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

Abstract

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...