Permanganate alkynes

Internal alkynes are oxidized to acytoins by thalliuin(III) in acidic solution (A. McKil-lop, 1973 G.W. Rotermund, 1975) or to 1,2-diketones by permanganate or by in situ generated ruthenium tetroxide (D.G. Lee, 1969, 1973 H. Gopal, 1971). Terminal alkynes undergo oxidative degradation to carboxylic acids with loss of the terminal carbon atom with these oxidants. 

Out first example is 2-hydroxy-2-methyl-3-octanone. 3-Octanone can be purchased, but it would be difficult to differentiate the two activated methylene groups in alkylation and oxidation reactions. Usual syntheses of acyloins are based upon addition of terminal alkynes to ketones (disconnection 1 see p. 52). For syntheses of unsymmetrical 1,2-difunctional compounds it is often advisable to look also for reactive starting materials, which do already contain the right substitution pattern. In the present case it turns out that 3-hydroxy-3-methyl-2-butanone is an inexpensive commercial product. This molecule dictates disconnection 3. Another practical synthesis starts with acetone cyanohydrin and pentylmagnesium bromide (disconnection 2). Many 1,2-difunctional compounds are accessible via oxidation of C—C multiple bonds. In this case the target molecule may be obtained by simple permanganate oxidation of 2-methyl-2-octene, which may be synthesized by Wittig reaction (disconnection 1). 

When compound (443), which contains alkene and alkyne moieties, was reacted with benzonitrile oxide, both an isoxazoline (444) and isoxazole (445) were produced, with the former predominating. Oxidation of (444) with permanganate produced 3-phenyl-2-isoxazoline-5-carboxylic acid (446) (67ZOR82i). The reaction of 1-trimethylsilylbut-l-yne-3-ene produced only a compound which reacted at the alkenic unit. Oxidation of the adduct also produced (446) (68ZOB1820). These reactions are shown in Scheme 102. 

Treating alkynes with ozone or with basic potassium permanganate leads to... 

The Phase-Transfer-Assisted Permanganate Oxidation of Alkenes and Alkynes... 

The versatility of permanganate as an oxidant has been greatly enhanced in the past decade by the observation that it can be solubilized in nonaqueous solvents with the aid of phase transfer agents (1). The literature contains descriptions for the use of this procedure for the oxidation of alkenes (2-13), alkynes (13-18), aldehydes (19), alcohols (20), phenols (21,22), ethers (23), sulfides (24,25), and amines (20,26). The dehydrogenation of triazolines has also been achieved by the use of permanganate and a phase transfer agent (27). ... 

When alkynes are oxidized by aqueous permanganate the carbon-carbon triple bond is cleaved, as in Equation 1, with the formation of two moles of carboxylic acid. 

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. 

N-Substituted amides derived from 2-chloro- or4-chloronicotinic acid react with CH-acidic nitriles in the presence of base to yield amino derivatives of [l,6]naphthytid-5(6//)-ones and [2,7]naphthyrid-l(2//)-ones <1997JHC397>. 3-(l-Alkylamino)pyridines react with electron-deficient alkynes (acetylene dicatboxylates) in the presence of acid to give l,2-dihydro[2,7]naphthyridine-3,4-dicarboxylates in up to 72% yield compounds unsubstituted at C-1 were readily oxidized with potassium permanganate to naphthyridine-l-ones <2005TL3953>. 

A very efficient, stereospecific synthesis of DL-ribose was based26 on the use of l,l-diethoxy-5-(tetrahydropyran-2-yloxy)-2-pentyn-3-ol as the substrate. Catalytic hydrogenation of this alkyne to the cts-alkene was accompanied by cyclization, to give 2-ethoxy-2,5-dihydro-5-(tetra-hydropyran-2-yloxy)furan (35). cis-Hydroxylation of the double bond in 35 was effected with potassium permanganate, yielding the ethyl DL-ribofuranoside derivative 36, which was hydrolyzed to DL-ribose. 

Reaction of an alkene with hot basic potassium permanganate (KMn04) results in cleavage of the double bond, and formation of highly oxidized carbons. Therefore, unsubstituted carbon atoms become CO2, mono-substituted carbon atoms become carboxylates, and di-substituted carbon atoms become ketones. This can be used as a chemical test (known as the Baeyer test) for alkenes and alkynes, in which the purple colour of the KMn04 disappears, and a brown Mn02 residue is formed. 

Alkynes are oxidized to diketones by cold, dilute and basic potassium permanganate. 

Ozonolysis of alkynes followed by hydrolysis gives similar products to those obtained from permanganate oxidation. This reaction does not require oxidative or reductive work-up. Unsubstituted carbon atoms are oxidized to CO2, and mono-substituted carbon atoms to carboxylic acids. For example, ozonolysis of 1-butyene followed by hydrolysis gives propionic acid and carbon dioxide. 

Tris(dimethylamino)sulfonium difluo-rotrimethylsilicate, 336 Xenon(II) fluoride, 345 Alkyl bromides Potassium permanganate, 258 Sodium bromide, 46 Tetraethylammonium bromide, 46 Alkyl iodides Aluminum iodide, 17 Potassium permanganate, 258 Sodium iodide, 46 Tetraethylammonium iodide, 46 Alkynes (see also Acetylenic carbonyl compounds, Diynes, Enynes, Propar-gyl alcohols)... 

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. 

Exercise 11-15 Alkynes react more slowly than alkenes with permanganate and usually give dicarbonyl compounds. An example follows ... 

Finally, [Method (c)], treatment of an internal alkyne with potassium permanganate under PTC conditions leads to a good yield of a 1,2-diketone.146 The reaction probably proceeds via hydroxylation of the triple bond, tautomeric rearrangement to the a-ketol, and oxidation to the diketone. The reaction is illustrated by the conversion of 1-phenylpent-l-yne to l-phenylpentane-1,2-dione (Expt 5.101). 

Baeyer reagent Cold, dilute potassium permanganate, which is used to oxidize alkenes and alkynes. 

Alkynes Bromine test Permanganate test Silver nitrate Sodium metal addition Sulfuric acid Positive for all alkynes Positive for all alkynes Positive for all terminal alkynes only Positive for all terminal alkynes only Positive for all alkynes... 

Ozonolysis of an alkyne, followed by hydrolysis, cleaves the triple bond and gives two carboxylic acids. Either permanganate cleavage or ozonolysis can be used to determine the position of the triple bond in an unknown alkyne (see Problem 9-24). 

With alkynes, either ozonolysis or a vigorous permanganate oxidation cleaves the triple bond to give carboxylic acids (Section 9-10). 

Oxidations of alcohols on solid potassium or sodium permanganate surfaces take place under mild conditions with high yield and easy workup. Solid sodium permanganate can also oxidize aldehydes and sulfides but not alkenes or alkynes the solid reagent is thus more selective than the oxidant in solution. Mechanistic aspects (including an observed need for trace quantities of water at the crystal surface,... 

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