Reducing Power of Photogenerated α-Hydroxy Radicals. Proton-Coupled Electron Transfer

Abstract

Proton-coupled electron transfer (PCET) has been reported in a number of systems. The oxidation product of a disubstituted α-hydroxy radical is a protonated ketone. In a case where electron transfer from such a radical to an electron acceptor is energetically unfavorable, it was of interest to probe whether electron transfer would occur if coupled with proton transfer to an added base. Through an experimental design based on fragmentation of benzopinacol radical cation, diphenylketyl radical (Ph2COH•, Eox = −0.25 V vs SCE) was photochemically generated as the only radical intermediate in a moderately polar solvent (1,2-dichlorethane). Direct electron transfer to 1,2,4,5-tetracyanobenzene (TCB, Ered = −0.65 V vs SCE) is endothermic by 0.4 eV and does not occur. In the presence of pyridine derivatives, however, PCET does, indeed, take place. In these termolecular reactions the electron is transferred to one molecule and the proton to another. Detailed kinetic studies by laser flash photolysis showed that a hydrogen-bonded complex between the ketyl radical and the pyridine base is formed, which then reacts with TCB, leading to TCB•-, benzophenone, and the protonated base. In these experiments, two TCB•- are formed per absorbed photon in a very clean reaction. The equilibrium constants for complex formation decrease with decreasing pKa of the base (∼18, 6, and 2 M-1 for 2,6-lutidine, 3-chloropyridine, and 2-chloropyridine, respectively). When the driving force for the overall reaction is ∼0.2−0.4 eV, the PCET rate constant reaches 1.5 × 109 M-1 s-1, which is one-fifth of the diffusion-controlled limit in dichloroethane. However, as the reaction becomes nearly isoenergetic, the PCET rate constant drops by a factor of 4. The deuterium isotope effect of ∼3.2 for the PCET reaction with 2-chloropyridine as base is consistent with a concurrent electron/proton transfer.