INTRAMOLECULAR HYDROGEN-ATOM TRANSFER IN 2-ALKYLBENZOYLOXYL RADICALS AS STUDIED BY TRANSIENT ABSORPTION KINETICS AND PRODUCT ANALYSES ON THE PHOTODECOMPOSITION OF BIS(2-ALKYLBENZOYL) PEROXIDES

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 produ ced by an intramolecular hydrogen-atom transfer from the neighboring 2 -methyl group; the rate constant was 1.7x10(7) s(-1) at 23 degrees 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 in 2-MeCH(2)C(6)H(4)CO(2) . and 2-PhCH(2)C(6)H(4)CO(2) . are much higher than that in 2-CH3C6H4CO2 ., since the parent peroxides, ( 2-MeCH(2)C(6)H(4)CO(2))(2) and (2-PhCH(2)C(6)H(4)CO(2))(2), exhibited only 350-nm bands ascribable to the corresponding 2-carboxybenzyl radi cals, even immediately after laser excitation. The pulsed-laser photol yses of the above-mentioned peroxides afforded PhCH(2)R, 2-HOCOC6H4CH( R)-CH2CN, and 2-HOCQC(6)H(4)CHRCHRC(6)H(4)CO(2)H-2 (R=H, Me, and Ph) a s the main products in acetonitrile. The formation of PhCH(2)R is expl ained in terms of the contribution of two-bond fission of the O-O and C(alpha)-C bonds of the peroxide in the excited singlet state in compe tition with O-O bond cleavage followed by an intramolecular hydrogen-a tom transfer.