Single‐Electron Transfer Reactions Enabled by N‐Heterocyclic Carbene Organocatalysis

Abstract

Over the past decades, N‐heterocyclic carbene (NHC) organocatalysis has undergone a flourish of development on the basis of closed‐shell reaction paths. By contrast, the emerging area of single‐electron transfer (SET) reactions enabled by NHC catalysis still remain underdeveloped, but offer plenty of opportunities to develop new catalytic modes and useful synthetic methods. A number of interesting transformations were triggered by the SET process from the electron‐rich Breslow intermediates to various single‐electron acceptors. In additions, recent studies revealed that the Breslow radical cations could also be generated by single‐electron reduction of the electron‐deficient acyl azolium intermediates. These discoveries open a new avenue for NHC organocatalysis to harness radical reactions. The present review will focus on the exciting advancements in the dynamic area of radical NHC organocatalysis.

Polyaniline‐Supported Tungsten‐Catalyzed Green and Selective Oxidation of Alcohols

“…Therefore, it could be concluded that TEMPO might disturb the reaction, but could not restrain it, showing that the oxidation might proceed via the ionic reaction route majorly, other than the initially expected free radical reaction process initiated by the single electron transfer reaction of catalytic tungsten. [17] Table 1. Conditional optimizations for benzylalcohol oxidation.…”

Radical transformations for allene synthesis

“… 35 Moreover, NHC-catalyzed radical reactions received increasing attentions as well. 36 Very recently, NHC catalysis has been employed for radical synthesis of allenes.…”

Asymmetric organocatalysis involving double activation

“…In contrast to the well-developed electron pair transfer processes catalyzed by NHCs, the single electron transfer processes represent another strategy which has been rapidly developed in recent years, especially in combination with photochemistry. Two major pathways are involved in the formation of radical intermediates from carbonyls via NHC catalysis, one is the single electron oxidation of Breslow intermediates to give radical cations, and the other is the single electron transfer reduction to generate ketyl radicals [109,110] (Scheme 33). Some elegant examples have been uncovered based on both strategies, unfortunately, usually in a racemic manner [111][112][113].…”