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
作者:Jun Wang、Masahiro Tsuchiya、Katsumi Tokumaru、Hirochika Sakuragi
DOI:10.1246/bcsj.68.1213
日期:1995.4
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 °C, the activation energy and frequency factor being 17 kJ mol−1 and 1010.5 s−1, respectively. The rates for intramolecular hydrogen-atom transfer in 2-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.
双(2-甲基苯甲酰基)过氧化物在乙腈中受到 308 纳米波长的脉冲激光激发后,在 500-800 纳米波长处产生了一条由 2-甲基苯甲酰基自由基产生的宽吸收带。在该吸收带衰减的同时,另一条波长为 350 纳米的吸收带也随之增长,该吸收带是由邻近的 2-甲基发生分子内氢原子转移而产生的 2-羧基苄基自由基引起的;23 ℃ 时的速率常数为 1.7 × 107 s-1,活化能和频率因子分别为 17 kJ mol-1 和 1010.5 s-1。2-MeCH2C6H4CO2- 和 2-PhCH2C6H4CO2- 分子内氢原子转移的速率远高于 2-CH3C6H4CO2- 分子内氢原子转移的速率,因为母体过氧化物 (2-MeCH2C6H4CO2)2 和 (2-PhCH2C6H4CO2)2 即使在激光激发后立即也只显示出相应 2-羧基苄基自由基的 350 纳米波段。在乙腈中,上述过氧化物经脉冲激光光解后的主要产物为 PhCH2R、2-HOCOC6H4CH(R)CH2CN 和 2-HOCOC6H4CHRCHRC6H4CO2H-2(R = H、Me 和 Ph)。PhCH2R 的形成可以解释为过氧化物在激发单线态中的 O-O 键和 C(α)-C 键的双键断裂与 O-O 键断裂竞争,然后发生分子内氢原子转移。