Oxidation osmylation

Organometals and metal hydrides as electron donors in addition reactions 245 Oxidative cleavage of carbon-carbon and carbon-hydrogen bonds 253 Electron-transfer activation in cycloaddition reactions 264 Osmylation of arene donors 270... 

The olive-green osmium(VI) octaethylporphyrin complex 0s02(0EP) (IR i/(Os—O) 825 cm-1) is representative of a number of osmyl porphyrins [185] they can readily be transformed into a number of osmium porphyrins in lower oxidation states (Figure 1.73). 

Next is the construction of the D ring. The TMS enol ether of compound 111 undergoes oxidation with m-CPBA, providing the C-5a trimethylsilyloxy ketone 112. Addition of methyl Grignard reagent to the ketone group and subsequent dehydration provides compound 113. Osmylation of the C=C double... 

Dihydroxylation. The key step in the synthesis of a natural mycotoxin from the dehydropentacyclic precursor 1 requires dihydroxylation of the nuclear double bond. Direct osmylation with catalytic 0s04 and N-methylmorpholine N-oxide... 

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. 

Isolated double bonds can be oxidatively cleaved in systems containing a conjugated diene moiety if it is protected as a tricarbonyl(diene)iron complex44. Dienal 39 was acquired in 49% yield by a two-step osmylation-periodate cleavage sequence (equation 27). In contrast, ozonolysis of the polyene complexes is reported to lead to destruction of the complex. 

Substituted 4,5-dihydro-5-vinylisoxazoles (40), obtained by regio- and stereospecific cycloaddition of nitrile oxides to dienes, undergo smooth osmium-catalyzed c/s-hydro-xylation to give amino-polyol precursors (equation 28)45. The reaction is anti selective, the diastereomeric ratios ranging from 73 27 up to 99 1. Highest stereoselectivities were observed when R3 was methyl. Thus, whereas osmylation of 40a afforded a 78 22 mixture of 41a and 42a, respectively, in 80% overall yield, similar treatment of 40b resulted in a 92 8 mixture of 41b and 42b, respectively, in 70% overall yield. The cycloaddition-osmylation sequence allows control of the relative configuration of up to 4 contiguous asymmetric centers. 

Preliminary studies on catalytic osmylation were reported by Kokubo and co-workers who used bovine serum albumin, 0s04, and t-butylhydroperoxide as the oxidant. Turnover numbers up to 40 and an e.e. for 2-phenylpropene of 68% was achieved [23], Apparently the protein binds to osmium via nitrogen donors, but as different sites may be available this may lower the e.e. 

Many ruthenyl and osmyl complexes with O donor ligands are known. The ruthenium compounds are usually prepared from [RuOJ, which can be generated by the oxidation of RuCl3 xH20 or [RUO2] with [104] . The osmium compounds are usually prepared from [OSO4] or K2[0s(0)2(0H)4]. 

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. 

An asymmetric osmylation method has been developed by Sharpless and coworkers. 0s04 modified by a dihydroquinidine auxiliary (cinchona alkaloid derivatives)449,455 158 or chiral diamines449,457 160 such as 59 and 60 used in stoichiometric oxidation may yield cis diols with excellent optical purity 460... 

The selective oxidation of hydrocarbons, particularly that of alkanes, remains a challenge. It is not surprising, therefore, that the problems of oxidation processes are addressed in several books,1043-1045 reviews,1046-1057 and a journal special issue,1058 as well as in international conferences1059-1064 devoted to the topic. For the advances in chirally catalyzed oxidation processes, including asymmetric epoxidation and osmylation, Sharpless was one of the recipients of the 2001 Nobel Prize in Chemistry. 

According to Shaipless, two cycles operate in the catalytic reaction (Scheme 39) (88c, 9CF). The first cycle is highly enantioselective, whereas the second is poorly enantioselective. Hydrolysis of the key intermediate formed from B and oxidant is not very fast. The second osmylation of olefinic substrate occurs as the intermediate enters the undesired catalytic cycle. Therefore, slow addition of olefinic substrates to minimize the second cycle is essential for obtaining high ee. Use of potassium hexacyanoferrate(III) as oxidant in a 1 1 tert-butyl alcohol-water two-layer system can suppress the second cycle and lead to high enantioselectivity (91). This procedure allows the convenient synthesis of 3-lactams from 2-octenoate. 

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. 

Diastereoselective epoxidations.6 Epoxidation of the ergoline 1 with m-chloro-perbenzoic acid provides the a-epoxide (2) with high diastereoselectivity and yield. In contrast, epoxidation via the bromohydrin provides the (1-oxide (2) in equally high diastereoselectivity. Osmylation of 1 also provides the 5a,6a-diol selectively (7 1) with similar preference for the a-face. 

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%... 

An unusual tetrahedral osmyl thiosulfato complex, [Os(0)2 (S203)2]2 , was recently prepared by reaction of Os04 in water with Na2S203. The X-ray crystal structure revealed a cis-dioxo structure with 0=0s=0 angle and Os=0 distances of 127.2(2)° and 1.692(3) A, respectively (256). In CH2C12, a reversible Os(VI/V) couple at - 1.10 V vs Cp2Fe+/0 and an irreversible oxidative wave were observed. 

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