Electron transfer and bond breaking: Recent advances

After a reminder of concerted/stepwise mechanistic dichotomy and other basic concepts and facts in the field, a series of recent advances is discussed. Particular emphasis is laid on the interactions between the fragments formed upon bond cleavage. These interactions may persist even in polar solvents and have important consequences on dissociative electron transfer kinetics and on the competition between concerted and stepwise pathways. Cleavage of ion radicals and its reverse reaction are examples of single electron transfer reactions concerted with bond cleavage and bond formation, respectively. The case of aromatic carbon-heteroatom bonds is particularly worth examination since symmetry restrictions impose circumventing a conical intersection. Reductive dehalogenases are involved in 'dehalorespiration' of anaerobic bacteria in which the role of dioxygen in aerobic organisms is played by major polychloride pollutants such as tetrachloroethylene. They offer an interesting illustration of how the coupling of electron transfer with bond breaking may be an important issue in natural processes. Applications of dissociative electron transfer concepts and models to mechanistic analysis in this class of enzymes will be discussed.