Versatile Synthesis of Chiral Aminophosphine Phosphinites (AMPPs) as Ligands for Enantioselective Hydrogenation

Abstract

New chiral aminophosphine phosphinite (AMPP) ligands with different P-aryl substituents were prepared from (1R,3S,4S)-2-azabicyclo[2.2.1]hept-3-ylmethanol by a new and chemoselective synthetic approach. The ligands showed good enantioselectivity (up to 91%) in the Rh-catalyzed enantioselective hydrogenation of benchmark substrates.Among enantioselective catalytic reactions in an industrial scale, the hydrogenation with rhodium complexes bearing chiral P-ligands is of pivotal importance. 1 Since the design of more efficient and selective catalysts is far from being understood, the synthesis and test of new ligands is still one of the most promising method for the optimization of enantioselective hydrogenations. This situation is forced by the increasing level of modern high-throughput methods, which allow the fast testing of series of catalysts and reaction conditions. For this approach the availability of broad arrays of related ligands is indispensable.For the establishment of comprehensive ligand libraries, aminophosphine phosphinites (AMPPs) 2 are of particular value. 3 They can be synthesized from chiral amino alcohols in one step. An overwhelming part of the latter can be easily derived from chiral amino acids, which are available in a large variety, sufficient quantity and high optical purity from nature or industrial processes. In the past several research groups have shown the versatility of this approach. 4 In due course several aminophosphine phosphinites as efficient ligands for enantioselective hydrogenations were disclosed.Generally, the synthesis of aminophosphine phosphinites is carried out by condensation reaction of the amino alcohol with dialkyl-or diarylchlorophosphines in the presence of a tertiary amine as a base. Hydroxyl and amino group can be esterified simultaneously. However, on the other hand under these conditions the selective synthesis of 'mixed' ligands with different substituents on both phosphorous atoms, which opens up another level of variation, frequently fails due to the similar reactivity of amino and hydroxyl group. As an alternative the 'borane methodology' was suggested, which allows even the preparation of P-stereogenic AMPPs. 5 However, this method requires several additional steps. A protocol for a convenient one-pot procedure could be the transphosphinylation reaction of phosphinous amides (aminophosphines). 6 To the best of our knowledge this methodology has never been used for the synthesis of 'mixed' P-ligands.In order to show the high potential of this methodology, herein we report our results in the preparation of AMPPligands of type 1 (Figure 1) starting from the amino alcohol (1R,3S,4S)-2-azabicyclo[2.2.1]hept-3-ylmethanol (3) (Scheme 1). 7 The amino alcohol 3 represents a prolinol analogue bearing a rigid backbone. 8 In particular the group of Andersson investigated the potential of this unnatural enantiopure material as a chiral auxiliary and ligand. 9 It is available in a multi-gram scale now. 10 Since a related aminophosphine phosphinite ligand ProloPHOS 2 d...