Percarboxylic acid

The epoxidation of olefins, as well as other oxidative procedures, require the use of percarboxylic acids. Two of the more easily prepared and more stable compounds are given below. 

Caution This reaction must be conducted behind a safety shield. By employing a beaker to contain the reactants, destructively high pressures in case of explosion will more readily be vented. 

A 500-ml, three-necked, round-bottom flask is fitted with a mechanical stirrer, a thermometer, and a wide-stern (powder) funnel. The flask is cooled in an ice-salt bath and charged with 125 ml (approx. 0.5 mole) of 15% sodium hydroxide solution. When the stirred solution reaches -10°, 30% hydrogen peroxide (57.5 g, 52.5 ml, approx. 0.5 mole) previously cooled to -10° is added in one portion. The pot temperature rises and is allowed to return to —10° whereupon 37.5 g (0.25 mole) of phthalic anhydride (pulverized) is added rapidly with vigorous stirring. Immediately upon dissolution of the anhydride, 125 ml (approx. 0.25 mole) of cooled (-10°) 20% sulfuric acid is added in one portion. (The time interval between dissolution of the anhydride and the addition of the cold sulfuric acid should be minimized.) The solution is filtered through Pyrex wool and extracted with ether (one 250-ml portion followed by three 125-ml portions). The combined ethereal extracts are washed three times with 75-ml portions of 40% aqueous ammonium sulfate and dried over 25 g of anhydrous sodium sulfate for 24 hours under refrigeration. 

The dried ether solution contains about 30 g (65%) of monoperphthalic acid and is approx. 0.26 to 0.28 M. It may be used directly for oxidation reactions (cf. Chapter 1, Section IV), or stored under refrigeration. Evaporation of the ether under reduced pressure (no heat) affords the crystalline product, mp 110° (dec). 

All reactions employing diazomethane must be carried out in a hood behind a safety shield  

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The reaction of peracids with ketones proceeds relatively slowly but allows for the conversion of ketones to esters in good yield. In particular, the conversion of cyclic ketones to lactones is synthetically useful because only a single product is to be expected. The reaction has been carried out with both percarboxylic acids and Caro s acid (formed by the combination of potassium persulfate with sulfuric acid). Examples of both procedures are given. 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

Alternative to m-chloroperbenzoic acid.1 This oxidant has been introduced as a suitable replacement for m-chloroperbenzoic acid, which is no longer available from commercial sources because of hazards in the manufacture. Actually MMPP is a safer reagent than MCPBA, which is shock-sensitive and potentially explosive. MMPP is soluble in water and in low-molecular-weight alcohols. The by-product, magnesium phthalate, is water-soluble and easily removed. It is generally more stable than other percarboxylic acids. It can replace MCPBA for the usual classic oxidations epoxidation, Baeyer-Villiger reactions, and oxidation of amines to N-oxides. 

FIGURE 21. Association of percarboxylic acid 50 in the solid state (the peroxide protons were not localized directly and were therefore omitted from the graphics) ... 

The percarboxylic acid proton of 3-oxo-l,2-benzisothiazole-2(377)-peroxypropanoic acid 1,1-dioxide (51) (Pnma, 0—0 = 1.469, C—O—O—H = 180.0°) was located on the difference Fourier map . Hydrogen bonding in the peracid 51 (Figure 22) occurs from the peracid proton to the carbonyl O of the saccharin entity (O O = 2.618 A) to provide chains of peracid molecules that are stacked via additional C—H O contacts (not shown in Figure 22) in sheets along the b axis. 

Organic peracids are the most reliable and commonly used oxidizing reagents for various purposes in organic chemistry. Compared to common percarboxylic acids such as performic acid, peracetic acid, perbenzoic acid and m-chloroperbenzoic acid, the first sulfur peroxy acid, i.e. monopersulfuric acid , was reported earlier, already in 1891. 

