Osmylation alkene

The first interaction in alkene osmylation is believed to involve a weak charge transfer complex (/ max 400-500 nm) between the 71-donor alkene and the acceptor metal oxide, which usually collapses instantaneously. When the substrate is the bulky adamantylineadamantane or an aromatic hydrocarbon, breakdown of the complex becomes a slow or forbidden process and its detection is possible1. Further decomposition of the charge transfer complex is believed to go through the corresponding metastable ion pair which eventually collapses to the final adduct. 

Studies undertaken in connection with the amine-catalyzed asymmetric alkene osmylation have recently clarified the peculiar mechanism of the hexacyanoferrate/CO2 mediated osmium re-oxidation in biphasic conditions36. Reversal of the osmate oxidation/hydrolysis sequence with respect to the previously described R3NO-mediated conditions was noted with this system. Thus, the monodiolate(amine) osmium(VI) ester 9 appears to be first hydrolyzed, releasing the diol and the amine ligand to the organic phase, and the resulting [0s02(0H)4]2 into the aqueous phase. 

Many examples of polycyclic alkene osmylations have been reported in the literature in connection with the syntheses of specific target molecules such as alkaloids, prostanoids, or steroids. However, carefully determined diastereomer ratios are usually not available. Due to the varied nature of these substrates, it is not possible to formulate definite rules for diastereo-face differentiation except in specific cases. Thus, for example, the exclusive exo reactivity of bridged systems such as bicyclo[2.2.1]heptene derivatives (norbornene-type) is well known as Alder s rule of exo addition63. 

The relatively bulky osmium complex is sterically demanding and osmylation generally involves reaction at the less hindered face of an alkene. Osmylation of 254 to 255 by Rigby, in a synthesis of (+)-pancratistatin, shows the cis-delivery of the reagent and the selectivity for delivery from the less hindered face.357 in this case, the osmium complex formed on the top of the molecule despite the free hydroxyl group on the bottom face. A conformational drawing of 254 will show that the alkene unit and the lactam unit effectively flatten the two rings, and the top face is more accessible. 

Unfortunately, the fast rates of 0s04 addition to most alkenes preclude the observation of D/A complexes, and they are not readily characterized. However, a variety of aromatic electron donors form similar (colored) D/A complexes with Os04 that are more persistent and the observation of ArH/ 0s04 complexes forms the basis for examining the electron-transfer paradigm in osmylation reactions. 

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. 

Asymmetric osmylation of alkenes.3 In the presence of 1 equiv. each of 1 and 0s04, alkenes undergo highly enantioselective ris-dihydroxylation. Highest enantiofacial selectivity (90-99%) is shown in osmylation of trans-di- and trisub-... 

Ring A diosterols.3 The ring A diosterols (3 and 4) of triterpenes can be prepared from the A2-alkene (1) by osmylation to form the two possible cis-diols (2), which on Swern oxidation give the a-diketone (3). The same diketone is also obtained by Swern oxidation of the 2(3,3a-diol, the product of peracid oxidation followed by acid cleavage. The diketone 3 rearranges to the more stable diosphenol (4) in the presence of base. 

Greg Fu supervised the proofreading and provided the structural formula based on the x-ray data for the new Sharpless catalytic reagent for osmylation of alkenes. Martita F. Barsotti is the photographer for the picture of some present and former co-workers for Reagents. 

The first observation that RuO is usable for the reaction was made by Sharpless et al. in 1976 in a footnote to a paper on osmylation of alkenes. It was found that E- and Z-cyclododecene with stoich. RuO /EtOAc at -78°C gave the threo and erythro diols, but the procedure was not deemed viable owing to the low yields obtained and the necessity for working at low temperatures [157],... 

The [2+2] Mechanism Already in 1977 Sharpless proposed a stepwise [2+2] mechanism for the osmylation of olefins in analogy to related oxidative processes with d°-metals such as alkene oxidations with CrO,Cl2 [23, 24], Metallaoxetanes [25] were suggested to be formed by suprafacial addition of the oxygens to the olefinic double bond. In the case of osmylation the intermediate osmaoxetane would be derived from an olefm-osmium(VIII) complex that subsequently would rearrange to the stable osmium(VI) ester. 

Enantioselective dihydroxy lotion.2 Highly enantioselective osmylation of rraiif-disubstituted and monosubstituted alkenes obtains with Os04 oxidations in the presence of 1 equiv. of this chiral 2,2 -bipyrrolidine ligand at —78°. Note, however, that the enantioselectivity for osmylation of cis-disubstituted alkenes is only —65%... 

In contrast, treatment of [0s04] with alkenes (R) or acetylenes in the presence of tertiary nitrogen bases (L = pyridine or isoquinoline) yielded the octahedral osmyl species (23) and (24). From measurements of the ionic products of the sparingly... 

Asymmetric dihydroxylation of alkenes (14, 235-239). Further study1 of this reaction reveals that the optical yields of products can be markedly improved by slow addition (5-26 hours) of the alkene to the catalyst in acetone-water at 0° with stirring. The enantioselectivity can also be increased by addition of tetraethylam-monium acetate, which facilitates hydrolysis of osmate esters. The report suggests that the first product (1) of osmylation can undergo a second osmylation to provide 2, with reverse enantioselectivity of the first osmylation. 

Dihydroxylation of alkenes. C6H5B(OH)2 is useful for capture of the diols formed on osmylation with 0s04 and N-methylmorpholine N-oxide, particularly unstable or water-soluble diols. Osmylation with the borane can also be conducted in a nonaqueous medium with enhanced rates.1... 

Oxo ester complexes with nitrogenous bases (L). Although most of these species contain the osmyl unit there are so many of them that it is appropriate to deal with them in a separate reaction. Most of the complexes have an Os L ratio of 1 2 (the majority taking the form 0s02(02R)py2) and so we consider such species first and include those complexes of the type 0s02(02R)(L—L), where L —L is a bidentate N donor (usually 2,2 -bipyridyl). Within this first section we consider first the species derived from alkenes R or diols R(OH)2, viz. 0s02(02R)L2 or 0s02(02R)L—L, and then species derived from dienes, trienes and alkynes. We then deal with the much smaller body of complexes where the Os L ratio is 1 1. 

Nitrophenylselenation of the primary alcohol in 291, followed by in situ oxidative elimination, furnished alkene 292 in 95% yield. Stereoselective osmylation of the exocyclic alkene in 292, using 0s04 -NM0 gave diol 293, which was additionally characterized as its pentaacetate 294, formed in 65% yield. Acid hydrolysis of 294 furnished the target aminocyclopentitol (+)-5 in 87% yield. 

The addition of the 0s04— N donor complex 3 to alkenes is a much faster process and accounts for the rate acceleration of alkene stoichiometric osmylations in the presence of this and other amines10. 

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