Osmium osmates

In the laboratory vicinal diols are normally prepared from alkenes using the reagent osmium tetraoxide (OSO4) Osmium tetraoxide reacts rapidly with alkenes to give cyclic osmate esters... 

Overall the reaction leads to addition of two hydroxyl groups to the double bond and IS referred to as hydroxylation Both oxygens of the diol come from osmium tetraox ide via the cyclic osmate ester The reaction of OSO4 with the alkene is a syn addition and the conversion of the cyclic osmate to the diol involves cleavage of the bonds between oxygen and osmium Thus both hydroxyl groups of the diol become attached to the same face of the double bond syn hydroxylation of the alkene is observed... 

The residue, which contains Ir, Ru, and Os, is fused with sodium peroxide at 500°C, forming soluble sodium mthenate and sodium osmate. Reaction of these salts with chlorine produces volatile tetroxides, which are separated from the reaction medium by distillation and absorbed into hydrochloric acid. The osmium can then be separated from the mthenium by boiling the chloride solution with nitric acid. Osmium forms volatile osmium tetroxide mthenium remains in solution. Ruthenium and osmium can thus be separately purified and reduced to give the metals. 

The cleavage of the intermediate osmate ester has presented problems in the past, and a variety of procedures have been developed, including the use of mannitol and strong aqueous base, refluxing aqueous alcoholic sodium bisulfite,formaldehyde and ascorbic acid. A much milder method involves simply bubbling hydrogen sulfide into a solution of the osmate ester osmium dioxide is precipitated rapidly, and the desired organic... 

Because osmium tetroxide is expensive, and its vapors are toxic, alternate methods have been explored for effecting vic-glycol formation. In the aliphatic series, olefins can be hydroxylated with hydrogen peroxide with the use of only a catalytic amount of osmium tetroxide. Anhydrous conditions are not necessary 30% hydrogen peroxide in acetone or acetone-ether is satisfactory. The intermediate osmate ester is presumably cleaved by peroxide to the glycol with regeneration of osmium tetroxide. When this reaction was tried on a A -steroid, the product isolated was the 20-ketone ... 

Unfortunately, a serious problem with the osmium tetroxide reaction is that Os04 is both very expensive and very toxic. As a result, the reaction is usually carried out using only a small, catalytic amount of OsO, in the presence of a stoichiometric amount of a safe and inexpensive co-oxidant such as A -methylmorpholine N-oxide, abbreviated NMO. The initially formed osmate intermediate reacts rapidly with NMO to yield the product diol plus... 

The most widely used reagent for oxidation of alkenes to glycols is osmium tetroxide. Osmium tetroxide is a highly selective oxidant that gives glycols by a stereospecific syn addition.39 The reaction occurs through a cyclic osmate ester that is formed by a [3 + 2] cycloaddition.40... 

Mono-, di-, and trisubstituted olefins undergo osmium-catalyzed enantioselective dihydroxylation in the presence of the (R)-proline-substituted hydroquinidine 3.9 to give diols in 67-95% yields and in 78-99% ee.75 Using potassium osmate(VI) as the catalyst and potassium carbonate as the base in a tm-butanol/water mixture as the solvent, olefins are dihydroxylated stereo- and enantioselectively in the presence of 3.9 and potassium ferricyanide with sodium chlorite as the stoichiometric oxidant the yields and enantiomeric excesses of the... 

The osmium-catalyzed dihydroxylation reaction, that is, the addition of osmium tetr-oxide to alkenes producing a vicinal diol, is one of the most selective and reliable of organic transformations. Work by Sharpless, Fokin, and coworkers has revealed that electron-deficient alkenes can be converted to the corresponding diols much more efficiently when the pH of the reaction medium is maintained on the acidic side [199]. One of the most useful additives in this context has proved to be citric acid (2 equivalents), which, in combination with 4-methylmorpholine N-oxide (NMO) as a reoxidant for osmium(VI) and potassium osmate [K20s02(0H)4] (0.2 mol%) as a stable, non-volatile substitute for osmium tetroxide, allows the conversion of many olefinic substrates to their corresponding diols at ambient temperatures. In specific cases, such as with extremely electron-deficient alkenes (Scheme 6.96), the reaction has to be carried out under microwave irradiation at 120 °C, to produce in the illustrated case an 81% isolated yield of the pure diol [199]. 

