Potassium Permanganate1-4

KMnO4

[7722-64-7]  · KMnO4  · Potassium Permanganate  · (MW 158.04)

(oxidant; conversion of arenes into carboxylic acids,10,11 a-ketones,12-15 or a-alcohols;14 degradation of aromatic rings;3 preparation of diols,17,18 ketols,5,19,20,22 and a-diketones22-24 from nonterminal alkenes; preparation of carboxylic acids,27 aldehydes26 and 1,2-diols28 from terminal alkenes; oxidation of alkynes to a-diones;29,30 oxidation of enones to 1,4-diones;31 conversion of 1,5-dienes into substituted tetrahydrofurans32,33 or lactones;34 conversion of primary and secondary alcohols into carboxylic acids1,2,37 and ketones,1-4,9,35 respectively; oxidation of allylic alcohols to a,b-unsaturated ketones35 and other unsaturated alcohols and a,o-diols to lactones;36,37 oxidation of aliphatic thiols to disulfides and aromatic thiols to sulfonic acids;4 oxidation of sulfides and sulfoxides to sulfones,4,38-40 sulfinic acids to sulfonic acids,43 sulfites to sulfates,44 and thiones to ketones;45 preparation of tertiary nitroalkanes from the corresponding amines;47 oxidation of tertiary amines to amides or lactams;48-50 allylic oxidations when used in conjunction with t-butyl hydroperoxide;51 preparation of iodoaromatic compounds when used with I2 and sulfuric acid;52 oxidation of nucleic acids to the corresponding diols and ketols;53 oxidation of guaiol and related compounds to rearranged ketols;54 oxidation of poly(vinyl alcohol) to poly(vinyl ketone);56 oxidation of nitroalkanes to aldehydes or ketones; oxidation of imines to nitrones)

Alternate Name: potassium manganate(VII).

Physical Data: d 2.70 g cm-3; decomposition 237 °C.

Solubility: water (at 20 °C) 63.8 g L-1; sol acetone, methanol.

Form Supplied in: purple solid; commercially available.

Handling, Storage, and Precautions: stable at or below rt. Because it is a strong oxidant it should be stored in glass, steel, or polyethylene vessels. Sulfuric acid should never be added to permanganate or vice versa. Permanganate acid, an explosive compound, is formed under highly acidic conditions.

Introduction.

Permanganate is an inexpensive oxidant that has been widely used in organic syntheses. Its most common salt, KMnO4, is soluble in water and as a consequence oxidations have traditionally been carried out in aqueous solutions or in mixtures of water and miscible organic solvents such as acetone, acetic acid, acetonitrile, benzonitrile, tributyl phosphate, or pyridine. The discovery that KMnO4 can, with the aid of phase-transfer agents, be readily dissolved in nonpolar solvents such as CH2Cl2, and the recent observation that is adsorption onto a solid support produces an effective heterogeneous oxidant, has further expanded its usefulness.

The general features of the reactions of permanganate dissolved in aqueous solutions, or in organic solvents with the aid of a phase-transfer agent, and as a heterogeneous oxidant will be briefly described, followed by specific examples.

Aqueous Permanganate Oxidations.

Potassium permanganate is a general, but relatively nonselective, oxidant when used in aqueous solutions. When an organic compound contains only one site at which oxidation can readily occur, this reagent is a highly efficient and effective oxidant. For example, oleic acid is converted into dihydroxystearic acid in quantitative yield when oxidized in a dilute aqueous solution of KMnO4 at 0-10 °C.5

If the aqueous solution is made acidic by addition of mineral acid, the rate of reaction increases, most probably because of formation of permanganic acid1 which is known to be a very strong oxidant.6 The rate of the reaction is also accelerated by addition of sodium or potassium hydroxide. It has been proposed that this acceleration may be due to ionization of the organic reductant; for example, conversion of an alcohol into an alkoxide ion.1 However, similar observations for the oxidation of compounds such as sulfides, which lack acidic hydrogens, suggests that other factors may be involved.7

Under acidic conditions, permanganate is reduced to soluble manganese(II) or -(III) salts, thus allowing for a relatively easy workup. However, under basic conditions the reduction product is a gelatinous solid, consisting primarily of manganese dioxide, that is difficult to separate from the product. As a consequence, for laboratory scale preparations the reaction product is not isolated until after the MnO2 has been reduced by addition of HCl and sodium bisulfite. For large scale (industrial) processes, MnO2 is removed either by filtration or by centrifugation.

