Supramolecular Catalysis: Investigation of Acid-Base Interactions to Promote the Assembly of Monodentate P-Ligands

In recent years, monodentate phosphorus ligands have held center stage in asymmetric catalysis, thanks to their outstanding activity, selectivity and convenient, fast and practical synthesis. Bidentate ligands are of course still very important, even though their synthesis is usually more tedious and expensive. Supramolecular bidentate ligands, based on the assembly of monodentate ligands possessing complementary functionalities, combine the essential features of bidentate ligands (preorganization and rigidity) with the advantages of monodentate ligands (practicality). Metal-ligands interactions (Reek, van Leeuwen, Takacs), hydrogen bonds (Breit, Reek, Nishibayashi), and ionic interactions (van Leeuwen) have been so far the main non-covalent forces used to promote the recognition of monodentate ligands.1 Here we report two new approaches for the assembly of monodentate phosphites in Rh-complexes: i) arene - interactions, using a binary combination of an electron-poor (with a perfluroarene or dinitroarene unit) and an electron-rich ligand (with a methoxyarene unit); ii) acid-base interactions, using a binary combination of an acidic (bearing a carboxylic acid) and a basic (bearing a tertiary amine) chiral binaphtholic phosphite. The preferential formation of the Rh-heterocomplexes was investigated by 31P-NMR and the ligand binary combinations were also tested in the Rh-catalyzed asymmetric hydrogenation of functionalized olefins (methyl acetamidoacrylate and dimethyl itaconate). In both cases, enhanced ee’s (up to 99 %) were obtained using selected heterocobinations with respect to the corresponding homocombinations. We thank MiUR for a postdoctoral fellowship (to L. Pignataro, PRIN prot. 2006030449), the European Commission [RTN Network (R)Evolutionary Catalysis MRTN-CT-2006-035866] for financial support and for a predoctoral fellowship (to B. Lynikaite) and a postdoctoral fellowship (to J. Cvengroš). C. Gennari gratefully acknowledges Merck Research Laboratories for the Merck's Academic Development Program Award.