Osmium tetroxide osmylation

Oxidation of phenol by tris(l,10-phenanthroline)osmium(III) is second order in Os(III) and phenol and inverse second order in Os(II) and acidity. A mechanism is inferred in which the phenoxyl radical is produced through a rapid proton-coupled electron transfer (PCET) pre-equilibrium, followed by rate-limiting phenoxyl radical coupling. Application of Marcus theory indicated that the rate of electron transfer from phenoxide to osmium(III) is fast enough to account for the rapid PCET preequilibrium, but it did not rule out the intervention of other pathways such as concerted proton-electron transfer or general-base catalysis DFT studies, at B3LYP/LACVP level, of the oxidation of ethylene by osmium tetroxide, osmyl hydroxide, and osmyl chloride indicated that in the reaction of osmium tetroxide, the [3 4- 2] addition pathway leading to a five-membered metallacycle intermediate is more favourable than the [24-2] addition. The reaction with osmyl hydroxide is less favourable. In the reaction with osmyl chloride, the [24-2] addition pathway is more favourable than the [3 4-2] addition. ... 

The history of asymmetric dihydroxylation51 dates back 1912 when Hoffmann showed, for the first time, that osmium tetroxide could be used catalytically in the presence of a secondary oxygen donor such as sodium or potassium chlorate for the cA-dihydroxylation of olefins.52 About 30 years later, Criegee et al.53 discovered a dramatic rate enhancement in the osmylation of alkene induced by tertiary amines, and this finding paved the way for asymmetric dihydroxylation of olefins. 

This new process has one unexpected benefit the rates and turnover numbers are increased substantially with the result that the amount of the toxic and expensive 0s04 is considerably reduced (usually 0.002 mole %). The rate acceleration is attributed to formation of an Os04-alkaloid complex, which is more reactive than free osmium tetroxide. Increasing the concentration of 1 or 2 beyond that of 0s04 produces only negligible increase in the enantiomeric excess of the diol. In contrast quinuclidine itself substantially retards the catalytic reaction, probably because it binds too strongly to osmium tetroxide and inhibits the initial osmylation. Other chelating tertiary amines as well as pyridine also inhibit the catalytic process. 

In this report we describe the conversion of cyclohexene to cis-diol 2 in 90% yield in a catalytic osmylation using 1 mole equivalent of N-methylmorpholine N-oxide (1) to regenerate the less than 1 mole % of osmium tetroxide catalyst. This procedure avoids the a-ketol by-products encountered with the presently available catalytic processes, and provides the high yields of the stoichiometric reaction without the expense and work-up problems. 

Certain tertiary amines such as pyridine or a-quinuclidine accelerate the stoichiometric reaction between osmium tetroxide and olefins (86). An asymmetric olefin osmylation using stoichiometric amounts of cinchona alkaloids as the chiral ligands was described in 1980 (87a). Optical yields of up to 90% were attained with frans-stilbene as substrate. 

The potassium salts may he obtained by the action of a suitable reducing agent upon osmium tetroxide in the presence of a simple potassium salt or its hydroxide. Thus, for example, potassium osmyl nitrite results when the tetroxide is reduced by nitric oxide in the presence of potassium nitrite solution ... 

Osmyl sulphites readily lend themselves to preparation in this manner. Rosenheim and Sasserath2 first prepared sodium osmyl sulphite by passing sulphur dioxide through a solution of osmium tetroxide in caustic soda. 

Potassium Osmyl Nitrite, K2(0s02)(N02)4, results when osmium tetroxide is reduced with nitric oxide in the presence of potassium nitrite z... 

The crystals of potassium osmyl nitrite obtained by cooling of the warm, concentrated solution readily decompose even in closed tubes, if exposed to sunlight, the decomposition is rapid, needles of osmium tetroxide and brown nitrous fumes appearing ... 

Osmyl oxy salts have been made the subject of careful study by Wintrebert.2 They can frequently be prepared by the action of an oxidisable salt upon osmium tetroxide. Thus, potassium nitrite reacts with the tetroxide as follows ... 