Also fifteen years of painstaking work and the gradual improvement of the system, the Sharpless team announced that asymmetric dihydroxylation (AD) of nearly every type of alkene can be accomplished using osmium tetraoxide, a co-oxidant such as potassium ferricyanide, the crucial chiral ligand based on a dihydroquinidine (DHQD) (21) or dihydroquinine (DHQ) (22) and metha-nesulfonamide to increase the rate of hydrolysis of intermediate osmate esters 1811. 

When recovered from the mineral osmiridium, the mineral is fused with zinc to convert it into a zinc alloy. The alloy is then treated with hydrochloric acid to dissolve the zinc away leaving a finely divided material. This finely divided sohd then is fused with sodium peroxide and caustic soda to convert osmium and ruthenium into their water-soluble sodium salts, sodium osmate and sodium iridate, respectively. While osmium is fully converted to osmate salt, most ruthemium and a small part of iridium are converted to ruthenate and iridate, respectively. The fused mass is leached with water to separate metals from sohd residues. 

To separate osmium from ruthenium, the aqueous solution is acidified with nitric acid. While nitric acid oxidizes osmate ion to volatile osmium tetroxide, Os04, it converts ruthenium to a nitric oxide complex. Osmium tetroxide is removed from the solution by distillation in air and collected in an aqueous solution of caustic soda containing ethanol. Osmium tetroxide solution is heated with ammonium chloride, upon which osmium precipitates out as a complex chloride, 0s02(NH3)4Cl2. The precipitate is filtered, washed and decomposed by ignition with hydrogen to yield osmium metal. 

Osmium is stable in mineral acids even under boiling conditions. The metal, however, is attacked by fused alkalies, such as caustic soda and caustic potash, particularly in the presence of an oxidizing agent such as sodium peroxide, sodium hypochlorite, or sodium nitrite, forming osmates. 

Osmium tetroxide is obtained as an intermediate during recovery of osmium metal from osmiridium or other noble metal minerals (See Osmium). In general, oxidation of an aqueous solution of an osmium salt or complex, such as sodium osmate with nitric acid, yields the volatile tetroxide which may be distilled out from the solution. In the laboratory, the compound can be prepared by oxidation of the osmium tetrachloride, OsCh, or other halide solutions with sodium hypochlorite followed by distdlation. 

Reaction with sodium or potassium hydroxide forms trans-osmate ion, trans-[0s04(0H)2]2 , which is reduced by ethanol to form dark purple [0s i(0)2(0H)4]2 . On the other hand, reaction of osmium tetroxide with rubidium or cesium hydroxide in stoichiometric ratios usually form the ions, [0s04(0H)] and [0s0( j,-0H)0s04] in addition to trans-[0s04(0H)2]27... 

The treatment of this insoluble residue may vary. In one typical process, residue is subjected to fusion with sodium peroxide. Ruthenium and osmium are converted to water-soluble sodium ruthenate and osmate, which are leached with water. The aqueous solution is treated with chlorine gas and heated. The ruthenate and the osmate are converted to their tetroxides. Ruthenium tetroxide is distilled out and collected in hydrochloric acid. The tetroxide is converted into ruthenium chloride. Traces of osmium are removed from ruthenium chloride solution by boiling with nitric acid. 

H2C10O11OS3, Osmium, decacarbonyl-p-hydrido-/i-hydroxotri-, 34 216 H2C12K2O12OS4, Osmate(2-),... 

OS3C10H2O11, Osmium, decacarbonyl- -hydri-do-/i-hydroxotri-, 34 216 OS3C69O9, Osmium, nonacarbonyl(/i3- j. r] vq -fullerene-60)tri-, 34 226 Os3lrNOi3P2C49H3o, Iridate(l-), tridecacarbo-nyltriosmium-, bis(triphenylphosphoranyli-dene)ammonium, 34 208 OsgOgPCggHis, Osmium, octacarbonyl(triphe-nylphosphine) (fi -q q q -fullerene-60)tri-, 34 227 OS4C12H2K2O12, Osmate(2-),... 

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