Esters, osmic

Another method for the hydroxylation of the etliylenic linkage consists in treatment of the alkene with osmium tetroxide in an inert solvent (ether or dioxan) at room temperature for several days an osmic ester is formed which either precipitates from the reaction mixture or may be isolated by evaporation of the solvent. Hydrolysis of the osmic ester in a reducing medium (in the presence of alkaline formaldehyde or of aqueous-alcoholic sodium sulphite) gives the 1 2-glycol and osmium. The glycol has the cis structure it is probably derived from the cyclic osmic ester ... 

There are a number of reagents that, overall, add two OH groups to alkenes. Thus osmium tetroxide, 0s04, adds to yield cyclic osmic esters (46), which can be made to undergo ready hydrolytic cleavage of their Os—O bonds to yield the 1,2-diol (47) ... 

Cis 2-butene (48a) thus yields the meso 1,2-diol (47), i.e. the overall hydroxylation is stereoselectively SYN, as would be expected from Os—O cleavage in a necessarily cis cyclic ester (46). The disadvantage of this reaction as a preparative method is the expense and toxicity of 0s04. This may, however, be overcome by using it in catalytic quantities only, but in association with H202 which re-oxidises the osmic acid, (HO)2OsOz, formed to 0s04. 

Osmium tetroxide (0s04, sometimes called osmic acid) reacts with alkenes in a concerted step to form a cyclic osmate ester. Oxidizing agents such as hydrogen peroxide (H202) or tertiary amine oxides (R3N+—O-) are used to hydrolyze the osmate ester and reoxidize osmium to osmium tetroxide. The regenerated osmium tetroxide catalyst continues to hydroxylate more molecules of the alkene. 

Cyclic osmic esters have long been known to be involved in the osmium tetroxide-catalyzed cis-dihydroxylation of alkenes, but not arenes. The isolation of compound (18) by Wallis and Kochi following irradiation of the charge-transfer complex between osmium tetroxide and benzene is therefore of particular interest. This suggests that the corresponding use of catalytic quantities of osmixim tetroxide in conjunction with hydrogen peroxide could lead to the formation of polyhydroxylated cyclohexenes and -anes. 

Two reaction mechanisms have been proposed for these dihydroxylations (pathway a or b, Figure 7.23), either a concerted [3+2] cycloaddition of the olefins on osmium-diamine complex 7.33 or a stepwise reversible [2+2] cycloaddition followed by a rearrangement [559,1350], An X-ray crystal structure of the resulting osmic ester 2.89A shows its symmetrical structure. Houk s calculations [1351] are in favor of a concerted reaction, and his transition state model is reactant-like, with steric interactions dictating the face selectivity of osmylation. 

Osmic esters of type (I) are intermediates in Hofmann oxidation of olefins and they have been isolated by Criegee.122 They are cleaved to glycols by... 

There is a preparative advantage in hydrolysing the osmic ester by aqueous-alcoholic sodium sulfite solution, for then the osmium is precipitated as sparingly soluble sulfite complex. 

Hydroxylation of olefins by hydrogen peroxide in the presence of osmium tetroxide leads to cis-glycols because a cyclic osmic ester is formed as intermediate and the cis-glycol is formed by hydrolysis of that intermediate. Other catalysts afford fraws-hydroxylation, which occurs by way of an intermediate epoxide.138... 

Racemic cii-diols 48. and were prepared as previously described by catalytic osmic oxidation of acronycine (IJ and 6-demethoxyacronycine (17 ). respectively. Treatment of those diols with excess acetic anhydride afforded the corresponding diesters 6 and 67., respectively. When only one equivalent of anhydride was used, monoesters at the less hindered 2-position, such as acetate and benzoate 69. were obtained. Treatment of the monobenzoate 69, with excess acetic anhydride furnished the mixed ester JR-... 

The role of osmium tetroxide in cis hydroxylation has been established. A cyclic ester of osmic acid 94 is formed. Hydrolysis produces an Os... 

Partial formation of cis-glycols. A soln. of osmic acid in benzene added to 3,20-bis(ethylenedioxy)-6-methyl-5,16-pregnadiene-llj, 21-diol in benzene containing pyridine, stirred 30 min. at room temp., the resulting osmate ester decomposed by addition of Na-sulfite and KHGO3 in water followed by methanol, vigorously stirred for 3 hrs. 3,20-bis (ethylenedioxy)-6-methyl-5-pregnene-... 

A soln. of crotyl bromide (mixture with 3-bromo-l-butene) in abs. ether added slowly to Mg in ether at such a rate as to keep the mixture under reflux, stirring continued 1 hr. at room temp., the ether removed with dry Ng, the residue dissolved in tetrahydrofuran, a soln. of (+)-8-methyl-frans-hydrindan-4-ol-l-one in the same solvent added dropwise, stirred 2 hrs. at room temp., then allowed to stand overnight, the product treated with osmium tetroxide, the resulting osmic acid ester isolated by filtration and refluxed 3.5 hrs. with Na-sulfite in water-ethanol tetrol (Y 88%) dissolved in dioxane, and shaken 75 min. with Pb-tetraacetate under Ng dihydroxyaldehyde (Y 74.5%). H. H. Inhoffen, H. Burkhardt, and G. Quinkert, B. 92, 1564 (1959). 

Orthophosphoric acid esters, alkylation with - 22, 775 Orthosilicates s. Silicates Osmic acid s. Osmium tetroxide Osmium tetroxide OCv >CC Oxa... s. a. 0-Heterocyclics Oxacyclimmonium salts, 2-hydroxy- 22, 990, 3,5-Oxadiazines, 6-oxo-... 

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