Peroxy acids formation

Instead of peroxy acid in situ, performic acid in situ may also be used for epoxidation (3). In this case, no mineral acid is necessary to catalyze peroxy acid formation because formic acid itself is sufficiently acidic. However, it is also acidic enough to catalyze the epoxide ring opening (see p. 167). 

To escape the dilemma between using pure but risky preformed peroxy acids and in situ peroxy acid formation accompanied by a strong acid fliat may decrease selectivity, peroxy acids have been generated from aldehydes and molecular oxygen catalyzed by transition metal compoimds. This technique has also been applied to the epoxidation of oleic acid (7). Benzaldehyde was used as the peroxy acid source and Co " as the catalyst (Fig. 2) the yield was 83% at 100% conversion after 1.5 h. In a similar way, 0-marked epoxy fatty acid esters have been synthesized (8). 

A similar method for the self- epoxidation of plant oils, one based on perhy-drolysis, yields 88-96% epoxides with a selectivity of >92% (15) (Fig. 4). The method is characterized by the use of 35% hydrogen peroxide and the addition of a small amount of free fatty acids, which is necessary not for peroxy acid formation but to prevent the formation of mono- and diglycerols by (per-)hydrolysis. 

When an unsymmetrical ketone is treated with a peroxy acid, formation of the ester is regiose-lective for example ... 

Step 1 The peroxy acid adds to the carbonyl group of the ketone This step is a nucleophilic addition analogous to gem diol and hemiacetal formation... 

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

Oxygen donors like peroxy acids, ozone, and pyridine IV-oxides cause carbon-carbon cleavage, perhaps by formation of a perepoxide (43 Scheme 30) (81JCS(P1)1871). Other oxidants have also been reported to react with oxiranes (64HC( 19-1)228). 

Ameisen-. formic, -aldehyd, n. formaldehyde. -amylMther, -amylester, m. amyl formate, -ather, m. formic ether (ethyl formate), geist, m. (Pharm.) spirit of ants (a mixture of formic acid, alcohol and water), -naphta, n. = Ameisenather. -persaure, /. peroxy-formic acid, performic acid, ameisensauer, a. of or combined with formic acid, formate of. — ameisensaures Salz, formate. 

The peroxy acid transfers an oxygen atom to the alkene in a cyclic, single-step mechanism. The result is the syn addition of the oxygen to the alkene, with formation of an epoxide and a carboxylic acid. 

Support for this conclusion is provided by the hydroperoxide specificity of BP oxidation. The scheme presented in Figure 6 requires that the same oxidizing agent is generated by reaction of h2°2/ peroxy acids, or alkyl hydroperoxides with the peroxidase. Oxidation of any compound by the iron-oxo intermediates should be supported by any hydroperoxide that is reduced by the peroxidase. This is clearly not the case for oxidation of BP by ram seminal vesicle microsomes as the data in Figure 7 illustrate. Quinone formation is supported by fatty acid hydroperoxides but very poorly or not at all by simple alkyl hydroperoxides or H2C>2. The fact that... 

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

Analogously, the use of m-chloroperbenzoic acid (MCPBA) or the in situ formation of peroxy acids by means of a sacrificial aldehyde (typically benzaldehyde plus molecular oxygen as a source of perbenzoic acid [20, 21]) are not viable processes, from an industrial point of view, because of the very poor atom efficiency and the huge formation of benzoic acids as side products. 

These materials are very easily autoxidised and often have a low autoignition temperature. It is reported that many of the less volatile liquid aldehydes will eventually inflame if left exposed to air on an absorbent surface. The mechanism is undoubtedly similar to that giving rise to easy ignition in the air-oxidation of acetaldehyde and propionaldehyde initial formation of a peroxy-acid which catalyses the further oxidation[l]. Autoignition temperatures of lower aldehydes are much reduced by pressure, but appear to depend little on oxygen content. The effect is worst in the presence of free liquid, in which initial oxidation appears to occur, possibly catalysed by iron, followed by ignition of the vapour phase [2], An acetaldehyde/rust mix exploded at room temperature on increasing the air pressure to 7 bar. 

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

Just such an example of a planar TS has been reported recently by Sarzi-Amade and his coworkers " . who managed to locate only a planar-hke TS and a planar TS (the peroxy acid plane contains the C=C bond axis), for anti- and iyw-sesquinorbornenes. They are substrates that, because of steric constraints, cannot easily accommodate spiro-like TSs. These planar TSs are nonconcerted since they are strongly unsymmetrical and only one of the C—O bonds of the oxirane ring is significantly formed. IRC analysis, while confirming that formation of one C—O bond fully precedes that of the other, also suggests that aU this can take place without formation of intermediates, that is, within a nonconcerted one-step process . 

The total yields from epoxidation are in the range of 35-90. The rate of the reaction with peroxy acids is rather low one to seven days are usually necessary for completion of epoxide formation. 

Special aspects. Five special features of peroxy acid epoxula tion will be mentioned briefly at this point (1) formation of lactone ... 

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