Homogeneous and Heterogeneous Cluster-Based Catalysts for Asymmetric Reactions

Sammanfattning: Synthesis and characterization of new transition metal carbonyl clusters has been undertaken, and evaluation of the new clusters to act as catalysts/catalysts precursor in asymmetric reactions has been carried out. The catalytic hydrogenation of tiglic acid under relatively mild conditions, using diastereomeric clusters of the general formula [(µ-H)2Ru3(µ3-S)(CO)7(µ-1,2-L)] (L= chiral diphosphine of the ferrocene-based Walphos and Josiphos families) as catalysts, reveal different catalytic behavior in terms of conversion and enantioselectivity. The observed reversal in enantioselectivity when the chirality of the cluster framework is changed strongly supports catalysis by intact Ru3 clusters. A proposed catalytic cycle generated by CO loss from the parent catalyst has been investigated.

The ability of carbonyl clusters based on rhenium, ruthenium and osmium derivatised with chiral phosphines and phosphiranes to catalyse the asymmetric hydrogenation of tiglic acid shows high to excellent conversion rates with low to moderate enantioselectivity. A high conversion rate (up to 99%) and an enantioselectivity of 19% have been achieved with the cluster [(µ-H)4Ru4(CO)11(ArPH2)] {where ArP = (R)-(2'-methoxy-[1,1'-binaphthalen]-2-ylphosphine)} acting as catalyst/catalyst precursor; this is, to the best of our knowledge, the first example of an asymmetric hydrogenation induced by a chiral primary phosphine.

A chiral solid catalyst derived from the immobilization of [H4Ru4(CO)10(µ-1,2-W001)] (W001 = chiral diphosphine ligand of the Walphos family) onto functionalized MCM-41 has been shown to exhibit excellent conversion rates (> 99%) and as high enantiomeric excesses (≈ 80%) as that of its homogeneous counterpart. The good catalytic properties are coupled with good recyclability of the mesoporous catalyst.