Breaking Bonds, Breaking Paradigms: Critical Insights into Inner-Sphere C(sp)-H Functionalization via Metal-Catalyzed Carbene and Nitrene Insertion.

This review systematically analyzes recent advances in transition metal-catalyzed carbene and nitrene insertion into unactivated aliphatic C(sp)-H bonds through inner-sphere mechanisms, offering a critical synthesis of mechanistic insights and synthetic applications from 2016 to 2024. By contrasting inner- and outer-sphere pathways, we elucidate how metal-substrate coordination governs regioselectivity, catalyst design, and substrate compatibility. Key discussions focus on breakthroughs in Rh(III), Pd(II), Co(III), Ir(III), and Ni(II) catalytic systems, emphasizing their distinct electronic and steric control strategies for directing C-H activation and migratory insertion. Notable achievements include the functionalization of sterically hindered substrates, enantioselective aminations via chiral ligand engineering, and cascade transformations enabled by metal-mediated β-elimination. We highlight emerging trends in sustainable catalysis using earth-abundant metals (e.g., Co, Ni), while addressing persistent challenges such as directing group dependency, catalyst deactivation, and limited substrate scope. The review further proposes strategic frameworks for future innovation, including (1) computational ligand optimization to enhance regiochemical control, (2) transient directing group strategies for native functional group tolerance, and (3) bifunctional catalyst design to differentiate electronically equivalent C-H bonds. By bridging mechanistic understanding with practical synthetic goals, this work establishes a roadmap for advancing precision C(sp)-H functionalization in complex molecule synthesis and industrial applications.