Oxidations of alkynes

Upon oxidation of an alkyne,the C C triple bond can be cleaved completely. AU three C-C bonds are oxidized to form three new C-O bonds,resulting in carboxylic acid products. This oxidation can be carried out either by ozonoiysis or with KMn04 (dihydroxylation occurs, followed by additional oxidative cleavage). 

For example, when 2-pentyne is treated with a cold, dilute solution of neutral permanganate, the product is 2,3-pentanedione. 

Terminal alkynes probably give a keto-aldehyde at first, but the aldehyde quickly oxidizes to an acid under these conditions. 

If the reaction mixture becomes warm or too basic, the diketone undergoes oxidative cleavage. The products are the salts of carboxylic acids, which can be converted to the free acids by adding dilute acid. 

For example, warm, basic permanganate cleaves the triple bond of 2-pentyne to give acetate and propionate ions. Acidification reprotonates these anions to acetic acid and propionic acid. 

Terminal alkynes are cleaved similarly to give a carboxylic acid and CO2. 

The two major characteristic oxidation processes of alkynes are their transformation to 1,2-dicarbonyl compounds and their cleavage reaction to carboxylic acids.710 The structure of the starting compounds has a decisive effect on the selectivity of oxidation. Since 1,2-dicarbonyl compounds proved to be intermediates in further oxidations, carefully controlled reaction conditions are often necessary to achieve selective synthesis. Certain oxidizing agents such as peroxyacids and ozone are nonselective oxidants. 

As with alkene cleavage the main reagent for alkyne oxidations is RuO. Oxidative cleavage of alkynes by a variety of reagents has been reviewed [4, 6, 12, 14, 15], The first oxidation of alkynes was noted by Pappo and Becker in 1956 they showed that l,2-fc/x(l-acetoxycyclohexyl)ethyne (2) (Fig. 1.5) gave the diketone. Minimal experimental details were given [195], 

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The molybdenum-catalyzed oxidation of alkynes by /-butyl hydroperoxide has been investigated 73JCS(P1)2851) (the epoxidation of alkenes by this system has become an important reaction Section 5.05.4.2.2(i)) but the formation of oxirenes was excluded. 

Probably the most surprising aspect of the putative oxidation of alkynes to oxirenes is the fact that evidence has recently been obtained that oxirenes may be formed, enzymatically, in living cells. The oxidation of labelled 4-biphenylylacetylene (94) to the carboxylic acid (95 Scheme 92) may involve an oxirene, since the alkyne gave acid with complete deuterium retention (path a) path (b) should have given unlabelled acid (80JA7373). 

The heterogeneous catalytic system iron phthalocyanine (7) immobilized on silica and tert-butyl hydroperoxide, TBHP, has been proposed for allylic oxidation reactions (10). This catalytic system has shown good activity in the oxidation of 2,3,6-trimethylphenol for the production of 1,4-trimethylbenzoquinone (yield > 80%), a vitamin E precursor (11), and in the oxidation of alkynes and propargylic alcohols to a,p-acetylenic ketones (yields > 60%) (12). A 43% yield of 2-cyclohexen-l-one was obtained (10) over the p-oxo dimeric form of iron tetrasulfophthalocyanine (7a) immobilized on silica using TBHP as oxidant and CH3CN as solvent however, the catalyst deactivated under reaction conditions. 

Diketones. The Sharpless procedure for oxidation of alkenes with NaIO catalyzed by Ru02 (11,462-463) is equally efficient for oxidation of alkynes to 1,2-diones. In fact, alkenes and alkynes react at a similar rate, but ether, epoxide, and ester groups are stable to the reagent. A 1-silylacetylene is oxidized to an acylsilane. Yields are moderate to high.1... 

The evidence supporting the suggestion that cyclic manganate(V) diesters are intermediates in the reaction between alkenes and permanganate is compelling (47), and Simandi (48) has also suggested that a similar intermediate, 2, may occur during the oxidation of alkynes as in equation 4. 

Oxidations of alkenes and alkynes have been reviewed, including mechanistic information in some cases. They include treatment of epoxidations [1-9], ketohydroxylations [7-9] and alkene cleavage [4, 6,10-14]. Oxidations of alkynes have been reviewed in [4, 12, 14, 15]. 

The same catalytic system (SeOi/TBHP) has also been used by Chabaud and Sharpless in the allylic oxidation of alkynes. The oxidation products resulting from the Se02-catalyzed allylic oxidation with TBHP are the allylic alcohol, the allylic diol, the allylic ketone, the ketol and the enynone (Scheme 127). The main product of the reaction is either the alcohol or the diol, depending on the substrate employed (together 76-100% of the whole yield). The yields of allylic oxidation products together range from 15 to 88%. From the observed results with unsymmetrical alkynes it could be concluded that the reactivity sequence for the carbon attached to the triple bond of alkynes is CH2 CH > CH3. 

SCHEME 127. Se02-catalyzed aUyUc oxidation of alkynes using TBHP as terminal oxidant... 

Aldehydes are prepared by the hydroboration-oxidation of alkynes (see Section 5.3.1) or selective oxidation of primary alcohols (see Section 5.7.9), and partial reduction of acid chlorides (see Section 5.7.21) and esters (see Section 5.7.22) or nitriles (see Section 5.7.23) with lithium tri-terr-butox-yaluminium hydride [LiAlH(0- Bu)3] and diisobutylaluminium hydride (DIBAH), respectively. 

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

Oxidation of alkynes preparation of diketones and carboxylic acids... 

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