Novel Water-Soluble Bisphosphinite Chiral Ligands Derived from α,α- and β,β-Trehalose. Application to Asymmetric Hydrogenation of Dehydroamino Acids and Their Esters in Water or an Aqueous/Organic Biphasic Medium

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Abstract
Novel 2,3:4,6-di-O-isopropylidene-α-d-glucopyranosyl-(1,1)-4,6-O-isopropylidene-2,3-di-O-diphenylphosphino-α-d-glucopyranoside (2), 2,3:4,6-di-O-cyclohexylidene-α-d-glucopyranosyl-(1,1)-4,6-O-cyclohexylidene-2,3-di-O-diphenylphosphino-α-d-glucopyranoside (4), and 2,3:4,6-di-O-cyclohexylidene-β-d-glucopyranosyl-(1,1)-4,6-O-cyclohexylidene-2,3-di-O-diphenylphosphino-β-d-glucopyranoside (11) were prepared from the corresponding α,α- or β,β-trehalose. The ligands were transformed into cationic Rh complexes, such as [Rh(α-d-glucopyranosyl-(1,1)-2,3-di-O-diphenylphosphino-α-d-glucopyranoside)(cod)]BF4 (3) and [Rh(β-d-glucopyranosyl-(1,1)-2,3-di-O-diphenylphosphino-β-d-glucopyranoside)(cod)]BF4 (12) bearing free hydroxy groups. These complexes were soluble in water and were efficient catalysts for the asymmetric hydrogenation of dehydroamino acids and their esters in water or an aqueous/organic biphasic medium with high enantioselectivity (up to 99.9% ee). Aqueous biphasic systems offer an easy separation of the aqueous catalyst phase from the product phase and allow recycling of the catalyst phase without the loss of enantioselectivity.

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