Tetroxide OsO

There are a number of reagents that, overall, add two OH groups to alkenes. Thus osmium tetroxide, OsO, 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) ... 

One of the important properties of osmium is the formation of gases when the metal is exposed to air. These fumes are extremely toxic, which limits osmiums usefulness. Osmium is a hard, tough, brittle, bluish-white metal that is difficult to use except in a powder form that oxidizes into osmium tetroxide (OsO ), which not only has objectionable odor but also is toxic. 

Whereas hydrolysis of epoxides leads to the trans diaxial addition of water and the formation of trans glycols (1,2-diols), cis glycol formation involves the addition of osmium(VIII) oxide (osmium tetroxide, OsO ) or cold dilute aqueous potassium manganate(Vll) (potassium permanganate) to an alkene. 

For most cases staining with osmium or rhu-tenium tetroxide (OsO or RuO, respectively) yields sufficient results. OsO reacts with isolated double bonds (C=0, C=C) such as the double bonds in polyisoprene and polybutadiene, but it does not react with conjugated double bonds [Parker and Vesely, 1990]. 

Several methods of studies have been developed. Osmium-staining technique, pioneered by Kato [1967], is one of the most successful methods for observing crazing in rubber toughened plastics. It depends upon a reaction between osmium tetroxide, OsO, and double bonds in PBD and other unsaturated polymers. However, it is not suitable for saturated rubbers. 

Osmium tetroxide, OsO. This is undoubtedly the most important compound or complex of osmium, and it was as the tetroxide that osmium was first separated and, as explained on p. 522, gave its name to the element. It is industrially important both in the fine organic chemicals industry (for the cis hydroxylation of alkenes) and in biological chemistry and medicine for the fixation or... 

The reaction of alkenes with osmium tetroxide (OsO ) is an example of an oxidation reaction (Following fig.). In this case the alkene is not split, but, a 1,2-diol is obtained which is also called a glycol. The reaction involves the formation of a cyclic intermediate where the osmium reagent is attached to one face of the alkene. On treatment with sodium bisulphite, the intermediate is cleaved such that the two oxygen atoms linking the osmium remain attached. Due to this both the... 

ESCA measurements ESCA(Electron Spectroscopy for Chemical Analysis) spectra were obtained on a Shimazu ESCA Model 650-B electron spectrometer using MgKa radiation. Samples of membranes cast from chloroform (Cf) - trifluoro-ethanol (TFE) 10 1 mixture, followed by fixation and staining of membranes by osmium tetroxide (OsO ) were coated onto plane glass plate, and fixed to the probe by doublesided tape. 

FIGURE 17. ESCA spectra of the products of the reaction of each polymer membrane with osmium tetroxide OsO, vapor (a) PCBL-1, (b) LBL-1, (c) LBL-2, and (d) LBL-5. 

OSMIUM TETROXIDE OsO, 0 mk acid Hydrochloric acid, easily Oiddized organic materiab Not combustible ... 

Caution. Osmium tetroxide, OsO, is a severe poison and must be handled in a well-ventilated hood. 

Osmiiun tetroxide, OsO, (osmic acid) is highly toxic if inhaled, ingested, or absorbed through the skin. Vapors can react with corneal tissues and cause blindness. There is a possible risk of irreversible effects. Wear appropriate gloves and safety goggles and always use in a chemical fume hood. Do not breathe the vapors. 

Since 1980, mthenium tetroxide, RuO, has been used for staining a number of heterophase polymers for tern (221) it seems to be a more versatile staining agent than OsO. For instance, in SAN modified with acrylate mbber, where the mbber phase is fully saturated, an excellent contrast between the mbber and the matrix has been achieved (222). Crystalline polymers have been stained with RuO (223), and excellent cra2e stmctures have been revealed (221). The stain may be prepared by dissolving RuCl - 3H2O in aqueous sodium hypochlorite for immediate use (224). 

The double bonds of avermectins react with y -chloroperbenzoic acid to give 3,4-, 8,9-, and 14,15-epoxides. The 8,9-epoxide is the primary product and can be isolated in good yield (45). The 8,9-epoxide was opened by aqueous acids to the 8,9-diol (46). The 3,4-diol can be obtained readily and regiospecificaHy by osmium tetroxide oxidation. Neither peracids nor OsO will attack the 22,23-double bond. 

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 bulk metal is stable in oxygen at ordinary temperatures but reacts at 200°C, forming osmium tetroxide. When further heated to 800 to 1,500°C in air or oxygen, the tetroxide converts to volatde trioxide, OsOs. When the metal is heated in a stream of 0s04 vapor, osmium dioxide is produced ... 

Osmium tetroxide reacts with amines, undergoing partial substitution of NR2 (where R=alkyl group) with O to form compounds such as OsOs (NR) and OsO(NR)3. 

Diammino-osmo-hydroxide, [OsO(NH3)2](OH)2, is prepared from osmium tetroxide and excess of concentrated aqueous ammonia. The yellow liquid obtained is heated in a closed vessel at 50° C. till it becomes dark brown in colour and a black powder is deposited. It is then exposed to the atmosphere and evaporated at low temperature, when a blackish-brown powder is deposited. The dry powder decomposes explosively on heating, with evolution of nitrogen. It dissolves in acids yielding the corresponding salts, and from these the base may be precipitated on the addition of aqueous alkali hydroxide. On boiling with a solution of alkali, however, the base is decomposed, ammonia is liberated, and a precipitate of osmium dioxide remains.1... 

Now in the foregoing reactions the osmium is used in the form of its tetroxide, in which its valency is 8. The first action is that of the reducing agent, which lowers its valency to six. Then comes the formation of the osmyl derivative. For example, the preparation of potassium osmyl oxalate takes place in two stages, namely, (1) the reduction of 0s04 to OsOs, which, in the presence of the potassium hydroxide, yields potassium osmate, K20s04 and (2) interaction of the last named with 2 molecules of oxalic acid to form the osmyl derivative. These stages may be expressed as follows ... 

Osmium Di-ammine Hydroxide, OsO(NH3)2(OH)2, results when the tetroxide is dissolved in concentrated aqueous ammonia and heated in a closed vessel to 50° C. On opening the vessel and evaporating the excess of ammonia, osmosammine hydroxide is obtained as a dark brown powder.2 It unites with acids to form salts, and upon being heated readily decomposes. When boiled with alkalies ammonia is evolved and hydrated osmium dioxide remains. ... 

Obviously the chemistry here is dominated by that of the tetroxide. We briefly consider the hexa-and penta-oxo species, then Os04, followed by species of the type Os04 L and OsO, . The few tri-, di- and mono-oxo complexes of osmium(VIII) conclude the section, (see Table 23). 

Check to make sure that the osmium tetroxide is fully dissolved. Osmium crystals must be left overnight in distilled H O to dissolve, and then mixed the following morning with 0.2 M phosphate buffer to make a working solution of 1% OsO. Remember to wear gloves and work in the hood when handling osmium... 

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,... 

Ruthenium tetroxide is a stronger oxidizing staining agent than OsO, which enables RUO4 to react also with phenyl groups, e.g. in polystyrene. Furthermore, ruthenium tetroxide has been used to stain polyethylene and polypropylene in addition to the list of polymers stained by OSO4. 

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