Intramolecular Hydrogen-Atom Transfer in 2-Alkylbenzoyloxyl Radicals as Studied by Transient Absorption Kinetics and Product Analyses on Photodecomposition of Bis(2-alkylbenzoyl) Peroxides

Article abstract of DOI:10.1246/bcsj.68.1213

The pulsed-laser excitation of bis(2-methylbenzoyl) peroxide at 308 nm in acetonitrile afforded a broad absorption band at 500-800 nm due to 2-methylbenzoyloxyl radicals.The decay of this band accompanied the growth of another band at 350 nm due to 2-carboxybenzyl radicals produced by an intramolecular hydrogen-atom transfer from the neighboring 2-methyl group; the rate constant was 1.7*10⁷ s^-1 at 23 deg C, the activation energy and frequency factor being 17 kJ mol^-1 and 10^10.5 s^-1, respectively.The rates for intramolecular hydrogen-atom transfer in2-MeCH2C6H4CO2. and 2-PhCH2C6H4CO2. are much higher than that in 2-CH3C6H4CO2., since the parent peroxides, (2-MeCH2C6H4CO2)2 and (2-PhCH2C6H4CO2)2, exhibited only 350-nm bands ascribable to the corresponding 2-carboxybenzyl radicals, even immediately after laser excitation.The pulsed-laser photolyses of the above-mentioned peroxides afforded PhCH2R, 2-HOCOC6H4CH(R)CH2CN, and 2-HOCOC6H4CHRCHRC6H4CO2H-2 (R=H, Me and Ph) as the main products in acetonitrile.The formation of PhCH2R is explained in terms of the contribution of two-bond fission of the O-O and C(α)-C bonds of the peroxide in the excited singlet state in competition with O-O bond cleavage followed by an intramolecular hydrogen-atom transfer.