Intramolecular C–H Activation Reactions of Ru(NHC) Complexes Combined with H2 Transfer to Alkenes: A Theoretical Elucidation of Mechanisms and Effects of Ligands on Reactivities

Recent experimental studies have identified Ru­(II) NHC complexes that are highly reactive in the tandem intramolecular C­(sp3)–H activation of an N-alkyl substituent to form a metallacycle and transfer hydrogenation of alkenes. These complexes are promising candidates for tandem catalytic processes that depend on a reversible uptake of hydrogen (“borrowing hydrogen catalysis”). We have elucidated the reaction mechanisms by density functional theory calculations and investigated ligand effects on reactivity. The reaction of ruthenium dihydride complex [Ru­(H)2(NHC)­(CO)­(PPh3)2] (1) with ethylene occurs via dissociative ligand exchange to replace one of the phosphine ligands with ethylene, followed by hydride migration and reductive elimination to form a Ru(0) intermediate. Subsequent C–H activation occurs via an oxidative addition mechanism. Bulkier NHC and phosphine ligands facilitate the dissociation of phosphine, which leads to a lower overall barrier. In addition, the N-iPr substituted NHC ligand promotes the C–H oxidative addition/ruthenacyclization due to the release of steric strain caused by the N-iPr group and the substituents on the NHC backbone. The reaction with the monohydride monochloride complex [RuHCl­(NHC)­(PPh3)2] (2) occurs via ligand exchange and hydride migration to form an alkyl ruthenium­(II) complex. The type of phosphine ligand determines whether the subsequent intramolecular C–H activation proceeds via an associative or a dissociative mechanism. In the associative pathway, C–H activation occurs via a concerted σ-bond metathesis mechanism, which directly transfers the hydrogen atom from the C–H bond of the N-alkyl group on the NHC to the alkyl ligand on ruthenium. In the dissociative pathway, C–H activation occurs via stepwise C–H oxidative addition to form a Ru­(IV) intermediate followed by reductive elimination of the alkane product.