Phase-Transfer Assisted Permanganate Oxidations.2

KMnO4 may be dissolved in nonpolar solvents such as benzene or CH2Cl2 by complexing the potassium ion with a crown ether or by replacing it with a quaternary ammonium or phosphonium ion. Although most reactions observed are similar to those found in aqueous solutions, the ability to dissolve permanganate in nonpolar solvents has greatly increased the range of compounds that can be oxidized.

The first example of a phase-transfer assisted permanganate oxidation involved the complexing of the potassium ion by a crown ether in benzene;8 however, it was later found that the use of quaternary ammonium or phosphonium salts was less expensive and just as efficient.2

Phase transfer into a nonpolar solvent can occur either from an aqueous solution or from solid KMnO4. Evaluation of various phase-transfer agents for these purposes has indicated that benzyltributylammonium chloride is highly efficient for transfer from aqueous solutions while alkyltriphenylphosphonium halides, tetrabutylammonium halides, and benzyltriethylammonium halides are all effective for the transfer from solid KMnO4.2 Adogen 464, an inexpensive quaternary ammonium chloride commercially available, is usually satisfactory for both purposes.

Quaternary ammonium and phosphonium permanganates can also be used as stoichiometric oxidants. For descriptions of their properties, refer to the separate articles on Methyltriphenylphosphonium Permanganate and Benzyltriethylammonium Permanganate.

Heterogeneous Permanganate Oxidations.

The use of permanganate, activated by adsorption on a solid support, as a heterogenous oxidant has further increased the scope of these reactions. CH2Cl2 or 1,2-dichloroethane (if a high reflux temperature is required) are the preferred solvents and Alumina, silica, or hydrated Copper(II) Sulfate are the most commonly used solid supports. The selectivity of the oxidant is dramatically altered by use of a solid support. For example, although carbon-carbon double bonds are very easily cleaved in homogeneous permanganate solutions, secondary allylic alcohols can be cleanly oxidized to the corresponding a,b-unsaturated ketones without disruption of the double bond under heterogeneous conditions.9

In addition to increased selectivity, the use of permanganate under heterogeneous conditions allows for easy product isolation. It is necessary only to remove spent oxidant by filtration followed by flash evaporation or distillation of the solvent. Products isolated in this way are often sufficiently pure to permit direct use in subsequent synthetic procedures.

Benzylic Oxidations.

Permanganate oxidizes side chains of aromatic compounds at the benzylic position.3 In aqueous solution, carboxylic acids are usually obtained (eqs 1 and 2).10,11

The oxidation of alkylbenzenes proceeds through the corresponding a-ketones, which can occasionally be isolated (eqs 3 and 4).12,13

Under heterogeneous conditions where alumina (acid, Brockman, activity 1)14 or copper sulfate pentahydrate15 is used as the solid support, a-ketones and alcohols are obtained with little or no carbon-carbon cleavage (eqs 5-8).

Oxidation of Aromatic Rings.

Permanganate will oxidatively degrade aromatic rings under both acidic and basic conditions.3 The effect of acid and base on the reaction has been demonstrated by the oxidation of 2-phenylpyridine; under basic conditions the product is benzoic acid (presumably because the oxidant attacks the site of greatest electron density) (eq 9), while under acidic conditions (where the nitrogen would be protonated) the product is picolinic acid (eq 10).3

Polycyclic aromatic compounds are also oxidatively degraded to a single-ring polycarboxylic acid (eq 11).16

Oxidation of Nonterminal Alkenes.

