Synthesis and structural studies of rhodium(I)-catalytic precursors containing two furanoside diphosphines

We have prepared new rhodium(I) complexes with two furanoside diphosphines, 3,5,6-trideoxy-3,5-bis-diphenylphosphine-1,2-O-isopropylidene-β-l-idofuranose (1) and 3,5,6-trideoxy-3,5-bis-diphenylphosphine-1,2-O-isopropylidene-α-l-glucofuranose (2), which form six-membered chelate rings when they coordinate to rhodium. NMR spectroscopy of the Rh complexes and molecular mechanics calculations showed that the configuration of the stereocenter C-5 greatly influences the structure of these complexes, and thus their enantioselectivity. The pre-catalyst [Rh(cod)(2)]BF4, which gives the highest enantioselectivities in the asymmetric hydrogenation of alkenes, adopts the twist-λ conformation. By changing the configuration of the C-5, the complex becomes fluxional at room temperature and enantioselectivity drops considerably.