Direct Observation of Ultrafast Decarboxylation of Acyloxy Radicals via Photoinduced Electron Transfer in Carboxylate Ion Pairs

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Abstract
Charge-transfer (CT) photoactivation of the electron donor−acceptor salts of methylviologen (MV2+) with carboxylate donors (RCO2-) including benzilates [Ar2C(OH)CO2-] and arylacetates (ArCH2CO2-) leads to transient [MV•+, RCO2•] radical pairs. Femtosecond time-resolved spectroscopy reveals that the photogenerated acyloxy radicals (RCO2•) rapidly lose carbon dioxide by C−CO2 bond cleavage, in competition with back-electron transfer to restore the original ion pair, [MV2+, RCO2-]. The decarboxylation rate constants for ArCH2CO2• lie in the range (1−2) × 109 s-1, in agreement with previous reports. In striking contrast, the C−CO2 bond scission in Ar2C(OH)CO2• occurs within a few picoseconds (kCC = (2−8) × 1011 s-1). The rate constants for decarboxylation of these donors approach those of barrier-free unimolecular reactions. Thus, real-time monitoring of the decarboxylation of benziloxy radicals represents the means for the direct observation of the transition state for C−C bond scission.

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