Oxidation of unsaturated aldehydes

In the case of the oxidation of unsaturated aldehydes, the investigation is complicated by the fact that the aldehyde and the acid resulting from the transformation of peracid are liable to become polymerized. The double bond in a position a to the carbonyl group is not very reactive with regard to peracid, and so there is no epoxidation. 

In this category of aldehydes, the only ones examined have been croton-aldehyde, acrolein and methacrolein. 

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Not surprisingly, active M11O2 is able to oxidize unsaturated cyanohydrins, resulting in the generation of acyl cyanides. Interestingly, both the formation of the cyanohydrins by reaction of aldehydes with cyanide, and the hydrolysis of acyl cyanides with MeOH, resulting in the formation of methyl esters, can be carried out in situ with the MnC>2 oxidation. Thus, Corey et al. proved68 that aldehydes can be directly transformed into methyl esters by treatment with NaCN and active MnC>2 in a mixture of acetic acid and methanol. This represents a useful protocol for the oxidation of unsaturated aldehydes to esters. 

Few investigations of the gas phase oxidation of unsaturated aldehydes in the low temperature region have been reported. However, an analytical and kinetic examination of the combustion of crotonaldehyde [48] at 166 — probably somewhat above the low temperature region as... 

Oxidations of unsaturated aldehydes, as long as the oxidation affected only the aldehyde group, have been discussed with the oxidations of saturated and aromatic aldehydes. In this section, only such oxidations that affect the double bonds are described. 

Mechanism of selective partial oxidation of unsaturated aldehydes to corresponding carboxylic acids... 

In particular, the formal scheme of chain conversions for oxidation of unsaturated aldehydes is presented as follows ... 

Detailed review of studies on the auto- and catalytic-oxidation of unsaturated aldehydes in the liquid phase may be found in [2,9], Here we just note the main specific feature of the unsaturated aldehydes oxidation, briefly cited in Section 5.2. It implies that the free-radical chain process of the aldehyde group oxidation stimulates undesirable reactions, including the copolymerization of aldehyde with oxygen through the aldehyde s double bond, and in some cases also the copolymerization of the reaction products, the unsaturated acids and peroxyacids with oxygen. The oxidation reactions of unsaturated aldehydes are defined as the multicentered chain processes with two types of reaction centers acyl monomeric and polyperoxide free radicals. 

Figure 6.3. Flow graph illustrating the chain oxidation of unsaturated aldehyde.

As with oleate and linoleate, some volatile decomposition compounds are formed from linolenate hydroperoxides that cannot be explained by the classical A and B cleavage mechanisms, including acetaldehyde, butanal, 2-butyl furan, methyl heptanoate, 4,5-epoxyhepta-2-enal, methyl nonanoate, methyl 8-oxooctanoate, and methyl lO-oxo-8-decenoate. Some of these minor volatile oxidation products can be attributed to further oxidation of unsaturated aldehydes. Other factors contribute to the complexity of volatile products formed from hydroperoxides, including temperature of oxidation, metal catalysts, stability of volatile products and competing secondary reactions including dimerization, cyclization, epoxidation and dihydroperoxidation (Section E). 

The oxidation of unsaturated aldehydes provides an important source of additional aldehydes from the decomposition of hydroperoxides. The oxidation products of 2-nonenal include alkanals, glyoxal, and mixtures of a-keto aldehydes. The same products are formed from the oxidation of 2,4-heptadienal,... 

The large number of precursors of volatile decomposition products affecting the flavor of oils has been discussed in Chapter 4. Only qualitative information is available on the relative oxidative stability of hydroperoxides, aldehydes and secondary oxidation products. As observed with the unsaturated fatty ester precursors, the stability of hydroperoxides and unsaturated aldehydes decreases with higher unsaturation. Different hydroperoxides of unsaturated lipids, acting as precursors of volatile flavor compounds, decompose at different temperatures. Hydroperoxides of linolenate and long-chain n-3 PUFA decompose more readily and at lower temperatures than hydroperoxides of linoleate and oleate. Similarly, the alkadienals are less stable than alkenals, which in turn are less stable than alkanals. The short-chain fatty acids produced by oxidation of unsaturated aldehydes will further decrease the oxidative stability of polyunsaturated oils. For secondary products, dimers are less stable than dihydroperoxides, which are less stable than cyclic peroxides. 

