Potassium Osmium Research Articles

2- C-Trifluoromethyl substituted pentoses

The synthesis of racemic 2- C-trifluoromethyl ribose and racemic 2- C-trifluoromethyl arabinose is described. The four step reaction sequence includes diastereoselective enol ether addition to methyl trifluoropyruvate, diastereoselective dihydroxylation of the CC double bond with potassium osmate, lactonization, and finally SMEAH reduction to give the lactol. 2- C-Trifluoromethyl pentoses exhibit unexpected anomeric and tautomeric stabilities.

Chemical and Electrochemical Asymmetric Dihydroxylation of Olefins in I(2)-K(2)CO(3)-K(2)OsO(2)(OH)(4) and I(2)-K(3)PO(4)/K(2)HPO(4)-K(2)OsO(2)(OH)(4) Systems with Sharpless' Ligand.

Iodine-assisted chemical and electrochemical asymmetric dihydroxylation of various olefins in I(2)-K(2)CO(3)-K(2)OsO(2)(OH)(4) and I(2)-K(3)PO(4)/K(2)HPO(4)-K(2)OsO(2)(OH)(4) systems with Sharpless' ligand provided the optically active glycols in excellent isolated yields and high enantiomeric excesses. Iodine (I(2)) was used stoichiometrically for the chemical dihydroxylation, and good results were obtained with nonconjugated olefins in contrast to the case of potassium ferricyanide as a co-oxidant. The potentiality of I(2) as a co-oxidant under stoichiometric conditions has been proven to be effective as an oxidizing mediator in electrolysis systems. Iodine-assisted asymmetric electro-dihydroxylation of olefins in either a t-BuOH/H(2)O(1/1)-K(2)CO(3)/(DHQD)(2)PHAL-(Pt) or t-BuOH/H(2)O(1/1)-K(3)PO(4)/K(2)HPO(4)/(DHQD)(2)PHAL-(Pt) system in the presence of potassium osmate in an undivided cell was investigated in detail. Irrespective of the substitution pattern, all the olefins afforded the diols in high yields and excellent enantiomeric excesses. A plausible mechanism is discussed on the basis of cyclic voltammograms as well as experimental observations.

Electrooxidative Fission of Carbon–Carbon Double Bond in a K2OsO2(OH)4/HIO4 Double Mediatory System

Abstract Electrooxidation of olefins in the presence of potassium osmate (0.04 molar amount) and periodic acid (0.5 molar amount) afforded the corresponding carbon–carbon double bond cleavage products in moderate to good yields, where the turnover number of periodic acid reached ca. 3.

Electrochemical Os-Catalyzed Asymmetric Dihydroxylation of Olefins with Sharpless’ Ligand

Abstract Asymmetric dihydroxylation of olefins using Sharpless’ ligand in the presence of potassium osmate [K2OsO2(OH)4] has been attained by recycling use of a catalytic amount of potassium ferricyanide [K3Fe(CN)6] in electrolysis media. The prominent advantage of the electrooxidation method is that minimum use of reoxidizing agent as well as K2OsO2(OH)4 may cause to improve enantioselectivities in comparison with the case of stoichiometric oxidant systems.

Potassium-modified osmium/alumina catalysts

Al 2O 3-supported osmium catalysts were prepared from Os 3(CO) 12 and from K 2[OsO 2(OH) 4]. The cluster-derived catalyst was modified with potassium by an additional impregnation with K 2CO 3. The materials were physically characterized by carbonyl infrared spectroscopy, temperature-programmed reduction and transmission electron microscopy. Isolated surface complexes were the majority species in the Os 3-derived catalysts, their oxidation state being +2 after thermal treatment while Os(0) carbonyl species were formed after H 2 reduction. These did not aggregate to metal particles due to the presence of the protecting CO ligands. Catalysts prepared from the potassium osmate compound behaved differently for different loadings. At low loading (1.6 wt.% Os) Os 2+ species were highly stabilized even after reducing treatments. CO turned out to be a more efficient reducing agent than H 2. In contrast, at high loading (4.2 wt.% Os) metal aggregation was observed after H 2 reduction, with average metal particle diameters of 1.8 nm. The major effect of potassium in these materials seemed to be blocking of the osmium species for CO adsorption at low temperatures (⩽300K). The formation of bicarbonate, carbonate and formate species was also detected during the adsorption of CO. These species may be involved as intermediates in catalytic reactions such as water-gas shift. Possible formation routes of these species and the role of potassium are discussed.

Chemical aspects of glycogen contrast-staining by potassium osmate.

To obtain contrast-staining of glycogen in electron microscopy, various contrast-enhancing additives can be used in combination with potassium osmate. Examples are potassium ferrocyanide and certain nitrogen heterocyclic compounds, such as triazoles. In the reaction sequence leading to contrast-stained glycogen, a primary reaction is the formation of glycogen osmate. This reaction was studied with isolated glycogen. On the basis of the stoichiometric findings, a molecular structure of the reaction product is proposed; apparently, osmate is bound by glycogen because of the presence of suitably located hydroxyl groups. The resulting compound is not itself sufficiently electron dense, but it binds 1,2,4-triazole as an additional ligand. Secondary reactions can result in additional osmium binding, and finally to osmium (IV) deposits, leading to contrast.

