The homogeneous hydrogenation of esters and related compounds by a ruthenium triphosphine complex
The homogeneous hydrogenation of esters and related compounds has been achieved by a catalyst-generated in-situ from the precursors Ru(acac)(_3) and the tripodal phosphine ligand l,l,l-tris(diphenylphosphinomethyl)ethane (Triphos). The catalyst is susceptible to deactivation in the presence of primary alcohols by their dehydrogenation to an aldehyde, which is subsequently decarbonylated to produce the [Ru(Triphos)H(_2)(CO)] complex, characterised by (^1)H and (^31)P NMR spectroscopy, elemental analysis, IR and X- ray crystallography. The co-ordinated carbonyl ligand of [Ru(Triphos)H(_2)(CO)] effectively blocks the co-ordination site required by the substrate for further hydrogenation, thus deactivating the catalyst. Reactivation of the deactivated catalyst can be achieved, by introducing water to the reaction mixture, and removing the carbonyl ligand through the water gas shift reaction. The inclusion of water within the reaction solvent results in catalyst reactivation during the hydrogenation process. Investigation of the hydrogenation of alkenes, in the absence of reagents susceptible to decarbonylation, resulted in the isolation of the solid dinuclear complex [(Triphos)RuH(μ-H)(_2)HRu(Triphos)], which was characterised by X-ray crystallography and (^31)P NMR spectroscopy. Its information suggests that the active catalyst is [Ru(Triphos)H(_2)].The separation of the products from the catalyst and unconsumed reactants has been achieved by distillation of the product solution of the hydrogenation reaction. The recovered catalyst is sufficiently robust to be capable of multiple recycles, replicating the original and repeated result without loss of activity or selectivity.