Nonterminal alkenes can be converted into 1,2-diols, ketols, or diketones by choice of appropriate conditions. The reaction, which proceeds by syn addition of permanganate to the double bond as indicated, gives the corresponding cis-diol under aqueous alkaline conditions (eq 12).17

Syn addition can also be achieved in nonaqueous solvents with the aid of a phase-transfer agent (PTA). Subsequent treatment with aqueous base gives 1,2-diols in good yields2 (eq 13).18 Equally good results were reported when the reaction was carried out in aqueous t-butyl alcohol.18

Under neutral conditions the product obtained from the oxidation of alkenes is the corresponding ketol.5 Good yields are obtained when aqueous acetone containing a small amount of acetic acid (2-5%) is used as the solvent. The function of acetic acid is to neutralize hydroxide ions produced during the reduction of permanganate. The oxidations of 5-decene and methyl 2-methylcrotonate provide typical examples (eqs 14 and 15).19,20

Heterogeneous oxidations of alkenes with a small amount of t-butyl alcohol and water present to provide an omega phase21 results in the formation of a-ketols in modest to good yields (eqs 16 and 17).22

Under anhydrous conditions, 1,2-diones are formed in good yields when alkenes are oxidized by permanganate. Appropriate conditions can be achieved by using acetic anhydride solutions (eq 18)23 or by dissolving permanganate in CH2Cl2 with the aid of a phase-transfer agent (eq 19).24

Similar yields are obtained under heterogeneous conditions, where workup procedures are much easier.22

The carbon-carbon double bonds of alkenes can also be oxidatively cleaved to give carboxylic acids in good yield by use of the Lemieux-von Rudloff reagent (aqueous potassium periodate containing catalytic amounts of permanganate).3,25 Under heterogeneous conditions, either aldehydes or carboxylic acids are obtained, depending on the conditions used (eqs 20 and 21).26

Oxidation of Terminal Alkenes.

Although oxidation of terminal alkenes by permanganate usually results in cleavage of the carbon-carbon double bond to give either a carboxylic acid27 or an aldehyde,26 1,2-diols can be obtained through use of a phase-transfer assisted reaction (eqs 22-24).2,28

Oxidation of Alkynes.

Oxidation of nonterminal alkynes results in the formation of a-diones. Good yields are obtained when aqueous acetone containing NaHCO3 and MgSO4,29 or CH2Cl2 containing about 5% acetic acid,30 is used as the solvent (eqs 25-27). A phase-transfer agent to assist in dissolving KMnO4 must be used when CH2Cl2 is the solvent. Terminal alkynes are oxidatively cleaved, yielding carboxylic acids containing one carbon less than the parent alkyne.

Oxidation of Enones to 1,4-Diones.

Enones react with nitroalkanes (Michael addition) to form g-nitro ketones that can be oxidized in good yield to 1,4-diones under heterogeneous conditions (eq 28).31

Oxidation of 1,5-Dienes.

The oxidation of 1,5-dienes results in the formation of 2,5-bis(hydroxymethyl)tetrahydrofurans with the indicated stereochemistry (eq 29).32 When R6 in (eq 29) is chiral, a nonracemic product is obtained.33 Use of heterogeneous conditions results in the formation of lactones (eq 30).34

Oxidation of Alcohols and Diols.

Primary and secondary alcohols are converted to carboxylic acids and ketones, respectively, when oxidized by aqueous permanganate under either acidic or basic conditions (eq 31).1 Similar results are obtained with phase-transfer assisted oxidations in organic solvents such as CH2Cl2 (eq 32).2

Heterogeneous oxidations are very effective with secondary alcohols (eq 33)35 and provide the added advantage that allylic secondary alcohols can be converted to the corresponding a,b-unsaturated ketones without disruption of the double bond (eq 34).9 Unsaturated secondary alcohols in which the double bond is not adjacent to the carbon bearing the hydroxy group are resistant to oxidation (eq 35) unless an omega phase21 is created by adding a small amount of water (50 mL per g KMnO4). The products are lactones under these conditions (eq 36).36

