Combinations of acidic and basic monodentate binaphtholic phosphites as supramolecular bidentate ligands for enantioselective Rh-catalyzed hydrogenations

The combination of two chiral BINOL-derived monodentate phosphites, containing either a carboxylic acid (-COOH) or a tertiary amine (-NMe2), can be seen as a supramolecular bidentate P-ligand self-assembled through an acid-base interaction. A small library of chiral BINOL-derived monodentate phosphites containing either a carboxylic acid or a tertiary amine was synthesized in a few straightforward steps and in moderate to good yields. The ligand combinations were screened in the enantioselective rhodium-catalyzed hydrogenation of methyl 2-acetamidoacrylate. Selected heterocombinations of an amine-derived phosphite and of a carboxylic phosphite displayed a higher level of enantioselectivity compared to the corresponding homocombinations (up to 90% e.e.). The formation of the rhodium complexes was studied by IR- and 31P-NMR spectroscopy using both the single ligands (homocombinations) and the combinations of a carboxylic phosphite and a tertiary amine-derived phosphite (heterocombinations). In this way, the role of the acid-base interactions in determining the heterocomplex : homocomplexes ratios was investigated.