A modified synthesis of P-chiral ligands for asymmetric catalysis

In asymmetric reactions aiming the synthesis of compounds with high enantioselective purity the application of chiral phosphorus-ligands as co-catalysts for transition metals plays a pivotal role. In contrast to the vast... [ view full abstract ]

In asymmetric reactions aiming the synthesis of compounds with high enantioselective purity the application of chiral phosphorus-ligands as co-catalysts for transition metals plays a pivotal role. In contrast to the vast number of chiral bidentate diphosphanes bearing the chiral information in the backbone of the carbon skeleton only a few phosphorus ligands are known with a stereogenic P-atom (e.g. DiPAMP, Duanphos, BisP). This situation is quite astonishing because a chiral P-atom allows the closest contact between ligand and prochiral substrate through the coordination sphere of the metal and therefore the most efficient chirality transfer can be expected.

Based on the well-established method for the synthesis of P-chiral monophosphanes developed mainly by Jugé et al. we intended to synthesize P-stereogenic versions of the prominent, but achiral Xantphos- and DPEphos-type ligands by the simultaneous introduction of both P-chiral groups in the corresponding carbon backbone. Surprisingly, the targeted diphosphanes could not be obtained by this protocol. Only by a decisive modification of the Jugé approach, the desired ligands could be prepared. By the new pathway more than 60 individuals of the new chiral ligand type were accessible (see figure 1). First results to use them in asymmetric catalytic reactions (hydrogenation, allylation) will be simultaneously reported. In the rhodium catalyzed asymmetric hydrogenation of isophorone as a prochiral substrate with some of the new diphosphanes complete conversion, high chemoselectivity and up to 96 % of enantioselectivity, respectively, were achieved. Also in the Pd-catalyzed asymmetric alkylation of 1,3-diphenylallyl acetate enantioselectivities up to 96% could be obtained.

Figure 1. Overview about the synthesized P-chiral ligands.