms of the rate constants for oxidation of p-substituted benzpinacolones by dibenzoyl peroxide were linearly correlated with the oxidation potentials of the benzpinacolones. These results are consistent with a single-electron transfer (s.e.t.) pathway from benzpinacolones to dibenzoyl peroxide. The oxidation is first-order in each reactant and is promoted by TFA. The effect of TFA is accounted for by two factors, (i) assisted O–O bond cleavage of the peroxide radical anion by TFA, and (ii) the formation of protonated peroxide, a more powerful oxidizing species. The former factor is dominant at lower TFA concentrations (<0.05M), the latter at higher concentrations.

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

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 SN1–SN2 dichotomy in nucleophilic substitution, as far as the molecularity of the peroxide is concerned.

One-electron oxidation of closed-shell molecules. Part 1. Oxidative cleavage of para-substituted benzpinacolones by cerium(IV) ammonium nitrate

The stoicheiometry and kinetics of the CeIV oxidation of 1,2,2,2-tetrakis-(p-methoxyphenyl)ethanone (1a) have been investigated at 25.0 °C in acetic acid as solvent by the use of cerium(IV) ammonium nitrate as oxidant. The reaction occurs with the stoicheiometry 2 CeIV+ An3C·CO·An → 2 CeIII+ An3C++ An·CO+(An =p-methoxyphenyl), affording tris-(p-methoxyphenyl)methanol and anisic acid after work-up. The kinetics were monitored by u.v.–visible spectrophotometry of the tris-(p-methoxyphenyl)methyl cation, showing that the rate-determining step is first order in each reactant. Evidence was found for the formation of p-methoxybenzoyl radical by means of spin-trapping with 2,3,5,6-tetrakis(trideuteriomethyl)nitrosobenzene. The rate constants at 25.0 °C, the oxidation potential, as measured by cyclic voltammetry, and the charge transfer energy with tetracyanoethylene have been measured for five para-substituted benzpinacolones (substituents MeO, Me, and Cl), including (1a). A good correlation of the logarithms of the rates with the oxidation potentials or with the charge transfer energies was found, indicating that a single electron-transfer step between the substrate and CeIV is rate determining. The formation of a radical cation of the benzpinacolone in the rate-determining step and subsequent rapid cleavage into the triarylmethyl cation and the aroyl radical are proposed as a possible mechanism.