Direct Observation of Ultrafast Decarboxylation of Acyloxy Radicals <i>via</i> Photoinduced Electron Transfer in Carboxylate Ion Pairs
作者:T. Michael Bockman、Stephan M. Hubig、Jay K. Kochi
DOI:10.1021/jo9617833
日期:1997.4.1
radicals (RCO(2)(*)) rapidly lose carbon dioxide by C-CO(2) bond cleavage, in competition with back-electron transfer to restore the original ion pair, [MV(2+), RCO(2)(-)]. The decarboxylation rate constants for ArCH(2)CO(2)(*) lie in the range (1-2) x 10(9) s(-)(1), in agreement with previous reports. In striking contrast, the C-CO(2) bond scission in Ar(2)C(OH)CO(2)(*) occurs within a few picoseconds
电荷转移(CT)光活化的甲基紫精(MV(2+))与羧酸酯供体(RCO(2)(-))包括苯甲酸酯[Ar(2)C(OH)CO(2))的电子供体受体盐(-)]和芳基乙酸酯(ArCH(2)CO(2)(-))导致瞬态[MV(*)(+),RCO(2)(*)]自由基对。飞秒时间分辨光谱显示,光生的酰氧基(RCO(2)(*))通过C-CO(2)键断裂迅速失去二氧化碳,与背电子转移竞争以还原原始离子对[MV] (2 +),RCO(2)(-)]。与以前的报告一致,ArCH(2)CO(2)(*)的脱羧速率常数在(1-2)x 10(9)s(-)(1)范围内。与之形成鲜明对比的是,Ar(2)C(OH)CO(2)(*)中的C-CO(2)键断裂在几皮秒内发生(k(CC)=(2-8)x 10(11) s(-)(1))。这些供体的脱羧速率常数接近于无障碍单分子反应的速率常数。因此,对苯甲氧基自由基脱羧的实时监测代表了直接观察CC键断裂的过渡态的手段。