Intramolecular Hydrogen Atom Transfer from Unactivated C(sp3)−H Bond to Alkyl Radical

Abstract Regioselective C( sp 3 )−H bond functionalization in atom‐ and step‐economic mode has long been recognized as a prominent and challenging task due to its application in late‐stage modification of complex skeletons, natural products and drugs. As a complement to transition metal‐catalyzed C−H activation and carbene/nitrene insertion reactions, hydrogen atom transfer (HAT) emerges as a versatile tool for C( sp 3 )−H bond functionalization and showcases significant and distinctive superiority in terms of selectivity trend and substrate scope. Intramolecular HAT processes initiated by nitrogen‐ and oxygen‐centered radicals, aryl radicals, and vinyl radicals, have already been well developed in the past century. In contrast, the development of alkyl radical‐mediated HAT reactions lagged behind, although generation methods of corresponding alkyl radicals under mild conditions have gained rapid growth. Especially, selective and efficient HAT from unactivated C( sp 3 )−H to alkyl radical is highly challenging due to their comparable BDEs and radical polarity. This Concept highlights recent advance in regioselective functionalization of ubiquitous unactivated C−H bonds enabled by intramolecular H‐atom transfer to alkyl radicals.