Oxidative cleavage alkenes, osmium tetroxide

Recently it has been reported that oxidative cleavage of alkenes with catalytic osmium tetroxide is possible in the absence of water using the co-oxidant Oxone (KHSOs). In this case the diol is not formed and the intermediate osmate ester is oxidized by the Oxone and fragments to regenerate osmium tetroxide and release the carbonyl products. For example, the alkene 1-nonene gave octanoic acid (90% yield) under these conditions. 

The oxidation reactions of either permanganate or osmium tetroxide with an alkene lead to a vicinal diol. The functional group exchange for this process is that shown. In other words, a diol is obtained by dihydro lation of an alkene. Oxidative cleavage reactions such as ozonolysis eventually lead to an aldehyde, ketone, or carboxylic acid, depending on the substituents attached to the unit... 

For the same reasons as described for the ozonolysis of alkenes, the oxidative cleavage of vicinal diols by periodate is hmited as a synthetic method. The vicinal diol is seldom directly available, and it must be prepared from an aUtene. We also recall that the hydroxyl groups must be located in a cis configuration, or the molecule must have sufficient conformational freedom to bring the two hydroxyl groups into a gauche conformation. The vicinal diol is prepared from an alkene by oxidation with osmium tetroxide. 

Osmium tetroxide used in combination with sodium periodate can also effect alkene cleavage.191 Successful oxidative cleavage of double bonds using ruthenium tetroxide and sodium periodate has also been reported.192 In these procedures the osmium or ruthenium can be used in substoichiometric amounts because the periodate reoxidizes the metal to the tetroxide state. Entries 1 to 4 in Scheme 12.18 are examples of these procedures. Entries 5 and 6 show reactions carried out in the course of multistep syntheses. The reaction in Entry 5 followed a 5-exo radical cyclization and served to excise an extraneous carbon. The reaction in Entry 6 followed introduction of the allyl group by enolate alkylation. The aldehyde group in the product was used to introduce an amino group by reductive alkylation (see Section 5.3.1.2). 

Osmium tetroxide used in combination with sodium periodate can also effect alkene cleavage.135 Successful oxidative cleavage of double bonds using ruthenium tetroxide and sodium periodate has also been reported.136 In these procedures, the osmium or ruthenium can be used in substoichiometric amounts because the periodate reoxidizes the metal to the tetroxide state. Entries 1 4 in Scheme 12.17 are examples of these procedures. 

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... 

Exercise 16-37 An elegant modification of the two-step procedure to prepare ketones from alkenes by hydroxylation and oxidative cleavage of the diol formed uses a small amount of potassium permanganate (or osmium tetroxide, 0s04) as the catalyst and sodium periodate as the oxidizing agent ... 

The use of osmium tetroxide for the conversion of an alkene to a 1,2-diol is a well-established reaction [19-22]. The formation of an intermediate cyclic ester accounts for the cis-stereochemistry [21, 23-32] as reaction occurs on the least hindered face of the alkene [21, 30, 33-38]. This steric effect is amplified in cyclic substrates [39, 40]. The reaction conditions have to be carefully controlled to avoid oxidative cleavage of the diol product [28]. 

Osmium tetroxide can be used in substoichiometric amounts because the periodate oxidizes the osmium back to the tetroxide. The periodate therefore plays two roles, one to cleave the diol and the other to re-oxidize the osmium(VI), although it does not itself react with alkenes or aldehydes. Reaction often occurs at the least-hindered alkene and therefore provides a selective method for the oxidative cleavage of, for example, a vinyl group in the presence of a di- or trisubstituted alkene. If potassium permanganate is used in place of osmium tetroxide, then any aldehyde products are usually oxidized to give the corresponding carboxylic acids. 

Oxidative Cleavage of Alkenes. An alternative to the oxidative cleavage of alkenes using ozone or the Lemieux-Johnson protocol has been reported recently. Under the action of catalytic osmium tetroxide, with oxone as a reoxidant, a variety of substituted alkenes were cleaved efficiently to furnish carbonyl compounds (eq 37). Any of the aldehydes that are produced via this sequence are immediately oxidized in situ to give the corresponding acid clearly this does not happen for any ketones so produced. Even electron deficient alkenes such as o , -unsaturated carbonyl compounds could be conveniently oxidized, although the products then underwent a decarboxylation reaction to produce the corresponding diacid. 

Overall, ozone compares favorably with other approaches for oxidative alkene cleavage involving Osmium Tetroxide, Potassium Permanganate, Ruthenium(VIII) Oxide, Sodium Periodate, or chromyl carboxylates which are costly, toxic, involve metal wastes, and may require detailed workup procedures. 

As stated in the previous sections, the chief drawbacks in reactions using osmium tetroxide are its volatility, toxicity, and high cost. The less expensive and hazardous isoelectronic ruthenium tetroxide has long been appreciated for its powerful oxidation chemistry, especially oxidative cleavage of alkenes to aldehydes, ketones, and carboxylic acids [57]. Sharpless and Akashi showed for the first time that RUO4 can... 

Osmium tetroxide used in combination with sodium periodate can also effect alkene cleavage cleanly.Successful oxidative cleavage of double bonds using ruthenium... 

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