Potassium Osmyl Oxynitrite, K2(0s03)(N02)3.3II20, is readily obtained 1 as the result of the direct action of a concentrated solution of potassium nitrite on osmium tetroxide ... 

If nitric oxide is passed through a solution of osmium tetroxide in potassium hydroxide containing exactly one molecule of the former to two of the latter, potassium osmyl oxynitrite is formed, and may be isolated on concentration of the solution. The reaction probably proceeds as follows ... 

Potassium osmyl oxynitrite crystallises in the form of brownish black needles, which dissolve in aqueous potash, yielding potassium osmate. The crystals are only slightly soluble in cold water. At 35° C. they begin to lose their water of crystallisation and acquire a yellowish tint. When crushed in a mortar the odours of osmium tetroxide and nitrogen peroxide become apparent. When heated in a current of hydrogen to 200° C. the salt decomposes without explosion, free osmium remaining behind. 

Sodium Osmyl Oxynitrite, Na2(0s03)(N02)2, is apparently formed by the action of sodium nitrite on osmium tetroxide.1 It has not been isolated from the resulting solution, as evaporation yields merely a syrupy liquid. Addition of ammonium chloride to the solution changes its colour from deep brown to yellow, and a yellow crystalline powder, namely, Ammonium Osmyl Oxynitrite, (NH4)20s03.(N02)2, is deposited. 

Osmyl Tetra-ammine Chloride, 0sO2(NH3)4Cl2, and is obtained as a yellow, crystalline precipitate when ammonium chloride is added to aqueous potassium osinate.6 It is but slightly soluble in water, and boiling water decomposes it into osmium tetroxide and osmium di-ammine hydroxide. Its solution gives a violet coloration with potassium ferrocyanide, a reaction that may be utilised for the detection of osmium.7 It is insoluble in alcohol and in hydrochloric acid. When heated it decomposes completely, yielding a mass of spongy osmium. 

Osmylation of diastereomerically pure P-hydroxy sulfoximines, derived from 2a and cyclic enones, with a catalytic amount of osmium tetroxide (5 mol%) and trimethylamine V-oxide (1.5 equiv) gives diastereomerically pure triols which on thermolysis yield 2,3-dihydroxy cyclic ketones in high enantiomeric purity ( 100% ee). Osmylation occurs syn to the sulfoximine group.74... 

The osmylation of arenes (Ar) with osmium tetroxide is a particularly informative system with which to illustrate the close interrelationship between the thermal and photochemical activation of electron-transfer oxidation. For example, a colorless solution of osmium tetroxide in n-hexane or dichlorometbane upon exposure to benzene turns yellow instantaneously. With durene an orange coloration develops and a clear bright red solution results from hexamethylbenzene. The quantitative effects of the dramatic color changes are illustrated in Figure 3 by the spectral shifts of the electronic absorption bands that accompany the variations in aromatic conjugation and substituents. The progressive bathochromic shift parallels the decrease in the arene ionization potentials (/F) in the order benzene 9.23 eV naphthalene... 

Hydroxylation of the 15, 20 double bond in (266) by means of osmium tetroxide could not be accomplished satisfactorily however, osmylation of the iVb -oxide of (266) followed by reductive (HaS) work-up afforded a hydroxyvinblastine formulated as the /8-diol, i.e. the C-15 epimer of (262). Since the stereochemical assignments are at present tentative, it is unfortunate that an authentic specimen of vincadioline was not available for direct comparison. ... 

Osmylation and Epoxidation With osmium tetroxide, carbon nanotubes react as expected for a compound containing double bonds. The osmylation adduct with the respective double bond being replaced by two C-O-bonds is formed as shown in Figure 3.78. However, the process is normally conducted in a photochemical way here. The intermediates thus obtained can be transformed into hydroxylated nanotubes by hydrolysis. In doing so, it is advisable to effect a reoxidation of the resultant osmium(Vl) by hydrogen peroxide in order to minimize the consumption of osmium. The osmylation of carbon nanotubes is reversible so the process may also be employed for purification or separation steps. Contrary to an ozonoly-sis with subsequent reductive work-up, the osmylation does not give rise to holes in the side wall. Hence the electronic structure is less affected. 

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