Nucleophile-substrate interaction single-electron transfer

The nature of the interaction between an electron-rich species, a nucleophile Nu , and a substrate S capable of electrophilic reactivity has concerned chemists for a long time. A widely used mechanistic distinction is made between single-electron transfer (SET), leading to radical species (Scheme I, path a), and two-electron processes, involving direct coordination via a pair of unshared electrons (Scheme I, path b). 

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