Pyridinium Salts as Redox-Active Functional Group Transfer Reagents

In this Review, we highlight recent advances in the understanding and design of N-functionalized pyridinium scaffolds as redox-active, single-electron, functional group transfer reagents. We provide a selection of representative methods that demonstrate reactivity and fundamental advances in this emerging field. The reactivity of these reagents can be divided into two divergent pathways: homolytic fragmentation to liberate the N-bound substituent as the corresponding radical or an alternative heterolytic fragmentation that liberates an N-centered pyridinium radical. A short description of the elementary steps involved in fragmentation induced by single-electron transfer is also critically discussed to guide readers towards fundamental processes thought to occur under these conditions.