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

Abstract

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.