Electrons and Holes as Catalysts in Organic Electrosynthesis

The injection (or removal) of electrons into (or from) a substrate by an electrode can effectively catalyze various redox-neutral reactions that otherwise require harsh conditions and/or the use of reagents. Such processes involve the electrogeneration of an ionic or radical ionic species, which after a coupled chemical step either undergoes a backward electron exchange with the electrode (ECEb mechanism) or triggers a chain process in the bulk solution. Under these circumstances, sub-stoichiometric amounts of charge are sufficient to achieve a full conversion and conceptionally, the electrons and holes can be understood as being catalysts. This principle has been successfully applied to accomplish a number of redox-neutral transformations such as molecular rearrangements, Diels-Alder-type cycloadditions and radical substitution reactions (S(RN)1) in a mild and atom-economical fashion. Although examples have been reported but sporadically since the early 1970s, a number of exciting recent developments have led us to review and discuss these cases using unifying mechanistic concepts that are described herein.