Application of dihydride ruthenium carbene complexes for indirect Wittig reactions

The ability of dihydride ruthenium N-heterocyclic carbene complexes to catalyze transfer hydrogenation reactions has been exploited in the formation of new C-C bonds from alcs. via an indirect Wittig reaction. We initially demonstrated this process using Ru(IMes)(PPh3)2CO(H)2 1 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) as the in situ transfer hydrogenation catalyst. The catalyst temporarily oxidises an alc., thus allowing the Wittig reaction to proceed, then catalytically reduces the resulting alkene to afford the target alkane. While good yields were achieved at moderate temps. using complex 1 we have since worked to identify more active but stable catalysts, particularly through the introduction of alternative N-heterocyclic carbenes with alkyl substituents. Further applications have also been investigated including the formation of amines from alcs. via indirect aza-Wittig reactions.