Activation of an alkyl C-H bond geminal to an agostic interaction: an unusual mode of base-induced C-H activation

Deuterium labeling studies indicate that base-induced intramolecular C−H activation in the agostic complex 2-D proceeds with exclusive removal of a proton from the methyl arm of an iPr substituent on the N-heterocyclic carbene (NHC) ligand. Computational studies show that this alkyl C−H bond activation reaction involves deprotonation of one of the C−H bonds that is geminal to the agostic interaction, rather than the agostic C−H bond itself. The reaction is readily accessible at room temperature, and a computed activation barrier of ΔE ‡calcd = +11.8 kcal/mol is found when the NHC 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene is employed as the external base. Charge analysis reveals that the geminal hydrogens are in fact more acidic than the agostic proton, consistent with their more facile deprotonation.