The pyrazine ring is stable toward permanganate oxidation, and this explains a variety of pyrazinecarboxylic acids that have been prepared from quinoxalines or benzo-fused quinoxalines. In contrast, alkyl side chains on pyrazines are effectively oxidized by permanganate, selenious acid, selenium dioxide, or dichromate to afford the corresponding carboxylic acids (Section 8.03.7.1). Oxidation of pyrazines with hydrogen peroxide or percarboxylic acids gives pyrazine A -oxides and/or A, A -dioxides (Section 8.03.5.2). 

The ability of percarboxylic acids to transfer the terminal hydroxyl group as HO+ to relatively weak donors such as alkenes is due to activation by the carbonyl via polarity alternation. As expected, other types of peracids with an acceptor heteroatom replacing the carbonyl behave similarly. Perhaps unanticipated by many chemists is the usefulness of Si as the C = 0 equivalent [241]. 

Fev=0 and Mnv=0 have also recently emerged as plausible reactive intermediates in the oxidation of hydrocarbons by iodosylbenzene, amine N-oxide, percarboxylic acids, hypochlorites, etc. catalyzed by iron or manganese porphyrins. Current opinion favors Fev=0 species as the active oxidant in cytochrome P-450 monooxygenases.54 55... 

It is noteworthy that cytochrome P-450 can also function with peroxide oxygen sources other than 02 + 2e, viz, iodosylbenzene,81 alkyl hydroperoxides,78 or percarboxylic acids.82... 

A) Nucleophilic attack of the alkene on the electrophilic oxygen atom covalently bound to the metal, which is reminiscent of Bartlett s butterfly mechanism for epoxidation of alkenes by percarboxylic acids.229... 

Another one-step addition reaction to C=C double bonds that forms three-membered rings is the epoxidation of alkenes with percarboxylic acids (Figure 3.19). Most often, meta-chloroperbenzoic acid (MCPBA) is used for epoxidations. Magnesium monoperoxyphthalate (MMPP) has become an alternative. Imidopercarboxylic acids are used to epoxidize olefins as well. Their use (for this purpose) is mandatory when the substrate contains a ketonic C=0 double bond in addition to the C=C double bond. In compounds of this type, percarboxylic acids preferentially cause a Baeyer-Villiger oxidation of the ketone (see Section 14.4.2), whereas imidopercarboxylic acids selectively effect epoxidations (for an example see Figure 14.35). 

Fig. 3.19. Stereospecific cis-epoxidations of alkenes with percarboxylic acids.

In the transition state of the epoxidation of alkenes with a percarboxylic acid the C=C axis of the alkene is rotated out of the plane of the percarboxylic acid group by 90° ( spiro transition state ). In this process, four electron pairs are shifted simultaneously shifted. This very special transition state geometry make peracid oxidations of C=C double bonds largely insensitive to steric hindrance. The epoxidation given in Figure 3.20 provides an impressive example. 

Fig. 3.20. Due to the transition state geometry shown there is hardly any steric hindrance in cis-epoxidations with percarboxylic acids—as is impressively demonstrated by the example given here.

Examination of this mechanism suggests that the nature of the R group should not make much difference in the reaction. In fact, a number of different percarboxylic acids can be used to epoxidize alkenes, as illustrated in the following examples. As expected, the additions occur with syn stereochemistry. 

Peptide (Section 26.5) A term used for a polymer of amino acids that is smaller than a protein. Percarboxylic acid (Section 11.9) A carboxylic acid that has an extra oxygen and an oxygen-oxygen bond RCO,H. 

One example which deserves special mention is the use of a percarboxylic acid such as peracetic acid, generated in situ by autoxidation of the corresponding aldehyde, developed by Murahashi and coworkers, see Eq. (1) [25-27]. These reactions are generally considered to involve high-valent oxoruthenium complexes, generated by reaction of the percarboxylic acid with the ruthenium catalyst, as the active oxidant. 

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