C-H Activation Reactions of Ruthenium N-Heterocyclic Carbene Complexes: Application in a Catalytic Tandem Reaction Involving C-C Bond Formation from Alcohols

The mono NHC complexes Ru(NHC)(PPh3)2(CO)H2 (NHC = IMe4 (1,3,4,5-tetramethylimidazol-2-ylidene, 3); IEt2Me2 (1,3-diethyl-4,5-dimethylimidazol-2-ylidene, 4); ICy (1,3-dicyclohexyl-imidazol-2-ylidene, 5); IiPr2 (1,3-bis(isopropyl)-imidazol-2-ylidene, 6); IPr2 (1,3-bis(propyl)-imidazol-2-ylidene, 7)) were readily prepd. upon heating Ru(PPh3)3(CO)H2 with between 2 and 6 equiv of the appropriate NHC precursor. Reaction of 1,3-bis(isopropyl)-4,5-dimethylimidazol-2-ylidene (IiPr2Me2) with Ru(NHC)(PPh3)2(CO)H2 gave C-H-activated carbene complex Ru(CH2(Me)CIiPrMe2)(PPh3)2(CO)H (9). The Ru hydride N-alkyl heterocyclic carbene complexes were studied as catalysts for a tandem oxidn. Wittig redn. reaction to give C-C bonds from alcs. The C-H-activated carbene complex 9 proves to be the most active precursor catalyzing the reaction of PhCH2OH and Ph3P:CHCN in 3 h at 70 Deg. These results provide (a) a rare case in which N-alkyl carbenes afford higher catalytic activity than their N-aryl counterparts and (b) a novel example of the importance of NHC C-H activation in a catalytic cycle. The mol. structures of 6, 7 and 9 were detd. by x-ray crystallog.