Osmium tetroxide—Bis

Another series of publications from Ken s group compared kinetic isotope effects, computed for different possible transition structures for a variety of reactions, with the experimental values, either obtained from the literature or measured by Singleton s group at Texas A M. These comparisons established the most important features of the transition states for several classic organic reactions — Diels-Alder cycloadditions, Cope and Claisen rearrangements, peracid epoxidations, carbene and triazolinedione cycloadditions and, most recently, osmium tetroxide bis-hydroxylations. Due to Ken s research, the three-dimensional structures of many transition states have become nearly as well-understood as the structures of stable molecules. 

Osmium tetroxide-Bis(3-methyl-2,4-pentanedionato)nickel(ll), 0s04/Ni(mac)2. 

Many of the reactions already discussed for the preparation of bis-oxygenated pregnanes can also be used for the synthesis of 17,20,21-tris-oxygenated pregnanes by proper choice of substrate. Thus, reaction of a 17-vinyl-17-hydroxy steroid or a A -21-hydroxypregnene with osmium tetroxide will give the 17,20,21-triol, and the Stork reaction can be applied to 17a-hydroxy-20-keto steroids. 

Hydroxycortisone BMD) (48) A solution of 4 g of 17a,20 20,21-bis-methylenedioxypregn-4-ene-3,l 1-dione (cortisone BMD) (46) dissolved in 300 ml of t-butanol and 5 ml of water is treated with 34 ml of 35 % hydrogen peroxide and 0.45 g of osmium tetroxide predissolved in 36 ml of /-butanol. The resulting mixture is allowed to stand at room temperature for 2 days. Diol (47) which crystallizes during the reaction is collected by filtration and washed with /-butanol and water. The filtrate is diluted with ethyl acetate and washed sequentially with aqueous sodium chloride, aqueous 10% sodium bisulfite, aqueous 10% sodium bicarbonate and finally with water to neutrality. The solvent is evaporated and a second crop of the diol (47) is collected, providing a total of about 3.8 g. 

Carbocations as electron acceptors in aromatic EDA complexes 192 Bis(arene)iron(II) complexes with arene and ferrocene donors 198 Carbonylmetallate anions as electron donors in charge-transfer salts 204 Aromatic EDA complexes with osmium tetroxide 219... 

Using chromium-based oxidants 2,4-Dimethylpentane-2,4-diol chromate(VI) diester, 122 Trimethylsilyl chlorochromate, 327 Using other oxidizing agents Bis(tributyltin) oxide, 41 Hydrogen hexachloroplatinate(IV)-Copper(II) chloride, 145 4-Methoxy-2,2,6,6-tetramethyl-1 -oxopiperidinium chloride, 183 Osmium tetroxide, 222 Potassium nitrosodisulfonate, 258 Samarium(II) iodide, 270 From alkenes by addition or cleavage reactions... 

Trimethylsilyldiazomethane, 327 Silyl substituted arenes Bis(trimethylsilyl)acetylene, 97 Chromium carbene complexes, 82 Titanium(IV) chloride-Diethylalu-minum chloride, 309 Other organosilanes Osmium tetroxide-Trimethylamine N-oxide-Pyridine, 223 Tributyltin chloride, 315 Di- x-carbonylhexacarbonyldicobalt, 99 Trimethylsilyl trifluoromethanesul-fonate, 329... 

Osmium tetroxide-N-Methyl-morpholine N-oxide, 222 Osmium tetroxide-Trimethylamine N-oxide-Pyridine, 223 Palladium Compounds Benzylchlorobis(triphenylphosphine)-palladium(II), 30 Bis (ace tonitrile) chloronitropalla-dium(II)-Copper(II) chloride, 33 Bis(acetonitrile)dichloropalladium(II), 33, 211, 236... 

To improve the position selectivity in the AD of oligoprenyl compounds bis-cinchona alkaloid ligand 8 was introduced by Corey 15,6]. Its design was based on the [3+2]-cycloaddition model for the AD mechanism, which will be discussed in Section 6E. 1.2. The two 4-heptyl ether substituents of the quinolines are supposed to assist fixation of the substrate in the binding cleft. Additionally, the jV-methylquinuclidinium unit and the linking naphthopyridazine were introduced to rigidify the osmium tetroxide complex of 8 [6],... 

Osmium tetroxide on treatment with the chelating diamine L2 (L2 = [R,R]-trans-1,2 bis[N-pyrrolidino]cyclohexane) gives 0s04L2. What is the electron count in this complex, and does this have any relevance to the ADH mechanism ... 

Ujaque, G., Maseras, F., Lledos, A. Theoretical Characterization of an Intermediate for the [3 + 2] Cycloaddition Mechanism in the Bis(dihydroxy- quinidine)-3,6-Pyridazine.Osmium Tetroxide-Catalyzed Dihydroxylation of Styrene. J. Org. Chem. 1997, 62, 7892-7894. 

As with the mechanism of osmium tetroxide addition, the origin of the enantioseiectivity in the AD process has also been a subject of much debate. An early proposal by Corey et al. [48], suggested that the AD reaction proceeded through the p-oxo-bridged bis-osmium(VIII) complex shown in Fig. 11. 

This proposal was later refuted by Kolb et al. [65] based on the fact that the rate of the AD reaction is first order in both osmium tetroxide and alkene, while Corey s proposal would require a second-order component for osmium tetroxide. Furthermore, Kolb et al. showed that the second alkaloid moiety in the bis-alkaloid ligands is not necessary for high enantioseiectivity, precluding any interaction between both quinuclidine nitrogen atoms and osmium tetroxide. The two prevalent models for explaining the introduction of enantioseiectivity in the AD process both operate with one active osmium tetroxide molecule per ligand. 

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