Supramolecular hybrid bidentate ligands in asymmetric hydrogenation

In this study we introduce a novel class of supramolecular bidentate hybrid ligands and their application in the rhodium-catalysed asymmetric hydrogenation of prochiral olefins. A new supramolecular strategy is reported in which the two nonequivalent phosphorus atoms are linked covalently to a chiral scaffold, and the supramolecular interactions are used to control the second coordination sphere of the transition-metal catalyst. The supramolecular assembly is formed in situ by selective interaction between the nitrogen-donor atoms and the zinc(II) template, which is essential for obtaining high activity and selectivity. The investigations of the different zinc(II) and ruthenium(II) templates on the reaction parameters revealed a dependence of the activity and selectivity on the association constant between the supramolecular template and the pyridyl ligands. The scope of the supramolecular assemblies was explored in the Rh-catalysed asymmetric hydrogenation of α-dehydroamino acid esters and Roche ester derivatives. High activities and good to excellent enantioselectivities up to 99 % ee were obtained with the supramolecular ligands. Application of the supramolecular strategy on the basis of variations of the steric and electronic properties of zinc(II) templates demonstrate that these changes can influence key reaction parameters such as activity and selectivity