Alkynes osmium tetroxide

Further chemistry of alkenes and alkynes is described in this chapter, with emphasis on addition reactions that lead to reduction and oxidation of carbon-carbon multiple bonds. First we explain what is meant by the terms reduction and oxidation as applied to carbon compounds. Then we emphasize hydrogenation, which is reduction through addition of hydrogen, and oxidative addition reactions with reagents such as ozone, peroxides, permanganate, and osmium tetroxide. We conclude with a section on the special nature of 1-alkynes— their acidic behavior and how the conjugate bases of alkynes can be used in synthesis to form carbon-carbon bonds. 

Vinyl and ethynyl groups attached to an imidazole ring can be catalytically reduced to the saturated (or less unsaturated) species and cleaved by oxidation. The corresponding 4-carbaldehyde is formed in 71% yield when l-methyl-2,5-diphenyl-4-styrylimidazole is oxidized with osmium tetroxide. However, they may not react like aliphatic alkenes and alkynes not all addition reactions occur normally, Michael additions are known, and the compounds can act as dienophiles in DielsAlder reactions (e.g., Scheme 132). 

Despite the development in recent years of new reagents for oi anic synthesis, osmium tetroxide, used either stoichiometrically or catalytically, remains the reagent of choice for syn hydroxylation of al kenes. Surprisingly, the hydroxylation of alkynes to give 1,2-dicarbonyl compounds has been much less studied, but it has considerable potential for use in the synthesis of complex molecules. 

Osmium tetroxide also catalyzes the oxidation of organic sulfides to sulfones with NMO or trimethylamine iV-oxide (see Osmium Tetroxide-N-MethylmorphoUne N-Oxide). In contrast, most sulfides are not oxidized with stoichiometric amounts of OSO4. Oxidations of alkynes and alcohols with OSO4 without and in the presence of cooxidants have also been reported. However, these reactions have not found wide synthetic applications because of the availability of other methods. 

Hydroboration of alkynes leads to an enol after treatment with NaOH/HgOg, and tautomerization gives an aldehyde from terminal alkynes, or a ketone from internal alkynes 44, 78, 83. Dilute potassium permanganate and osmium tetroxide react with alkenes to give a manganate ester or an osmate ester, respectively. Both of these products are decomposed under the reaction conditions to give a vicinal (l,2)-cis-diol 49, 50, 83, 96. 

Oxidation of the products by ruthenium(VIII) oxide with the TIPS group still in place gives a mixture of carboxylic acid and an a-keto acyl silane (silyl a-diketone (eq 13), an otherwise rare class of compounds compare the oxidation of disubstituted alkynes to 1,2-diketones by Ru04 9 and the osmium tetroxide-f-butyl hydroperoxide oxidation of TMS-aUcynes. ... 

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