Manganese Dioxide1

MnO2

[1313-13-9]  · MnO2  · Manganese Dioxide  · (MW 86.94)

(useful selective oxidizing reagent for organic synthesis; oxidation of allylic alcohols to a,b-ethylenic aldehydes or ketones;2 conversion of allylic alcohols to a,b-ethylenic esters or amides;3 oxidation of propargylic alcohols,4 benzylic or heterocyclic alcohols,5 and saturated alcohols;6 oxidative cleavage of 1,2-diols;7 hydration of nitriles to amides;8 dehydrogenation and aromatization reactions;9 oxidation of amines to aldehydes, imines, amides, and diazo compounds10)

Alternate Names: manganese oxide; manganese(IV) oxide.

Physical Data: mp 535 °C (dec); d 5.03 g cm-3.

Solubility: insol H2O and organic solvents.

Form Supplied in: dark brown powder, widely available. The commercial active MnO2 used as oxidizing reagent for organic synthesis is a synthetic nonstoichiometric hydrated material. The main natural source of MnO2 is the mineral pyrolusite, a poor oxidizing reagent.

Structure of Active Manganese Dioxide.1,11,12

The structure and the reactivity of active manganese dioxides used as oxidizing reagents in organic synthesis closely depends on their method of preparation (see below). Active manganese oxides are nonstoichiometric materials (generally MnOx; 1.93 < x < 2) and magnetic measurements reveal the presence of lower valency Mn species, probably MnII and MnIII oxides and hydroxides. Thermogravimetric analysis experiments show the existence of bonded and nonbonded water molecules (hydrated MnO2). On the basis of ESR studies and other experiments, a locked-water associated structure (1) has been proposed for the apomorphous precipitated MnO2.12

In addition, variable amounts of alkaline and alkaline earth metal derivatives are detected by atomic adsorption analysis. X-ray studies have shown that the structures of active MnO2 are quite complex; they are either amorphous or of moderate crystallinity (variable proportions of b- and g-MnO2).

Preparation of Active MnO2, Oxidizing Power, and Reproducibility of the Results.1,13

Pyrolusite (natural MnO2) and pure synthetic crystalline MnO2 are poor oxidants.1 The oxidation of organic compounds requires an active, specially prepared MnO2 and several procedures have been reported.1,13 According to the method of preparation, the structure, the composition, and therefore the reactivity of active MnO2 are variable. On this account, the choice of a procedure is of considerable importance to obtain the desired oxidation power and the reaction conditions must be carefully controlled to obtain a consistent activity. The active manganese dioxides described in the literature are generally prepared either by mixing aqueous solutions of KMnO4 and a MnII salt (MnSO4, MnCl2) between 0 and 70 °C under acid, neutral, or basic conditions13a-e or by pyrolysis of a MnII salt (carbonate, oxalate, nitrate) at 250-300 °C.13d,f In this case the activity of the resulting material can be increased by washing with dilute nitric acid.13f A similar treatment has also been used to activate pyrolusite (natural MnO2).13h On the other hand, it has been reported that the efficiency of an active MnO2 depends on the percentage of the g-form present in the material.13i Indeed, active g-MnO2 is sometimes clearly superior to the classical active MnO2 prepared according to Attenburrow.13j

It worthy of note that the percentage water content strongly influences both the oxidizing power and the selectivity (oxidation of multifunctional molecules) of active MnO2. Thus it is well known that the wet material (40-60% H2O) obtained after filtration must be activated by drying1,13 (heating to 100-130 °C for 12-24 h13a-d or, better, at 125 °C for 52 h).13e Indeed, an excess of water decreases the oxidation power1e,13k since, according to the triphasic mechanism generally postulated,12 it would prevent the adsorption of the substrate to the oxidatively active polar site on the surface of MnO2.1a,c,e On the other hand, it is very important not to go past the point of complete activation since the presence of hydrated MnO2 species is essential to obtain an active reagent. For this reason, the drying conditions must be carefully controlled.1,13a,d,g,k Alternatively, the wet material can be activated by azeotropic distillation since this mild procedure preserves the active hydrated species.131 Thus azeotropic distillation has been used to remove the water produced during the oxidation reaction to follow the rate of MnO2 oxidations.13e Finally, the active MnO2 mentioned above contains various metallic salts as impurities. According to their nature, which depends on the method of preparation, they can also influence the oxidizing power of the reagent (for instance, permanganate).13d Finally, it should be noted that the preparation of active MnO2 on carbon13m or on silica gel13n as well as the activation of nonactive MnO2 (pure crystalline MnO2) by ultrasonic irradiation13o has also been reported. Some typical procedures to prepare active MnO2 are reported below.

Preparation of Active MnO2 from KMnO4 under Basic Conditions (Attenburrow).13a

A solution of MnSO4.4H2O (110 g) in H2O (1.5 L) and a solution of NaOH (40%; 1.17 L) were added simultaneously during 1 h to a hot stirred solution of KMnO4 (960 g) in H2O (6 L). MnO2 precipitated soon after as a fine brown solid. Stirring was continued for an additional hour and the solid was then collected with a centrifuge and washed with water until the washings were colorless. The solid was dried in an oven at 100-120 °C and ground to a fine powder (960 g) before use.

