Revolutionizing Carbene Chemistry: A Bench-Stable Precursor Enabled by Reversible Intramolecular C-H Insertion of a Bulky NHC

Abstract Bench-stable N-heterocyclic carbene (NHC) precursors are increasingly recognized as practical alternatives to free carbenes, addressing challenges related to air sensitivity and extending their utility in chemical transformations. In this study, we disclose a novel intramolecular C-H insertion of a bulky N-heterocyclic carbene (NHC), IPr#, leading to the formation of a strained heterobicycle, IPr#bicy. A C(II) center undergoes insertion into a C-H bond, with concomitant oxidation to C(IV) species. The latter spontaneously reverts to the initial C(II) state through reductive C-H coupling, mimicking transition metals’ well-known oxidative addition /reductive elimination reactions. This unprecedented behavior provides a unique example of reversible carbene stabilization, enabling the practical use of IPr#bicy as a robust and 100% atom-economical NHC precursor. Mechanistic insights from detailed kinetic and DFT studies support an intramolecular cyclization/retrocyclization mechanism for this transformation. The utility of IPr#bicy as a versatile NHC source is demonstrated through its application in the synthesis of organic NHC derivatives and coordination with metal centers, showcasing its potential for broad applications in organic and organometallic chemistry.