Osmate states

Olefins react directly at the electron-rich and rather electron-deficient oxygens. If the dimer is much more reactive toward olefins than the monomer, only a small fraction of the alkaloid-Os04 complex need be present as a dimer (94a). Houk developed a symmetrical five-membered transition-structure model on the basis of X-ray crystal structures of Os04-amine complexes and osmate ester products and ab initio transition structures of analogous reactions (Scheme 40). The MM2 calculations based on this [3 + 2] reaction model reproduce the stereoselectivities of the stoichiometric reactions observed with several chiral diamines (94b). The transition state may be stabilized by tt-tt interaction of the alkene substrate and the ligand aromatic ring (95). 

Two key improvements have been made very recently (96). Scheme 41 summarizes the current state of art, which has been marked by the discovery of the phthalazine class of ligands, (DHQD)2-PHAL and (DHQ)2-PHAL, and the acceleration of osmate ester hydrolysis in the presence of organic sulfonamides, the turnover-limiting step of the reaction of nonterminal olefins. 

Once the [3+2] mechanism was accepted as the operative one, to investigate the transition states associated with the formation of the osmate ester in a chiral system, one must take into account all of the different ways that an olefin can approach the catalyst. These different paths were classified according to the criteria shown in Fig. 12. The olefin binds to an axial and to an equatorial oxygen, providing three different families of reaction paths labeled as regions A, B and C. When the olefin has one substituent, it can be placed in four different orientations , which are labeled as I, II, III, IV. The... 

Potassium osmate may be obtained in a pure state by fusing osmium with potassium nitrate and hydroxide, dissolving the melt in water, and precipitating with alcohol. The potassium osmate thus obtained has a greyish violet colour, and is not quite pure. It is therefore heated to 120° C. with a mixture of sulphuric acid and chromium trioxide, whereby osmium tetroxide is produced. This volatilises and is collected in 10 per cent, potash solution, from which it is obtained in a pure, crystalline form on addition of alcohol.3... 

The interaction of the osmate ion [0s02(0H)4]2 , discussed in Section 18-F-ll, with aqueous HC1 leads via complex transformations to species in oxidation states III to VI some with 0s=0 or OH groups, e.g.,... 

Although all of these methods allow release of the desired diols from the corresponding osmates(VI), the oxidation state of the final osmium species differs ... 

On the basis of the available X-ray crystal structures of Os04 -amine complexes and osmate esters, and ab initio transition structures of analogous reactions, a modified MM2 force field has been created in order to model the transition state of such osmylations. The calculations based on this model reproduce the observed stereoselectivities quite well78. 

Some history. The development of the current state of the art makes an interesting story. The reaction of osmium tetroxide with olefins was discovered14 in the 1930s. The reaction first gives an osmate ester 86. This ester is a stable species but can be hydrolysed to yield the diol 80. 

Shaipless, Singleton, and Houk obtained additional data favoring a [3-I-2] mechanism fi-om studies involving experimental kinetic isotope effects (KIE) and transition-structure/KIE calculations [39]. Their results predict a highly symmetrical transition-state and a [3+2] cycloaddition as the rate-determining step instead of a rate-limiting ring expansion of an osmaoxetane to the osmate ester [40]. 

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