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36.7.1.1.1.2 Variation 2: Using Reusable Osmium(VIII) Oxide on Ion Exchanger

DOI: 10.1055/sos-SD-036-00589

Nativi, C.; Roelens, S.Science of Synthesis, (200836759.

A development of the catalytic asymmetric dihydroxylation (AD) of alkenes is ligand heterogenization (on silica gel,[‌13‌‌15‌] or soluble[‌16‌,‌17‌] or insoluble [‌18‌,‌19‌] polymeric supports), followed by osmium complexation. This method allows the recovery and reuse of the expensive and toxic osmium catalyst. An alternative ion-exchange technique can also be used; the catalyst is immobilized on layered double hydroxides (LDH) through the exchange of the osmium(VIII) oxide dianion (Scheme 3).[‌20‌] The exchangerosmium(VIII) oxide catalyst is successfully employed for the asymmetric dihydroxylation of several alkenes 4, to afford syn-diols 5, with higher yields and comparable enantiomeric excesses to those obtained using the corresponding homogeneous conditions. Various co-oxidants, such as 4-methylmorpholine N-oxide, potassium hexacyanoferrate(III), and molecular oxygen, can be used for the asymmetric dihydroxylation reaction under heterogeneous conditions. Among the heterogeneous osmium catalysts, the osmium(VIII) oxide dianion exchanged onto layered double hydroxides[‌21‌] displays excellent activity with 4-methylmorpholine N-oxide as the co-oxidant; however, it has been reported that the use of potassium hexacyanoferrate(III) and molecular oxygen deactivate LDHosmium(VIII) oxide over a number of recycling steps.[‌20‌] The resinosmium(VIII) oxide reagent shows good performance in terms of catalyst recovery and reusability, irrespective of the co-oxidant employed.

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