Good yields of carboxylic acids are obtained from primary alcohols under heterogeneous conditions (KMnO4/CuSO4.5H2O) only when a base such as KOH or Cu(OH)2.CuCO3 is intermixed with the solid support.37 Under these conditions the reagent has also been reported to be selective for primary alcohols.37

The oxidation of a,o-diols under heterogenous conditions results in the formation of lactones. A good example is found in the preparation of 3-hydroxy-p-menthan-10-oic acid lactone (eq 37).37

Oxidation of Organic Sulfur Compounds.

Aromatic thiols are oxidized by permanganate to the corresponding sulfonic acids while aliphatic thiols usually give disulfides, which are resistant to further oxidation.4 Sulfides and sulfoxides are easily oxidized in CH2Cl2 to the corresponding sulfones under both homogeneous38,39 and heterogeneous conditions (eqs 38-42).40

Permanganate oxidizes sulfoxides more readily than sulfides, as indicated by the products obtained from the oxidation of compounds containing both sulfide and sulfoxide functional groups (eqs 43 and 44).41,42

The greater ease of oxidation of sulfoxides is also responsible for the observation that gem-disulfides are oxidized to monosulfones.42 Monosulfoxides, although not isolated, are likely to be intermediates in these reactions (eq 45).

Oxidation of sulfinic acids results in the formation of sulfonic acids,43 while sulfites give sulfates (eqs 46 and 47).44

Cyclic thiones are readily oxidized to the corresponding ketones by permanganate (eq 48).45

Oxidation of Amines.

The synthetic usefulness of permanganate as an oxidant for aliphatic amines is decreased by the fact that a complex mixture of products is often obtained.4,46 Good yields of tertiary nitroalkanes can, however, be obtained from the oxidation of the corresponding amines (eq 49).47

Primary and secondary amines react with permanganate in buffered, aqueous t-butyl alcohol to give aldehydes and ketones (eq 50).46

Amides (or lactams, if the amine is cyclic) are obtained from the oxidation of tertiary amines (eqs 51 and 52).48-50

Miscellaneous Oxidations.

Use of permanganate in conjunction with t-Butyl Hydroperoxide results in allylic oxidation (eq 53).51

Aromatic compounds are oxidized to aryl iodides when treated with permanganate, Iodine, and Sulfuric Acid (eq 54).52

Chemical modification of nucleic acids by treatment with permanganate results in oxidation of the D5 double bond to give either diols or ketols (eq 55).53

Guaiol and related compounds can be oxidized to rearranged ketols using aqueous glyme as the solvent (eq 56).54

cis-2,5-Dihydro-2,5-dimethoxyfuran is oxidized to the corresponding a-diol in preference to the trans compound (eqs 57 and 58).55

Oxidation of poly(vinyl alcohol) by permanganate results in the formation of poly(vinyl ketone) (eq 59).56

Treatment of D5-unsaturated steroids with KMnO4/CuSO4.5H2O in CH2Cl2 containing catalytic amounts of t-butyl alcohol and water results in formation of the corresponding 5b,6b-epoxide (eq 60).22,57

The oxidation of D7-cholesterol acetate by KMnO4 under neutral or slightly basic conditions results in formation of all-cis-epoxydiol (eq 61).58

Aliphatic nitro compounds are converted into the corresponding oxo compounds on treatment with basic permanganate.59,60 Because these reactions are carried out under basic conditions, it is likely that anions are intermediates, as suggested in eqs 62-64.

Nitrones can be obtained from the oxidation of imines by KMnO4 in a two-phase CH2Cl2/H2O solution containing a phase-transfer agent (PTA) such as tetrabutylammonium chloride (eq 65).61

Related Reagents.

Potassium Permanganate-Copper(II) Sulfate; Sodium Periodate-Potassium Permanganate.


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Donald G. Lee

University of Regina, Saskatchewan, Canada



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