Carboxylic peroxycarboxylic acids

Functional groups that stabilize radicals would be expected to increase susceptibility to autoxidation. This is illustrated by two cases that have been relatively well studied. Aldehydes, in which abstraction of the aldehyde hydrogen is fecile, are easily autoxidized. The autoxidation initially forms a peroxycarboxylic acid, but usually the corresponding carboxylic acid is isolated because the peroxy acid oxidizes additional aldehyde in a... 

For the mechanism of azolide hydrolysis under specific conditions like, for example, in micelles,[24] in the presence of cycloamyloses,[25] or transition metals,[26] see the references noted and the literature cited therein. Thorough investigation of the hydrolysis of azolides is certainly important for studying the reactivity of those compounds in chemical and biochemical systems.[27] On the other hand, from the point of view of synthetic chemistry, interest is centred instead on die potential for chemical transformations e.g., alcoholysis to esters, aminolysis to amides or peptides, acylation of carboxylic acids to anhydrides and of peroxides to peroxycarboxylic acids, as well as certain C-acylations and a variety of other preparative applications. 

The transformation of carboxylic acids and their functional derivatives to the corresponding peroxycarboxylic acids or diacyl peroxides are generally known reactions.200-201 Among the hydroperoxy derivatives, trifluoroperacetic acid is a frequently used epoxidizing reagent in organic chemistry and is usually prepared in situ.200-201... 

Other methods for preparing peroxycarboxylic acids include (/) autoxidation of aldehydes. (2) reaction of acid chlorides, anhydrides, or boric-carboxylic anhydrides with hydrogen or sodium peroxide, and (3) basic hydrolysis or perhydrolysis of dtacyl peroxides. 

Another early discovery was that CALB accepts H202 as nucleophile to produce peroxycarboxylic acids from esters or carboxylic acids (perhydrolysis activity can also be found in other serine hydrolases) [46, 47]. The in situ formed peracid can subsequently be used to epoxidize an alkene by (non-enzymatic) Prileshajev epoxidation. Hence, oleic acid incubated with CALB and H202 will produce 9,10-epoxyoctadecanoic acid [48]. Other alkenes can be epoxidized by H202 and a catalytic amount of carboxylic acid (and CALB) (Scheme 13.2) [49],... 

Peroxycarboxylic acid esters are utilized as initiators for radical polymerization and are interesting intermediates for the decarboxylation of carboxylic acids. They are generally prepared by the acylation of hydro peroxides with acid chlorides, acid anhydrides, or imidazolides in the presence of a base. Condensation of carboxylic acids (20) with /-butyl-hydroperoxide (21) has been smoothly achieved by the use of diethyl phophorocyanidate and NEt3 under mild conditions giving f-butyl peroxycarboxylates (22) in good yield.9... 

Peroxycarboxylic acid RCOiH. a carboxylic acid with an additional, electrophilic oxygen. 

With in situ generated peroxycarboxylic acids from urea-H202, carboxylic acid anhydrides and N-methylmorpholine N-oxide as an additive, chiral derivatives of Mn-salen complex I were found to oxidize effectively dihydronaphthalene and indene (126). The combination employing maleic anhydride has led to good results, up to 87 % ee and 71 % yield. Generally, lowering the temperature from 2 to —18°C had marginal positive effect on the yield and enantiomeric excess of the epoxides. 

Peroxycarboxylic acids. Peracids are generally prepared by reaction of carboxylic acids with H2O2 in a strongly acidic medium (HaSOi, CHsSOjOH, 1,458). Lefort el al. now report that they can be made in a neutral medium by reaction of acyl chlorides with 857o H20i in THF, usually with added pyridine, which can markedly improve the yields in some instances. Diacyl peroxides are sometimes obtained as by-products. The peracids are obtained in 55-70% yield. ... 

