Combinations of Acidic and Basic Monodentate Binaphtholic Phosphites as Supramolecular Bidentate Ligands for Enantioselective Rh-Catalyzed Hydrogenations

A small library of chiral BINOL-derived monodentate phosphites containing either a carboxylic acid (-COCH, A1-A3) or a tertiary amine (-NMe2, B1-B4) was synthesized. The ligand combinations were screened in the enantioselective rhodium- catalyzed hydrogenation of methyl 2-acetamidoacrylate. The heterocombinations of the amine-derived phosphite (R)-B2 and of a carboxylic-phosphite [(R)-A1, (R)-A2, (R)-A3] displayed a slightly higher level of enantioselectivity compared to the corresponding homocombinations [up to 90% ee using (R)-A1/(R)-B2]. The nature and extent of the interaction between the acidic and basic ligands in the rhodium complexes were studied by IR and P-31-NMR spectroscopy. The formation of an intramolecular salt in the Rh-heterocomplex, between the carboxylic acid and the tertiary amine, was suggested by the IR spectra. The selective formation of the Rh-heterocomplex was quantitatively assessed by P-31-NMR spectroscopy, using a modified acidic ligand (R)-A1-Me. In this way, a moderate (ca. 70:30) heterocomplex/homocomplexes ratio was determined. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)