[Ru(NHC)(xantphos)(CO)H2] complexes: intramolecular C–H activation and applications in C–C bond formation

Treatment of [Ru(PPh3)(xantphos)(CO)H2] (1) with the N-heterocyclic carbenes (NHCs) IEt2Me2 (1,3-diethyl-4,5-dimethylimidazol-2-ylidene), IiPr2Me2 (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) and IMes (1,3-dimesitylimidazol-2-ylidene) at elevated temperature affords the C-H activated carbene complexes [Ru(NHC)(xantphos)(CO)H] (2-4). In contrast, ICy (1,3-dicyclohexylimidazol-2-ylidene) reacts with 1 to give the substitution product [Ru(ICy)(xantphos)(CO)H2] (6), which can be converted into the C-H activated species [Ru(ICy)(xantphos)(CO)H] (7) upon thermolysis in the presence of H2CCHSiMe3. Addition of H2 to 2 yields [Ru(IEt2Me2)(xantphos)(CO)H2] (5), while H2 reacts with 7 to reform 6. Complexes 2-4 and 7 catalyse the Knoevenagel reaction of PhCH2OH with tBuC(O)CH2CN, although they prove to be somewhat less active than the phosphine complex 1.