Chiral Rh phosphine–phosphite catalysts immobilized on ionic resins for the enantioselective hydrogenation of olefins in water

Abstract

The asymmetric hydrogenation of prochiral enamides with Rh complexes bearing chiral phosphine–phosphite ligands (P–OP) supported on sulphonated polystyrene resins has been studied. The complexes have been supported by simple treatment of preformed [Rh(diolefin)(P–OP)](BF₄) with the Li salt of resins. In MeOH, high enantioselectivities were observed in the hydrogenation of methyl α-N-acetamido acrylate (4a), while they decreased for less reactive methyl α-N-acetamido cinnamate (4b). Moreover, a significant Rh leaching was observed in these reactions. In contrast, a low Rh leaching was observed in reactions performed in water. Catalyst optimization enabled by the highly modular structure of P–OP ligands led to efficient catalysts for the hydrogenation of 4a and 4b in water, with enantioselectivities over 95% ee and S/C values up to 2300 and 500, respectively. To investigate the synthetic potential of the present catalytic system, the hydrogenation of a set of β-aryl α-dehydroaminoacids (4c–4j) in water has also been studied. For these reactions good catalytic activity and high enantioselectivity (87–97% ee) were also observed. In the case of brominated substrates 4i and 4j an unexpected debromination reaction in water has been observed. This phenomenon seems associated with the use of the supported catalyst and water and could be minimized by the use of a catalyst formed from 3f. In addition, gel-phase ³¹P{¹H} NMR studies performed with a representative supported complex show an analogous reactivity to that displayed by its soluble tetrafluoroborate counterpart. Finally, a selective deuterium labelling of the product has been observed in hydrogenations of 4a and 4b in D₂O, similarly to that observed in the homogeneous hydrogenation of these enamides with water soluble catalysts.