Preparation of Active MnO2 from KMnO4 under Acidic Conditions.13k

Active MnO2 was made by mixing hot solutions of MnSO4 and KMnO4, maintaining a slight excess of the latter for several hours, washing the product thoroughly with water and drying at 110-120 °C. Its activity was unchanged after storage for many months, but it was deactivated by H2O, MeOH, thiols, or excessive heat (500 °C). MnO2 was less active when prepared in the presence of alkali and ineffective when precipitated from hot solutions containing a large excess of MnSO4.

Preparation of Highly Active MnO2 from KMnO4.1a

A solution of MnCl2.4H2O (200 g) in H2O (2 L) at 70 °C was gradually added during 10 min, with stirring, to a solution of KMnO4 (160 g) in H2O (2 L) at 60 °C in a hood. A vigorous reaction ensued with evolution of chlorine; the suspension was stirred for 2 h and kept overnight at rt. The precipitate was filtered off, washed thoroughly with H2O (4 L) until pH 6.5-7 and the washing gave a negligible chloride test. The filter cake was then dried at 120-130 °C for 18 h; this gave a chocolate-brown, amorphous powder; yield 195-200 g. Alternatively, the wet cake was mixed with benzene (1.2 L) and H2O was removed by azeotropic distillation giving a chocolate-brown, amorphous powder; yield 195 g. The last procedure gave a slightly less active material.

Preparation of Active MnO2 by Pyrolysis of MnCO3.13f

Powdered MnCO3 was spread in a one-inch thick layer in a Pyrex glass and heated at 220-280 °C for about 18 h in an oven in which air circulated by convection. The initially tan powder turned darker at about 180 °C, and black when maintained at over 220 °C. No attempt was made to determine lower temperature or time limits, nor the upper limit of temperature. The MnO2 prepared as above was stirred with about 1 L of a solution made up of 15% HNO3 in H2O. The slurry was filtered with suction, the solid was washed on the Buchner funnel with distilled water until the washes were about pH 5, and finally was dried at 220-250 °C. The caked, black solid was readily crushed to a powder which retained its oxidizing ability even after having been stored for several months in a loosely stoppered container.

Preparation of g-MnO2.1a

To a solution of MnSO4 (151 g) in H2O (2.87 L) at 60 °C was added, with stirring, a solution of KMnO4 (105 g) in H2O (2 L), and the suspension was stirred at 60 °C for 1 h, filtered, and the precipitate washed with water until free of sulfate ions. The precipitate was dried to constant weight at 60 °C; yield 120 g (dark-brown, amorphous powder).

Preparation of Active MnO2 on Silica Gel.13n

KMnO4 (3.79 g) was dissolved in water (60 mL) at rt. Chromatographic grade silica gel (Merck, 70-230 mesh, 60 g) was added with stirring, and the flask connected to a rotary evaporator to strip off the water at 60 °C. The purple solid was ground to fine powder and then added with vigorous stirring to a solution of MnSO4.H2O (9.3 g) in H2O (100 mL). The resulting brown precipitate was filtered with water until no more MnII ion could be detected in the wash water by adding ammonia. After being dried at 100 °C for 2 h, each gram of this supported reagent contained 0.83 mmol of MnO2.

As shown above, a wide range of products of various activities are called active MnO2 and the results described in the literature are sometimes difficult to reproduce since the nature of the MnO2 which was used is not always well defined. Now, the commercial materials give reproducible results but they are not always convenient to perform all the oxidation reactions described in the literature. In addition, their origin (method of preparation) is not often indicated and comparison with the active MnO2 described in the literature is sometimes difficult. For all these reasons, the use of activated MnO2 has been somewhat restricted in spite of the efficiency and selectivity of its reactions since only an empirical approach and a careful examination of the literature allow selection of the suitable activity of MnO2 and the optimum reaction conditions for a defined substrate.

Oxidation of Organic Compounds with MnO2: Reaction Conditions.1,13

Solvent.

Oxidation of organic compounds with MnO2 has been performed in many solvents. The choice of the solvent is important; thus primary or secondary alcohols (or water) are unsatisfactory since they can compete with the substrate being adsorbed on the MnO2 surface and they have a strong deactivating effect.13k A similar but less pronounced influence has also been observed with various polar solvents such as acetone, ethyl acetate, DMF, and DMSO. However, these polar solvents, including water,13p acetic acid, and pyridine, can be used successfully at higher temperatures. This deactivating influence due to the polarity of the solvent can be used to control the reactivity of active MnO2 and sometimes to avoid side reactions or to improve the selectivity (for instance, allylic alcohol vs. saturated alcohol). Most of the reactions described in the literature were carried out in aliphatic or aromatic hydrocarbons, chlorinated hydrocarbons, diethyl ether, THF, ethyl acetate, acetone, and acetonitrile (caution: MeCN can react with highly active MnO2 or with classically activated MnO2 on prolonged treatment). In the case of the oxidation of benzylic13f and allylic13d alcohols, the best results have been obtained using diethyl ether (diethyl ether > petroleum ether > benzene). Caution: a spontaneous ignition has been observed when highly active MnO2 was used in this solvent.13f

Temperature and Reaction Time.

