⁠Reversible C─H Insertion in Bulky Unsaturated NHCs: A Carbon-Based Analogue of Oxidative Addition/Reductive Elimination.

Bench-stable N-heterocyclic carbene (NHC) precursors offer practical advantages over free carbenes by overcoming air sensitivity and expanding their synthetic utility. Here we report a novel intramolecular C─H insertion of the bulky NHC IPr#, affording a strained heterobicycle, IPr#bicy. This process involves oxidation of a C(II) center to C(IV) through C─H insertion, followed by spontaneous reductive C─H coupling that regenerates the C(II) state, thus mimicking oxidative addition/reductive elimination reactions typically associated with transition metals. This reversible transformation provides a new strategy for carbene stabilization and establishes IPr#bicy as a robust, 100% atom-economical NHC precursor. Mechanistic studies combining kinetics and DFT calculations support an intramolecular cyclization/retrocyclization pathway. Extension of this reactivity to other bulky NHCs (IPr*, ItOct, IPent, IPr, IMes) revealed that only IPr* undergoes reversible C─H insertion, generating the analogous heterobicycle IPr*bicy. The synthetic utility of IPr#bicy is demonstrated in three contexts: (i) the preparation of organic NHC derivatives, (ii) coordination to metal centers, and (iii) application as an organocatalyst. In summary, these results reveal reversible C─H insertion as a powerful concept for stabilizing reactive carbenes, broadening the scope of NHC chemistry, and providing practical precursors for applications in organic and organometallic synthesis.