Recycling, osmium tetroxide

Reaction Mixtures. Osmium tetroxide can be reduced to the dioxide by reacting it with an olefin to form the osmate ester and then bubbling hydrogen sulfide through the solution. The black precipitate of osmium dioxide is removed by filtration. Package Os02 for disposal or recycle.5... 

Recently, Yao showed that osmium tetroxide could be immobilized in an ionic liquid. The recyclability of osmium tetroxide was improved by the addition of DMAP. Both the catalyst and the ionic liquid were reused in six consecutive runs without significant reduction in yield [47]. Dihydroxylations in a solvent mixture of l-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), tert-butanol, and water with 0s04 (2 mol%), DMAP (2.4 mol%), and NMO (1.1 equivalents) as cooxidant afforded diols in good yield (73-99% depending... 

It is probably in asymmetric dihydroxylation, where the use of ionic liquids appears to be most promising. The decreased acute toxicity of osmium tetroxide due to its suppressed volatility certainly represents a great benefit for those who work with this reagent and its derivatives. Furthermore, high cost of both the osmium catalyst, as well as the chiral ligand, make recycling of the catalyst-ionic liquid particularly attractive. On the other hand, disposal of osmium-contaminated ionic liquids in an environmentally benign manner has yet to be addressed. 

Recycling of Osmium by Immobilization of Osmium Tetroxide. Several groups have been actively searching for immobilized forms of osmium tetroxide in order to overcome the problems of toxicity and volatility associated with this reagent. The following are some representative examples of immobilization methods for a catalytic system (OSO4 and/or cinchona alkaloid ligand). 

Immobilization of Osmium Tetroxide onto Ionic Polymer Support. The incorporation of OSO4 onto short-length PEGy-lated (PEG Poly(ethylene glycol)) ionic polymers was achieved by Janda and coworkers. Immobilized OSO4 exhibited excellent catalytic performance and recyclability in the AD reaction. The high recyclability could be attributed to the interaction between the induced dipole of OSO4 and the ions of the polymeric scaffolds in a manner similar to ionic liquids. Moreover, Os-bounded... 

The intermediate is hydrolyzed with aqueous hydrogen peroxide. Hydrogen peroxide re-oxidizes the osmium reagent back to osmium tetroxide. (Because osmium tetroxide is recycled, only a catalytic amount of this expensive and toxic oxidizing agent is needed.)... 

Diols.—The osmium tetroxide-catalysed y/c-hydroxylation of alkenes to provide cis- 1,2-diols, using t-butyl hydroperoxide in a basic medium (tetraethyl-ammonium hydroxide) to recycle Os to Os ", itself an advance over earlier methods, has been further improved by eliminating the need for strongly basic conditions. It has been reported that tetraethylammonium hydroxide may be replaced by the corresponding acetate if the solvent is simply changed from t-butyl alcohol to acetone, and that base-sensitive groups such as esters can survive the new conditions. 

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