Cellular membrane contrast and contrast differentiation with osmium triazole and tetrazole complexes.

Addition of heterocyclic nitrogen compounds to the classical osmium tetroxide postfixation medium, applied after glutaraldehyde fixation, results in enhanced membrane contrast in ultrathin sections of liver tissue. The addition of similar compounds to potassium osmate solutions, results in contrast differences in some cellular membranes. The membranes of the rough endoplasmic reticulum, the nuclear envelope and the plasma membrane acquire contrast, while the mitochondrial membranes do not. The apolar regions of membranes are contrasted when osmium tetroxide is combined with heterocyclic nitrogen compounds, whereas the polar regions are contrasted by combinations of potassium osmate with these compounds. This polar membrane contrast is probably due to the presence of an amino-group in the heterocyclic nitrogen compounds. Compounds without the amino-group do not contrast membranes, although the glycogen is contrasted. X-ray microanalysis served to establish the relative osmium content in contrasted glycogen, and showed that such nitrogen compounds play a role in complexation of cations in aldehyde-fixed tissues. Electron spectroscopy for chemical analysis (ESCA) measurements of isolated muscle glycogen show that after treatment with various osmium tetroxide or potassium osmate solutions, hexavalent and quadrivalent osmium species are present in the glycogen. The presence of (heterocyclic) nitrogen compounds in such solutions stabilizes certain osmium valency species, and this may account for the contrast observed.

The acidic behaviour of osmium(VIII) and osmium(VI)

Various methods yield a value of 6.3 · 10−13 for the apparent k1 and ∼ 4 · 10−15 for K2 of osmium(VIII) acid at 298 K. Spectrophotometric and potentiometric measurements yielded approximate values for the acidic constants of osmium(VI) acid: 3.2 · 10−9 and 4.0 · 10−11. The dependence of the pH of solution on the potassium osmate(VI) concentration can be interpreted in terms of the base inclusions in the preparation, rather than in terms of the dimerization of the osmium(VI).

The x-ray crystal structures of the oxo-osmium complexes, OsO2(OH)2phen (1) and Os2O6py4 (2)

Potassium osmate (trans-K2[OsO2(OH)4]) reacts with 1,10phenanthroline (phen) to give three oxo-osmium(VI) phenanthroline complexes; one is believed to be an oxo-bridged dimer (A) while the other two are thought to be closely related monomers of structures (B), (C) or (D) (1). We report here the crystal structure of the "copper-coloured" monomer (1) and find it to be of type (D). We also report the crystal structure of Os206py 4 (2) and show it to be a type (A) complex. This latter species is made by reaction of OsO4 and pyridine (py) in the presence of ethanol (z' 3) and has been variously formulated as OsO3py 2 (structure C) (2), OsO2(OH)epy2 (structure D) (4) or OszO6py 4 (structure A) (3-6). These compounds are relevant to the preparation of oxo-osmium(VI) esters of biologically important molecules (1' 7), and the structure of (1) is relevant to some recent controversy in the histological literature (8) over the oxidation state of osmium in fixed tissue. Crystals of (1) were grown by the method of Chang et al. [11 using slow crystallisation from the mother-liquor; (2) was prepared by the method of Subbaraman et al. (3) and very slowly recrystallised from water. For both compounds, crystals were kept damp in sealed capillaries during x-ray examination.

Reduction of osmium(VIII) to osmium(II) by reaction with an olefin and EDTA. Formation of an O S II—EDTA complex

The complex Na 6H 4[O S II (EDTA) 3] is formed by reaction between osmium tetroxide, an olefin and EDTA. The same complex is produced by reaction between potassium osmate and EDTA. The EDTA analogue, trans-1,2-diaminocyclohexane-N,N,N′,N′- tetraacetate, forms a similar complex.

A Hydroxylation Catalyst from Potassium Osmate

A Hydroxylation Catalyst from Potassium Osmate

Osmium tetroxide fixation of lipids for electron microscopy a possible reaction mechanism

1. 1. To elucidate the reaction mechanisms responsible for the fixation of cellular materials by osmium tetroxide, as used in electron microscopy, the products resulting from the interaction of OsO 4 with various unsaturated substances were studied. By a spectrophotometric method, crotonic acid, oleic acid, linoleic acid and lecithin (phosphatidyl choline) were found to take up approx. 1 mole OsO 4 per double bond. In dilute solutions in water-containing media cyclic osmic acid mono-ester formation occurs. Cyclic osmic mono-esters were also found to be products of the reaction between cis-diols and potassium osmate. 2. 2. Cyclic osmic esters in watery media behave like acids, which means they probably have a hydroxylated structure. The corresponding anions readily combine by electrostatic interaction with quaternary ammonium groups in bipolar phosphatides. 3. 3. The reaction between OsO 4 and membrane phosphatides probably begins with the formation of cyclic mono-esters, the corresponding anions moving almost simultaneously into the two polar strata of the membrane bilayer. Although nonlipid substances and dehydrating solvents lead to further reduction, the ultimate location of osmium dioxide deposits may be determined by the reaction between OsG4 and membrane lipids occurring initially.