Photoinduced Electron Transfer Double Fragmentation: An Oxygen-Mediated Radical Chain Process in the Cofragmentation of Aminopinacol Donors with Organic Halides

摘要

Investigations of photoinduced electron transfer reactions have led to the finding of several ion-radical fragmentation reactions in which relatively strong covalent bonds in the neutral (or starting molecule) rapidly cleave in the one-electron redox product. Among oxidized donors that undergo relatively rapid bond-breaking reactions are 1,2-diarylethanes, pinacols, diamines, and amino alcohols. One-electron reduction of acceptors such as ethers, esters, and organic halides can also result in bond cleavage; the cleavage of organic halides is particularly interesting since electrochemical studies as well as thermochemical calculations indicate that the carbon halide bond can have a negative bond energy in certain reduced halides and that the cleavage process must be extremely rapid. The efficiency of these reactions is determined by the relative rates of back electron transfer (k_(-et)), fragmentation (k_r), and separation of the ion-radical pair (k_(sep)). While some examples of fast bond cleavage of ion radical systems have been reported and high quantum efficiencies have been obtained, the quantum yield of fragmentation reactions is generally low due to the fast back electron transfer (k_(-et)), which can be in the range of ～10~9-10~(11) s~(-1) when singlet excited state quenching is involved.