At rt, the reaction time can vary from 10 min to several days according to the nature of the substrate and the activity of the MnO2.13a,d,e,f,l,p The reaction times are shortened by heating13d but the selectivity is very often much lower.13q

Ratio MnO2:Substrate.

The amount of active MnO2 required to perform the oxidation of an organic substrate depends on the type of MnO2, on the substrate, and on the particle size of the MnO2.13d,f,g With a classical material (100-200 mesh), the ratio varies from 5:1 to 50:1 by weight.

Oxidation of Allylic Alcohols.2

MnO2 was used as oxidizing reagent for the first time by Ball et al. to prepare retinal from vitamin A1 (eq 1).2a Since that report, the use of MnO2 for the conversion of allylic alcohols to a,b-ethylenic aldehydes has been extensively utilized. Interestingly, the configuration of the double bond is conserved during the reaction (eqs 2-4).2b,c In some cases a significant rate difference between axial and equatorial alcohols has been observed2d (eq 5).2e

MnO2 has been frequently used for the preparation of very sensitive polyunsaturated aldehydes or ketones (eq 6).2f

Numerous functionalized a,b-ethylenic aldehydes are readily obtained by chemoselective oxidation of the corresponding allylic alcohols (eqs 7-11).2g-k

The a,b-ethylenic ketone obtained by treatment of an allylic alcohol with MnO2 can undergo an in situ Michael addition (eqs 11 and 12).2l

Conversion of Allylic Alcohols to a,b-Ethylenic Esters and Amides.2b,3

This procedure was first described by Corey.2b,3 The key step is the sequential formation and oxidation of a cyanohydrin. In the presence of an alcohol or an amine the resulting acyl cyanide leads by alcoholysis or aminolysis to the corresponding a,b-ethylenic ester2b or amide3 (eqs 13-15).

Oxidation of Propargylic Alcohols.4

Propargylic alcohols are easily oxidized by MnO2 to alkynic aldehydes and ketones (eqs 16-18).4a-c In the example in eq 19 the unstable propargyl aldehyde is trapped as a Michael adduct.4d

Oxidation of Benzylic and Heterocyclic Alcohols.2b,5

Conjugated aromatic aldehydes or ketones can be efficiently prepared by treatment of benzylic alcohols with MnO2 (eq 20).5a Numerous functional groups are tolerated (eqs 21-24).5a-c

Benzyl allyl and benzyl propargyl alcohols have been oxidized successfully to ketones (eqs 25 and 26).5d,e

The oxidation reaction can be extended to heterocyclic alcohols (eq 27)5f and the Corey procedure gives the expected esters (eq 28).2b

Oxidation of Saturated Alcohols.6

Cyclic and acyclic saturated alcohols react with MnO2 to give the saturated aldehydes or ketones in good yields (eqs 29-32).6a,b

Oxidative Cleavage of 1,2-Diols.7

1,2-Diols are cleaved by MnO2 to aldehydes or ketones. With cyclic 1,2-diols, the reaction leads to dialdehydes or diketones (eqs 33 and 34)7 and the course of the reaction depends on the configuration of the starting material (eqs 34 and 35).7

Hydration of Nitriles to Amides.8

By treatment with MnO2, nitriles are readily converted to amides. MnO2 on silica gel is especially efficient to perform this reaction (eqs 36 and 37).8a,b

Dehydrogenation and Aromatization Reactions.9,13j,q

MnO2 has been widely used to carry out various dehydrogenation and aromatization reactions (eqs 38-41).9a-c In some cases the dehydrogenation can occur as a side reaction during, for instance, the hydration of nitriles (eq 37)8b or the oxidation of allylic alcohols.13q

It is interesting to note that the use of g-MnO2 is essential to achieve the following dehydrogenation reactions (eqs 42 and 43).13j

Oxidation of Amines to Aldehydes, Imines, Amides, and Diazo Compounds.10,13g,p

The oxidation of amines by MnO2 can lead to various products according to the structure of the starting material. Thus the formation of imines (eq 44),10 formamides (eqs 45 and 46),13g and diazo compounds (eq 47)13p have all been described.

Miscellaneous Reactions.13p,14

MnO2 has also been used to perform various oxidation reactions: the oxidative cleavage of a-hydroxy acids (eq 48),13p the oxidative dimerization of diarylmethanes (eq 49),14a or their conversion to diaryl ketones (eq 50),14a the oxidation of aldehydes to carboxylic acids,13p the preparation of disulfides from thiols,14b of phosphine oxides from phosphines,13p or of ketones from amines.14c

Related Reagents.

Barium Manganate; Nickel(II) Peroxide.


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Gérard Cahiez & Mouâd Alami

Université Pierre & Marie Curie, Paris, France



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