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 (-COOH, 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 31P-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 31P-NMR spectroscopy, using a modified acidic ligand (R)-A1-Me. In this way, a moderate (ca. 70:30) heterocomplex/ homocomplexes ratio was determined.