An oxy group, when acting as an acceptor (e.g., RO ), is soft. Therefore, it is easily understood why peroxycarboxylic acids are thermodynamically unstable and reactive molecules. Moreover, the stronger (harder) the carboxylic acid, the more reactive its corresponding peracid is since there exists a wider softness gap between OH and RCOO . Thus, trifluoroperacetic acid is a more potent oxidant than peracetic acid, and triazole-1-peroxycarboxylic acid (2) is about two hundred times more reactive than perbenzoic acid toward olefins. 

An excellent candidate for this intermediate would be a trioxolanone. By analogy with the primary ozonides, this species would decompose to carbon dioxide and a peroxycarboxylic acid. However, in the presence of a suitable oxene acceptor (A), the exothermicity of the liberation of carbon dioxide and a carboxylic acid could be exploited for efficient mono-oxygenation (293,294) (eq. 79). Similar oxene-donating properties have been demonstrated for analogous trioxolenes (295, 296). 

A group bonded to the initial ketone or aldehyde carbon migrates to oxygen, producing the ester (or lactone) as the peroxycarboxylic acid is released as a carboxylic acid. 

Following is a balanced equation for the epoxidation of cyclohexene by a peroxycarboxylic acid. In the process, the peroxycarboxylic acid is reduced to a carboxylic acid ... 

Unfortunately, direct epoxidation of alkenes by metal-free haloperoxidases led to racemic epoxides [1331, 1332]. Since the reaction only takes place in the presence of a short-chain carboxylic acid (e.g., acetate or propionate), it is believed to proceed via an enzymatically generated peroxycarboxylic acid, which subsequently oxidizes the alkene without the aid of the enzyme. This mechanism has a close analogy to the lipase-catalyzed epoxidation of alkenes (Sect 3.1.5) and halogenation reactions catalyzed by haloperoxidases (Sect. 2.7.1), where enzyme catalysis is only involved in the formation of a reactive intermediate, which in turn converts the substrate in a spontaneous (nonenzymatic) foUowup reaction. 

In contrast to the acidic conditions usually applied for in situ generation of peroxycarboxylic acids, they may be generated under virtually neutral conditions in a suitable organic solvent directly from the parent carboxylic acid and hydrogen peroxide via lipase catalysis (Scheme 3.29). 

Sodium peroxide/magnesium sulfate Peroxycarboxylic acids from carboxylic acid chlorides... 

As a rule, alkenes do not react with 78-80 unless there is another reagent present—specifically, a transition metal. This reaction will not be discussed further. In sharp contrast, peroxycarboxylic acids such as 81 react directly with alkenes. Peroxycarboxylic acids 81 are named by adding the term peroxy to the name of the carboxylic acid (see Chapter 5, Section 5.9.3 and Chapter 16, Section 16.4). Using the common names, the peroxy analog of formic acid is peroxyformic acid (82), and others include peroxyacetic acid (83), peroxytrifiuo-roacetic acid (84), peroxybenzoic acid (85), and me a-chloroperoxybenzoic acid (abbreviated mCPBA, 86). Peroxycarboxylic acid 85 is a derivative of the aromatic carboxylic acid benzoic acid (PhCOOH), and the carboxylic acid precursor to 86 is clearly another aromatic carboxylic acid. (The nomenclature and structural features of benzoic acid and other aromatic carboxylic acid derivatives will be discussed in detail in Chapter 21, Section 21.2.) The salient feature of peroxyacids 82-86 is the presence of the electrophihc oxygen atom mentioned previously, which will react with an alkene. 

Interestingly, the same kind of carboxyl radical can be generated from the thermal decomposition of esters (usually the t-butyl ester as its reactivity is more easily controlled) of peroxycarboxylic acids. In this case, the peroxyester is not formed by oxidation of the corresponding ester but rather (Equation 9.85) by the reaction between the acid chloride of the carboxylic acid and the corresponding alkyl peroxide (often generated by direct oxygenation of the appropriate alkane. Chapter 5). 

Hydrocarbons from carboxylic acid chlorides via peroxycarboxylic acid ferf-butyl esters... 

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