Osmium-catalyzed asymmetric dihydroxylation of olefins in ionic liquids

Abstract

Effective osmium-catalyzed asymmetric dihydroxylation was observed with 1,4-bis(9- O -diquininyl)phthalazine, (DQ) 2 PHAL, as the chiral ligand in the ionic liquid, 1- n -butyl-3-methylimidazolium hexafluorophosphate [bmim][PF 6 ]. It was shown that the ligand undergoes dihydroxylation during the reaction, affording a more polar ligand which remains in the ionic liquid phase, when the latter is extracted with diethyl ether, thus facilitating catalyst recycling. In contrast, when 1,4-bis(9- O -dihydroquininyl)phthalazine, (DHQ) 2 PHAL, which cannot undergo dihydroxylation, was used as the chiral ligand, the catalyst was extracted into the organic layer. The osmium-catalyzed hydroxylation of trans-stilbene in aqueous acetone-ionic liquid medium was enantioselective (85% ee) in the presence of the chiral ligand (D2OHQ) 2 PHAL. The chiral ligand was formed from a precursor under the reaction conditions and was recycled together with the ionic liquid.