Inoue, Kida and Imoto [252] found that the oxidation of unsaturated aldehydes such as cinnamaldehyde and acrolein proceeded much more slowly than did oxidation of the saturated substrates in the presence of copper-iron-polyphthalocyanine. As in the case of the saturated acids the products were a mixture of the peracid and the corresponding carboxylic acid. Other groups have recently investigated the oxidation of unsaturated aldehydes in the presence of metal complexes [253-260]. Methacrylic acid and acetic acid were formed in the copper naphthenate catalyzed oxidation of methacrolein [255]. The oxidation of acrolein to acrylic acid was catalyzed by Co, Ni, Mn and Cu acetates [256]. It was found that at concentrations of acrolein in... 

Unsaturated organic acids(e.g., acrylic acid, CH2=CHC(0)0H and its derivatives) are produced as minor products in the ozonolysis of dienes, and from the OH-initiated oxidation of unsaturated aldehydes. The most important examples in the atmosphere involve the formation of methacrylic and acrylic acids from the ozonolysis of isoprene (Orzechowska and Paulson, 2005a) and the formation of methacrylic acid from the oxidation of methacrolein, itself a by-product of isoprene oxidation ... 

Similarly, monobasic forms of other trivalent phosphorus species have been used successfully in such conjugate addition processes, including monoesters of phosphonous acids375 425 426 and secondary phosphine oxides.427-429 The notable exception to the last of these species is the addition of the anion from diphenyl phosphine oxide to unsaturated aldehydes, which appears always to proceed by addition to the carbonyl carbon.427... 

V.C.8.1. Alkenes and Alcohol Functions. Although TS-1 and other titanosi-licates oxidize alcohols to the corresponding aldehydes and ketones, the rates are suppressed in the presence of compounds containing C=C bonds. CH3OH, for example, is not oxidized at all during epoxidations of alkene reactants. Higher alcohols, however, are partially oxidized. The oxidation of unsaturated alcohols in the presence of TS-1 is shown in Table XVII (193). 

More recently, the Noyori group described an organic solvent- and haUde-free oxidation of alcohols with aqueous H202 . The catalyst system typically consists of Na2W04 and methyltrioctylammonium hydrogen sulfate, with a substrate-to-catalyst ratio of 50-500. Secondary alcohols are converted to ketones, whereas primary alcohols, in particular substituted benzyUc ones, are oxidized to aldehydes or carboxylic acid by selecting appropriate reaction conditions This system also catalyzed the chemoselective oxidation of unsaturated alcohols, the transformation exemplified in equation 65, with a marked prevalence for the hydroxy function. 

Continued investigation revealed that the principal epoxidizing agents for combined oxidation of unsaturated compounds and aldehydes are not the corresponding peracids, but the radicals of acyl peroxides. 

Table 6. Oxidation of unsaturated primary alcohols to aldehydes at the nickel hydroxide electrode...

In fine chemical manufacturing, the application of promoted platinum catalysts is less known. Maxted and Akhar have reported that the addition of stannous, manganous, ceric and ferric chloride to platinum oxide (Adams catalyst) facilitates the hydrogenation of aldehydes, ketones and olefins (ref. 1). The selective hydrogenation of unsaturated aldehydes or ketones to unsaturated alcohols has been achieved by the addition of ferrous sulfate and zinc acetate to platinum catalysts (ref. 2). 

Lipid peroxidation (Figure 14.5) is the initiating reaction in a cascade of events, starting with the oxidation of unsaturated fatty acids to form lipid hydroperoxides, which then break down to yield a variety of end products, mainly aldehydes, which can go on to produce toxicity in distal tissues. For this reason cellular damage results not only from the breakdown of membranes such as those of the endoplasmic reticulum, mitochondria, and lysosomes but also from the production of reactive aldehydes that can travel to other tissues. It is now thought that many types of tissue injury, including inflammation, may involve lipid peroxidation. 

Facile decarbonylation of aldehydes with the Rh complex (Wilkinson complex) is known [43,44], The reaction is explained by the oxidative addition of aldehyde to Rh, followed by decarbonylation and reductive elimination. However, the Rh-catalysed intramolecular reaction of some unsaturated aldehydes proceeds without the decarbonylation, and cyclic ketones are obtained. Treatment of unsaturated aldehyde... 

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