New C2-Symmetric Diphosphite Ligands Derived from Carbohydrates: Effect of the Remote Stereocenters on Asymmetric Catalysis

The synthesis of new modular chiral diphosphite ligands with C₂-symmetry and carbohydrate backbone is reported. We also report here the synthesis of the corresponding rhodium complexes [Rh(COD)(L)]BF₄ (L=diphosphite). All these species have been characterised in solution by NMR spectroscopy and in some cases in the solid state by X-ray diffraction. The solution structures of the hydridorhodiumcarbonyl species [RhH(CO)₂(L)], where L=diphosphites 12a–14a, 12b, have been studied using high-pressure NMR spectroscopy. The configuration and substitution of the remote stereocentres in positions 2 and 5 of the tetrahydrofuran ring of the diphosphite ligands were observed to have a considerable influence on the results obtained in the rhodium-catalysed hydroformylation and hydrogenation reactions. Thus, the configuration of the major isomer obtained in the hydroformylation reaction may be controlled by changing the configuration of these stereocentres.