Potassium Monoperoxysulfate

2KHSO5.KHSO4.K2SO4

[37222-66-5]  · H3K5O18S4  · Potassium Monoperoxysulfate  · (MW 614.81)

(oxidizing agent for a number of functional groups, including alkenes,16 arenes,17 amines,26 imines,30 sulfides;37 used for the preparation of dioxiranes5)

Alternate Names: potassium caroate, potassium hydrogen persulfate, Oxone®.

Physical Data: mp dec; d 1.12-1.20 g cm-3.

Solubility: sol water (25.6 g 100 g, 20 °C), aqueous methanol, ethanol, acetic acid; insol common organic solvents.

Form Supplied in: white, granular, free flowing solid. Available as Oxone® and as Curox® and Caroat®.

Analysis of Reagent Purity: iodometric titration, as described in the Du Pont data sheet for Oxone®.

Handling, Storage, and Precautions: the Oxone triple salt 2KHSO5.KHSO4.K2SO4 is a relatively stable, water-soluble form of potassium monopersulfate that is convenient to handle and store. Oxone has a low order of toxicity, but is irritating to the eyes, skin, nose, and throat. It should be used with adequate ventilation and exposure to its dust should be minimized. Traces of heavy metal salts catalyze the decomposition of Oxone. For additional handling instructions, see the Du Pont data sheet.

Oxidation Methodology.

Oxone (2KHSO5.KHSO4.K2SO4) is a convenient, stable source of potassium monopersulfate (caroate), which serves as a stoichiometric oxidizing agent under a variety of conditions. Thus aqueous solutions of Oxone can be used to perform oxidations in homogeneous solution and in biphasic systems using an immiscible cosolvent and a phase-transfer catalyst. Recently, solid-liquid processes using supported Oxone reagents have been developed. Other oxidation methods involve the generation and reaction of a secondary reagent under the reaction conditions, as with the widely employed aqueous Oxone-ketone procedures, which undoubtedly involve dioxirane intermediates.1-4 In other instances, oxaziridine derivatives and metal oxo complexes appear to be the functional oxidants formed in situ from Oxone. Synthetically useful examples of these oxidations are grouped below according to the functional groups being oxidized.

Ketones and Other Oxygen Functions.

Various ketones can be converted to the corresponding dioxiranes by treatment with buffered aqueous solutions of Oxone (eq 1). Of particular interest are dimethyldioxirane5 (R1 = R2 = Me) and methyl(trifluoromethyl)dioxirane6 (R1 = Me, R2 = CF3) derived from acetone and 1,1,1-trifluoro-2-propanone, respectively. The discovery of a method for the isolation of dilute solutions of these volatile dioxiranes in the parent ketone by codistillation from the reaction mixture has opened an exciting new area of oxidation chemistry (see Dimethyldioxirane and Methyl(trifluoromethyl)dioxirane). Solutions of dioxiranes derived from higher molecular weight ketones have also been prepared.5,7

Interestingly, the reaction of a solid slurry of Oxone and wet Alumina with solutions of cyclic ketones in CH2Cl2 provokes Baeyer-Villiger oxidation to give the corresponding lactones (eq 2).8 The same wet alumina-Oxone reagent can be used to oxidize secondary alcohols to ketones (eq 3).9 Aldehydes are oxidized to acids by aqueous Oxone.5,10

Alkenes, Arenes, and Alkanes.

Aqueous solutions of Oxone can epoxidize alkenes which are soluble under the reaction conditions; for example, sorbic acid (eq 4)11 (the high selectivity for epoxidation of the 4,5-double bond here is noteworthy). Alternatively, the use of a cosolvent to provide homogeneous solutions promotes epoxidation (eq 5).11 Control of the pH to near neutrality is usually necessary to prevent hydrolysis of the epoxide. Rapidly stirred heterogeneous mixtures of liquid alkenes and aqueous Oxone solutions buffered with NaHCO3 also produce epoxides, as shown in eq 6.12

An in situ method for epoxidations with dimethyldioxirane using buffered aqueous acetone solutions of Oxone has been widely applied.1-4 The epoxidation of 1-dodecene is particularly impressive in view of the difficulty generally encountered in the epoxidation of relatively unreactive terminal alkenes (eq 7).13 A biphasic procedure using benzene as a cosolvent and a phase-transfer agent was utilized in this case. Equally remarkable is the epoxidation of the methylenecyclopropane derivatives indicated in eq 8, given the propensity of the products to rearrange to the isomeric cyclobutanones.14

The epoxidation of conjugated double bonds also proceeds smoothly with the Oxone-acetone system, as illustrated by eq 9.15 The conversion of water-insoluble enones can be accomplished with this method using CH2Cl2 as a cosolvent and a quaternary ammonium salt as a phase-transfer catalyst. However, a more convenient procedure utilizes 2-butanone both as a dioxirane precursor and as an immiscible cosolvent (eq 10).16 No phase-transfer agent is required in this case.

The epoxides of several polycyclic aromatic hydrocarbons have been prepared by the use of a large excess of oxidant in a biphasic Oxone-ketone system under neutral conditions, as shown for the oxidation of phenanthrene (eq 11).17 However, the use of isolated dioxirane solutions is more efficient for the synthesis of reactive epoxides, since hydrolysis of the product is avoided.5,18 A number of unstable epoxides of various types have been produced in a similar manner, as discussed for Dimethyldioxirane and Methyl(trifluoromethyl)dioxirane.

