Osmate , potassium

Potassium osmate (VI) dibydrate [19718-36-6] M 368.4. Hygroscopic POISONOUS crystals which are soluble in H2O but insol in EtOH and Et20. It decomposes slowly in H2O to form the tetroxide which attacks the eyes. The solid should be kept dry and in this form it is relatively safe. [Synthesis 610 1972.]... 

Mono-, di-, and trisubstituted olefins undergo osmium-catalyzed enantioselective dihydroxylation in the presence of the (R)-proline-substituted hydroquinidine 3.9 to give diols in 67-95% yields and in 78-99% ee.75 Using potassium osmate(VI) as the catalyst and potassium carbonate as the base in a tm-butanol/water mixture as the solvent, olefins are dihydroxylated stereo- and enantioselectively in the presence of 3.9 and potassium ferricyanide with sodium chlorite as the stoichiometric oxidant the yields and enantiomeric excesses of the... 

The hydroxylation of alkenes to diols with potassium osmate-oxidant mixtures has been described, with either hydrogen peroxide or sodium chlorate as the oxidant. The sodium chlorate method is not applicable where the diol is to be distilled from the mixture, because of the danger of explosive oxidation of the product diol by the chlorate. 

The osmium-catalyzed dihydroxylation reaction, that is, the addition of osmium tetr-oxide to alkenes producing a vicinal diol, is one of the most selective and reliable of organic transformations. Work by Sharpless, Fokin, and coworkers has revealed that electron-deficient alkenes can be converted to the corresponding diols much more efficiently when the pH of the reaction medium is maintained on the acidic side [199]. One of the most useful additives in this context has proved to be citric acid (2 equivalents), which, in combination with 4-methylmorpholine N-oxide (NMO) as a reoxidant for osmium(VI) and potassium osmate [K20s02(0H)4] (0.2 mol%) as a stable, non-volatile substitute for osmium tetroxide, allows the conversion of many olefinic substrates to their corresponding diols at ambient temperatures. In specific cases, such as with extremely electron-deficient alkenes (Scheme 6.96), the reaction has to be carried out under microwave irradiation at 120 °C, to produce in the illustrated case an 81% isolated yield of the pure diol [199]. 

In a 50 mL round-bottomed flask equipped with a magnetic stirrer bar were placed tert-butyl carbamate (545 mg) and n-propanol (6 mL). A solution of sodium hydroxide (183 mg) in water (12.2 mL) and tert-butyl hypochlorite (0.53 mL) were added to the solution. The resulting solution was stirred for 5 minutes and cooled to 0°C. Then a solution of DHQ2PHAL (71 mg) in n-propanol (6mL), a solution of 4-methoxystyrene in //-propanol (12.2 mL) and potassium osmate dihydrate (22.5 mg) were added sequentially to give a green solution. After 1 hour at 0 °C, the reaction mixture had turned from green to yellow. 

The potassium ferricyanide was Spectrum technical grade. Methane-sulfonamide (97%), (DHQ)2PHAL (95%), potassium osmate dihydrate and tert-butyl alcohol (99.3%) were obtained from Aldrich Chemical Company, Inc. 1-Phenylcyclohexene (98%) was obtained from Acros Organics. 

The oxidation of sulfides to sulfones has been the subject of extensive studies, since sulfones are useful synthons for the construction of various chemically and biologically significant moleculesJ Recently, a new catalytic system has been developed by exchanging potassium osmate onto chloride-saturated layered double hydroxides (Figure 9.1), which we have shown to be an efficient catalyst for the direct oxidation of sulfides to sulfones, using molecular oxygen as the stoichiometric oxidant and with delivery of two oxygen atoms simultaneously to the sulfide, reminiscent of olefin dihydroxylation reactions. 

Mg-Al-Cl LDH (1 g) was suspended in 100 mL (0.689 g, 1.87 mmol) of aqueous potassium osmate solution and stirred at 25 °C for 12 h under nitrogen atmosphere. The solid catalyst was filtered, washed thoroughly with 500 mL of water, and vacuum-dried to obtain 1.416g of LDH-OSO4 (1.34mmol of Os per g). 

Note Due to the high toxic nature of potassium osmate, it should be handled with extreme care in a chemical fume hood using chemical resistant gloves and safety goggles. 

Tetrammino-osmyl-sulphate, [0s02(NH3)4]SO4, may be prepared by mixing potassium osmate with ammonium sulphate. It is a yellow crystalline body which is sparingly soluble in cold water and decomposed by boiling water. 

The oxalate, [0s02(NH3)4]C204, is prepared in similar manner from potassium osmate and ammonium oxalate. It is a yellow crystalline substance. 

By the action of ammonia on potassium osmate a brown liquid is produced which probably contains another base of formula [Os(NH3) 2] (OH) 2. The liquid, which has not been fully investigated, is basic in character and yields salts with acids.2... 

Since the distribution of the chlorooxo-osmate species present in the hydrochloric acid solution that is loaded on the generator is affected by the acid concentration C5), a series of generators were prepared using a range of hydrochloric acid concentrations to dissolve the potassium osmate precipitate. The results obtained with these generators are included in Table... 

Alternate sources of osmium can be used and should be considered, as they offer advantages in ease of handling. Both osmium(III) chloride hydrate [OsCl xHjO] [30] and potassium osmate(VI) dihydrate [K2OsO2(0H)4] [31] have been used as replacements for osmium tetroxide, and the latter reagent is specified in the preceding standard set of conditions for catalytic AD. 

Methyl cinnamate (30) served as starting material for both routes. The AD was carried out with only 0.5 mol % of the (DHQ)2PHAL ligand and 0.2 mol % of potassium osmate(VI) dihydrate, which renders this process extremely cost-effective. Enantiomerically pure bis-hydroxyester 86 was obtained in 72% yield after one recrystallization. Conversion of 86 into A7-benzoyl-(2i ,3S)-3-phenylisoserine (88) was accomplished in only three steps giving the paclitaxel C-13 side chain in 23% overall yield. 

Alkenes and Potassium Osmate. Sodium chlorate should not be used in the hydroxylation of alkenes to diols in the presence of potassium osmate if the diol is to be distilled from the reaction mixture explosive oxidation may occur.3... 

The osmium complexes rans-[0sVI(py)3(0)2(H20)]2+ and rans-[OsVI (py)2(0)2X2] (X = Cl, Br) have been prepared by Griffith and Rossetti by the treatment of potassium osmate with pyridine followed by addi-... 

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