Catalytic asymmetric dihydroxylation of olefins with reusable OsO₄^2- on ion-exchangers: the scope and reactivity using various cooxidants

Abstract

Exchanger-OsO₄ catalysts are prepared by an ion-exchange technique using layered double hydroxides and quaternary ammonium salts covalently bound to resin and silica as ion-exchangers. The ion-exchangers with different characteristics and opposite ion selectivities are specially chosen to produce the best heterogeneous catalyst that can operate using the various cooxidants in the asymmetric dihydroxylation reaction. LDH-OsO₄ catalysts composed of different compositions are evaluated for the asymmetric dihydroxylation of trans-stilbene. Resin-OsO₄ and SiO₂-OsO₄ designed to overcome the problems associated with LDH-OsO₄ indeed show consistent activity and enantioselectivity in asymmetric dihydroxylation of olefins using K₃Fe(CN)₆ and molecular oxygen as cooxidants. Compared to the Kobayashi heterogeneous systems, resin-OsO₄ is a very efficient catalyst for the dihydroxylation of a wide variety of aromatic, aliphatic, acyclic, cyclic, mono-, di-, and trisubstituted olefins to afford chiral vicinal diols with high yields and enantioselectivities irrespective of the cooxidant used. Resin-OsO₄ is recovered quantitatively by a simple filtration and reused for a number of cycles with consistent activity. The high binding ability of the heterogeneous osmium catalyst enables the use of an equimolar ratio of ligand to osmium to give excellent enantioselectives in asymmetric dihydroxylation in contrast to the homogeneous osmium system in which excess molar quantities of the expensive chiral ligand to osmium are invariably used. The complexation of the chiral ligand (DHQD)₂PHAL, having very large dimension, a prerequisite to obtain higher ee, is possible only with the OsO₄^2- located on the surface of the supports.