Epoxidations have also been performed with other oxidizing agents generated in situ from Oxone. An intriguing method uses a catalytic amount of an immonium salt to facilitate alkene epoxidation in a process which apparently involves an intermediate oxaziridium species as the active oxidant.19 This procedure is carried out by adding solid Oxone and NaHCO3 to a solution of the alkene and catalyst in MeCN containing a very limited quantity of water (eq 12). Finally, Oxone is the stoichiometric oxidant in interesting modifications of the widely studied metal porphyrin oxidations, where it has obvious advantages over some of the other oxidants commonly used.20 The potential of this method is illustrated by the epoxidation reaction in eq 13.21 In this conversion, only 1.4 mol % of the robust catalyst tetrakis(pentafluorophenyl)porphyrinatomanganese chloride (TFPPMnCl) is required. The catalytic hydroxylation of unactivated hydrocarbons is also possible (eq 14).22 Other metal complexes promote similar oxidations.23-25

Nitrogen Compounds.

The aqueous Oxone-acetone combination has been developed for the transformation of certain anilines to the corresponding nitrobenzene derivatives, as exemplified in eq 15.26 This process involves sequential oxidation steps proceeding by way of an intermediate nitroso compound. In the case of primary aliphatic amines, other reactions of the nitrosoalkane species compete with the second oxidation step (for example, dimerization and tautomerization to the isomeric oxime), thereby limiting the synthetic generality of these oxidations.27 An overwhelming excess of aqueous Oxone has been used to convert cyclohexylamine to nitrocyclohexane (eq 16).27

Pyridine is efficiently converted to its N-oxide by the Oxone-acetone oxidant.5 Cytosine and several of its derivatives give the N3-oxides selectively upon reaction with buffered Oxone (eq 17).28 A similar transformation of adenosine 5-monophosphate yields the N1-oxide.29

The very useful N-sulfonyloxaziridines are conveniently prepared by treating N-sulfonylimines with Oxone in a biphasic solvent system (eq 18).30,31 Either bicarbonate or carbonate can be used to buffer this reaction, but reaction is much faster with carbonate, suggesting that the monopersulfate dianion is the oxidizing species (for illustrations of the remarkable chemistry of these oxaziridines, see N-(Phenylsulfonyl)(3,3-dichlorocamphoryl)oxaziridine).

The Oxone-acetone system has also been employed for the synthesis of simple oxaziridines from N-alkylaldimines (eq 19).32 Interestingly, the N-phenyl analogs produce the isomeric nitrones rather than the oxaziridines (eq 20). It is noteworthy that MeCN can replace acetone as the solvent in this procedure.

Finally, the chlorination of aldoximines gives the corresponding hydroximoyl chlorides, as shown in eq 21.33 The combination of Oxone and anhydrous HCl in DMF serves as a convenient source of hypochlorous acid, the active halogenating agent.

Sulfur Compounds.

Some of the earliest applications of Oxone in organic synthesis involved the facile oxidation of sulfur functions. For example, aqueous Oxone selectively oxidizes sulfides to sulfones even in highly functionalized molecules, as illustrated in eq 22.34 Sulfones can also be prepared by a convenient two-phase system consisting of a mixture of solid Oxone, wet Montmorillonite K10 clay, and a solution of the sulfide in an inert solvent.35

The partial oxidation of sulfides to sulfoxides has been accomplished in a few cases by careful control of the reaction stoichiometry and conditions.34 A biphasic procedure for sulfoxide formation from diaryl sulfides is shown in eq 23.36 However, a more attractive and versatile procedure uses a solid Oxone-wet alumina reagent with a solution of the sulfide.37 This method permits control of the reaction to form either the sulfoxide or the sulfone simply by adjusting the amount of oxidant and the reaction temperature, as illustrated in eq 24. These oxidations are compatible with other functionality.

Another intriguing method for the selective oxidation of sulfides to sulfoxides (eq 25) uses buffered Oxone in a biphasic solvent mixture containing a catalytic amount of an N-phenylsulfonylimine as the precursor of the actual oxidizing agent, the corresponding N-sulfonyloxaziridine.38 The oxaziridine is smoothly and rapidly formed by reaction of the imine with buffered Oxone and regenerates the imine upon oxygen transfer to the sulfide. The greater reactivity of the sulfide relative to the sulfoxide accounts for the preference for monooxidation in this procedure. The biphasic nature of this reaction prevents direct oxidation by Oxone, which would be less selective.

Oxone sulfoxidations can show appreciable diastereoselectivity in appropriate cases, as demonstrated in eq 26.39 Enantioselective oxidations of sulfides to sulfoxides have been achieved by buffered aqueous Oxone solutions containing bovine serum albumin (BSA) as a chiral mediator (eq 27).40 As little as 0.05 equiv of BSA is required and its presence discourages further oxidation of the sulfoxide to the sulfone. Oxone can be the active oxidant or reaction can be performed in the presence of acetone, trifluoroacetone, or other ketones, in which case an intermediate dioxirane is probably the actual oxidizing agent. The level of optical induction depends on structure of the sulfide and that of any added ketone. Sulfoxide products show ee values ranging from 1% to 89%, but in most examples the ee is greater than 50%.

1-Decanethiol is efficiently oxidized to decanesulfonic acid (97% yield) by aqueous Oxone.10 In a similar manner an acylthio function was converted into the potassium sulfonate salt, as shown in eq 28.41

Finally, certain relatively stable thioketones can be transformed into the corresponding thione S-oxides by the aqueous Oxone-acetone reagent (eq 29).42

Related Reagents.

Dimethyldioxirane; Methyl(trifluoromethyl)dioxirane.


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Jack K. Crandall

Indiana University, Bloomington, IN, USA



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