Asymmetric Catalysis in Water: Prospects and Problems of Using Hydroxyphosphines and Hydroxyphosphinites as Ligands

The efficiency and selectivity of Rh(I)-catalyzed asymmetric hydrogenation of dehydroaminoacids have been investigated using water-soluble organometallic catalysts prepared from three readily available carbohydrates, trehalose, D-salicin [2-(hydroxymethyl)phenyl β-D-glucopyranoside] and D-mannitol. Bisdiarylphosphinite- Rh(I) complexes prepared from trehalose give moderate selectivity in hydrogenations in aqueous media. However, investigations of partition coefficients of these complexes between water and organic solvents show that they have significant solubility in organic solvents, suggesting that increasing the number of hydroxyl groups alone (in this case 6) may not assure adequate water solubility to enable catalyst recycling. A series of water-soluble chelating bis-diarylphosphinite ligands with an additional tetraalkylammonium group have been prepared from D-salicin. These are excellent ligands for hydrogenation in organic solvents where the reaction is fast. However, reactions in neat aqueous or biphasic media are considerably slower, and under these conditions, only poor enantioselectivities are observed. Anecdotal evidence suggests that the hydrophobic diarylphosphinite moiety might be responsible for the low solubility and the attendant low reactivity. Competitive hydrolytic degradation of the P-O bond might be another significant problem. An attractive solution to the dual problems of instability in water and hydrophobicity is the use of polyhydroxyphospholanes bearing robust P-C bonds (vis-a-vis P-O bonds of the phosphinites), but with a reduced number of aryl groups on phosphorus. Cationic Rh-complexes of these ligands have been found to be excellent catalysts for organic and aqueous phase hydrogenation of dehydroaminoacids. The viability of catalyst recovery has been demonstrated in three different systems, including two cases where >99% ee can be achieved under recycling conditions.