Osmium oxide tetroxide

An interesting catalytic effect upon the alkaline ferricyanide oxidations of ketones is shown by osmium(VIII) tetroxide, the rate expression being... 

Osmic Acid Anhydride Osmium Oxide Osmium Tetraoxide Osmium Tetroxide OU1 Dry Agent ... 

Osmium tetroxide Osmium oxide (8) Osmium oxide, (T-4)- (9) (20816-12-0)... 

The crystals are decomposed by water, but osmie acid is not precipitated owing to oxidation of the osmium to tetroxide by the nitrous acid. With excess of potassium hydroxide potassium osmate is formed ... 

Osmium Tetroxide-Pyridine. Osmium(VI) tetroxide is one of the most extensively used probes for DNA structure. It is an example of a high-valent transition metal complex which, due to its uniquely reactive center, is able to functionalize specific bonds on DNA. This powerful oxidant is known to form cisoid osmate esters upon attack of an electron-rich double bond. By tuning the reactivity of OSO4 and its esters with the addition of other ligands, it has been possible to generate a family of reactive probes for exposed pyrimidine bases. 

Oxidation of alkenes with osmium(Vin) tetroxide (OSO4) (Chapter 6, Scheme 6.10) involves a cyclic intermediate and results in the addition of two hydroxyl groups to the same face [d5-, syn-, (Z)- or suprafacial] of the reacting double bond. However, it is usually desirable to avoid stoichiometric concentrations of expensive, heavy metal poisons. Therefore, the observation that catalytic quantities of osmium(Vni) tetroxide (OSO4) could effectively be used in the presence of a less noxious oxidizing agent (the N-oxide of N-methylmorpholine, NMO) was particularly important. The function of the latter then is to convert the osmium(VI) trioxide (OsOs), produced when the osmium(VIII) tetroxide is reduced (as it performs the oxidation), back to osmiiun(Vni) tetroxide (OSO4), that is,... 

Now, when the (Lewis acid) osmium(VIII) tetroxide (OSO4) is added to a solution containing either (DHQ)2-PHAL or (DHQD)2-PHAL, the metal oxide bridges the two basic nitrogens in the azabicy-clo[2.2.2]octyl systems so that only a narrow channel is available into which the unsymmetrically substituted alkene will fit. Thus, one face of the alkene is more readily oxidized than the other. 

A catalytic enantio- and diastereoselective dihydroxylation procedure without the assistance of a directing functional group (like the allylic alcohol group in the Sharpless epox-idation) has also been developed by K.B. Sharpless (E.N. Jacobsen, 1988 H.-L. Kwong, 1990 B.M. Kim, 1990 H. Waldmann, 1992). It uses osmium tetroxide as a catalytic oxidant (as little as 20 ppm to date) and two readily available cinchona alkaloid diastereomeis, namely the 4-chlorobenzoate esters or bulky aryl ethers of dihydroquinine and dihydroquinidine (cf. p. 290% as stereosteering reagents (structures of the Os complexes see R.M. Pearlstein, 1990). The transformation lacks the high asymmetric inductions of the Sharpless epoxidation, but it is broadly applicable and insensitive to air and water. Further improvements are to be expected. 

Chemical ingenuity in using the properties of the elements and their compounds has allowed analyses to be carried out by processes analogous to the generation of hydrides. Osmium tetroxide is very volatile and can be formed easily by oxidation of osmium compounds. Some metals form volatile acetylacetonates (acac), such as iron, zinc, cobalt, chromium, and manganese (Figure 15.4). Iodides can be oxidized easily to iodine (another volatile element in itself), and carbonates or bicarbonates can be examined as COj after reaction with acid. 

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