N-Heterocyclic Carbene Enabled Functionalization of Inert C(Sp³)-H Bonds via Hydrogen Atom Transfer (HAT) Processes

Developing methods to directly transform C(sp³) -H bonds is crucial in synthetic chemistry due to their prevalence in various organic compounds. While conventional protocols have largely relied on transition metal catalysis, recent advancements in organocatalysis, particularly with radical NHC catalysis have sparked interest in the direct functionalization of "inert" C(sp³) -H bonds for cross C-C coupling with carbonyl moieties. This strategy involves selective cleavage of C(sp³) -H bonds to generate key carbon radicals, often achieved via hydrogen atom transfer (HAT) processes. By leveraging the bond dissociation energy (BDE) and polarity effects, HAT enables the rapid functionalization of diverse C(sp³)-H substrates, such as ethers, amines, and alkanes. This mini-review summarizes the progress in carbene organocatalytic functionalization of inert C(sp³)-H bonds enabled by HAT processes, categorizing them into two sections: 1) C-H functionalization involving acyl azolium intermediates; and 2) functionalization of C-H bonds via reductive Breslow intermediates.