N‐Oxyl Radicals in Oxidative C−O Coupling: Free‐Radical Hydrogen Substitution and Addition to C=C Bonds

Abstract

N‐oxyl radicals occupy an important place in free‐radical oxidative CH‐functionalization being one of the most efficient redox‐organocatalysts for hydrogen atom abstraction (HAT). Their applications include aerobic radical chain autoxidation, CH‐functionalization with the formation of carbon‐carbon and carbon‐heteroatom bonds. The persistent nature of N‐oxyl radicals combined with their high reactivity in HAT results in their unique dual chemistry: the same radical can both propagate radical chain reaction (at low N‐oxyl concentrations) and effectively “terminate” carbon‐centered radicals (at higher N‐oxyl concentrations). The latter case opens a new synthetic application area of N‐oxyl radicals, in which they act as both hydrogen abstracting species and O‐reagents for cross‐coupling with carbon‐centered radicals thus produced. Apart from the C−H bond cleavage, reactive N‐oxyl radicals have been extensively used recently in C=C double bond functionalization via radical addition reactions. In this review, both free‐radical CH‐functionalization reactions with the introduction of N‐oxyl fragments and alkene difunctionalizations by N‐oxyls are covered with emphasis on the relationship between reaction conditions and selectivity.