Potassium permanganate, reaction with alkenes

For the ozonolysis of linear alkenes only alk-l-encs should be used to avoid product mixtures as the resulting formaldehyde or formic acid are readily separated. This type of reaction is also a successful method for the preparation of perfluorinated carboxylic acids.The advantage over the oxidation with potassium permanganate is that the process does not form solid byproducts which are diflicult to separate. A disadvantage of this procedure is the fact that two reaction steps arc needed to obtain the required product. 

Sodium periodate (sodium metaperiodate), NaI04 (mp 300 °C dec), which is commercially available, is applied mainly in aqueous or aqueous-alcoholic solutions. Like the free periodic acid, sodium periodate cleaves vicinal diols to carbonyl compounds [762], This reaction is especially useful in connection with potassium permanganate [763, 764] or osmium tetroxide [765], Such mixed oxidants oxidize alkenes to carbonyl compounds or carboxylic acids, evidently by way of vicinal diols as intermediates. Sulfides are transformed by sodium periodate into sulfoxides [322, 323, 766, 767, 768, 769, 770, 771, 772], and selenides are converted into selenoxides [773]. Sodium periodate is also a reoxidant of lower valency ruthenium in oxidations with ruthenium tetroxide [567, 774],... 

The results of some oxidations with potassium permanganate differ depending on the pH of the reaction. For example, stearolic acid gives 9,10-diketostearic acid at pH 7-7.5 (achieved with carbon dioxide) and azelaic acid on treatment at pH 12 [864]. In some reactions, potassium permanganate is used as a catalyst for oxidation with other oxidants, such as sodium periodate. Thus alkenes are cleaved to carbonyl compounds or acids via vicinal diols obtained by hydroxylation with potassium permanganate, followed by cleavage by sodium periodate [763, 552]. 

Elimination reactions leading to pyrazoles have also been reported for dialkoxyphosphinyl-substituted 4,5-dihydro-3//-pyrazoles, but a more common problem for these compounds is competitive alkene and cyclopropane formation and the difficulty of separating the rather intractable mixtures that result. " In the synthesis of 1-dimethoxyphosphinyl-1,2,2-trimethyl-cyclopropane (36), this problem was overcome by treating the crude product with potassium permanganate in aqueous acetone and selectively oxidizing away the alkenes. ... 

The reaction of alkenes with potassium permanganate (KMn04) at low temperature, or osmium tetroxide (0s04), leads to the syn- addition of two OH groups (i.e. the two OH groups add to the same face of the double bond). 

As the reaction occurs, the purple color of the permanganate ion is replaced by the brown precipitate of manganese dioxide. Because of this color change, the reaction can be used as a chemical test to distinguish alkenes from alkanes, which normally do not react with potassium permanganate. 

An example is the reaction of 2,3-dimethyl-2-butene with potassium permanganate to make two C-0 bonds. This product (126) is called a manganate ester. The concerted nature of the reaction of the alkene is important because 125 reacts to form 126 in such a way that the rate of bond formation for each C-0 bond is close, so they have a cis relationship. Indeed, cyclic product 126 has a cis relationship of the two atoms that form bonds to carbon. An alkene is normally added to potassium permanganate in an aqueous hydroxide solution. 

The addition of reagents X-Y to carbon-carbon ir-bonds may also proceed via a concerted mechanism in which each new a-bond is formed simultaneously on the same face of the ir-bond. The stereochemistry of such reactions is necessarily syn. For example, the reaction of potassium permanganate, which is purple, with an alkene such as cyclohexene proceeds via si/H-addition of permanganate ion across the ir-bond to give 39, which is colorless. Subsequent decomposition of 39 gives a ds-l,2-diol and manganese dioxide, the brown precipitate that is observed as the other product of the reaction (Eq. 10.19). This decoloration of potassium permanganate by alkenes forms the basis of the Baeyer qualitative test for the presence of carbon-carbon ir-bonds (Sec. 25.8B). 

Two common types of unsaturated compounds are alkenes and alkynes, characterized by the carbon-carbon double and triple bonds, respectively, as the functional group. The two common qualitative tests for unsaturation are the reaction of the compounds with bromine in dichloromethane and with potassium permanganate. In both cases, a positive test is denoted by decoloration of the reagent. There are no simple, direct ways to prepare solid derivatives of unsaturated aliphatic compounds having no other functional groups. 

Cyclic diketones can be the predecessors of quinoxaline macrocycles, obtained in one stage with the oxidation of cyclic alkenes with potassium permanganate (1971JA3303) or in two stages by the cyclotrimerization of cyclic alkynes and subsequent ozonolysis of compounds 18 (1986JOC3257). Along with the formation of diketone 4 the latter reaaion leads, to the unstable hexaketone 19. The reaction of the mixture of compounds 4 and 19 with the DAB leads to macrocycles 20 and 21 with one or three quinoxaline fr pients, respectively, with 10% and 3% yields, calculated in two st es.The synthesis of the macrocycle 20a from the analytically pure diketone 4a was in quantitative yield. 

The use of sofid supports in conjunction with permanganate reactions leads to modification of the reactivity and selectivity of the oxidant. The use of an inert support, such as bentonite (see Clays), copper sulfate pentahydrate, molecular sieves (qv) (151), or sifica, results in an oxidant that does not react with alkenes, but can be used, for example, to convert alcohols to ketones (152). A sofid supported permanganate reagent, composed of copper sulfate pentahydrate and potassium permanganate (153), has been shown to readily convert secondary alcohols into ketones under mild conditions, and in contrast to traditional permanganate reactivity, the reagent does not react with double bonds (154). 

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. 

The two tests employed for the detection of unsaturation are decolourisation of a dilute solution of bromine in dichloromethane, and reaction with dilute aqueous potassium permanganate. It is essential to apply both tests since some symmetrically substituted alkenes (e.g. stilbene, C6H5,CH=CH-C6H5) react only slowly under the conditions of the bromine test. With dilute potassium permanganate solution the double bond is readily attacked, probably through the intermediate formation of a ds-diol. 

The alkenes are distinguished from the alkanes by their solubility in concentrated sulphuric acid and their characteristic reactions with dilute potassium permanganate solution and with bromine. Characterisation may be based upon the determination of their physical and/or spectral properties. Characterisation by way of solid adducts with nitrosyl chloride has been quite widely used in the terpene field the preparation of adducts with 2,4-dinitrobenzenesulphenyl chloride is described below (see also Section 8.1.1, p. 1128). 

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