Aldehydes from hydroperoxides

Many impurities are present in commercial caprolactam which pass into the liquid wastes from PCA manufacture from which caprolactam monomer may be recovered. Also, the products of die thermal degradation of PCA, dyes, lubricants, and other PCA fillers may be contained in the regenerated CL. Identification of die contaminants by IR spectroscopy has led to the detection of lower carboxylic acids, secondary amines, ketones, and esters. Aldehydes and hydroperoxides have been identified by polarography and thin-layer chromatography. 

Another factor complicating the situation in composition of peroxyl radicals propagating chain oxidation of alcohol is the production of carbonyl compounds due to alcohol oxidation. As a result of alcohol oxidation, ketones are formed from the secondary alcohol oxidation and aldehydes from the primary alcohols [8,9], Hydroperoxide radicals are added to carbonyl compounds with the formation of alkylhydroxyperoxyl radical. This addition is reversible. 

Although the ozonolysis product exists in oligomeric form, the amount of acid used was calculated by assuming a theoretical yield of the corresponding monomeric aldehyde—methoxy hydroperoxide. p-Toluenesulfonic acid monohydrate, purchased from Aldrich Chemical Company, Inc., was not further purified. 

One-pot synthesis of campholenic aldehyde from a-pinenc has been achieved using t-butyl hydroperoxide and a Ti-containing mesoporous material of the I IMS-type. [80]. Selectivity to campholenic aldehyde was 82.4 % under the following. Conditions a-pincnc (5 mmol), t-butyl hydroperoxide (5 mmol, dried over MgSO,i), Ti-llMS (0.1 g) in acetonitrile (30 ml) stirred 24 h at 75 °C. 

We first screened several transition metal conqtlexes which have been shown to have activity in transferring oxygen from hydroperoxides onto organic substrates via peroxometal intermediates. If such an intermediate is capable of coordinating an alcohol moiety, this may produce a potentially active catalyst for alcohol oxidation. The results of the oxidation of the alcohol with t-BuOOH are listed in Table 1. The molybdenum conqjlex effected a conqrlete conversion of the t-BuOOH to t-BuOH (Run 1) at very low efficiency towards the formation of the aldehyde with the alcohol converted primarily to the acid which further reacted with the t-BuOH or the starting alcohol to give the esters. The use of the other complexes, with the exception of the ZrO(OAc)2 (Run 5), led to low conversions and low selectivity to the aldehyde. 

Figure 1 LC/ESI-MS analysis of oxidized PtdCho in oxidized LDL. Total positive ion current profile of oxidized LDL (A) single ion plots of representative PtdCho oxidation products. Peak identification is as given in figure. LDL was oxidized by incubation with 5pmoll CiS04 in 0.1 moll phosphate buffered saline (PBS) for 12h at 37°C. The total lipid extract of the oxidized LDL was dissolved in chloroform/methanol (2 1, v/v) and 20 pi of the sample containing 10 pg lipid was analyzed. Structural assignment for aldehydes and hydroperoxides are according to reference standards. Ions 832 and 830 were identified on the basis of retention time and molecular weight. Reproduced with permission of publisher from Ravandi A, Kuksis A, and Shaikh NA (2000) Arteriosclerosis, Thrombosis, and Vascular Biology 20. 467-477.

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,... 

Tijet, N., et al. (2001) Biogenesis of volatile aldehydes from fatty acid hydroperoxides Molecular cloning of a hydroperoxide lyase (CYP74C) with specificity for both the 9-and 13-hydroperoxides of linoleic and linolenic acids. Arch. Biochem. Biophys. 386, 281-289... 

However, in the water-free fat or oil phase of food, the homolytic cleavage of hydroperoxides presented above is the predominant reaction mechanism. Since option A of the cleavage reaction is excluded (Fig. 3.26), some other reactions should be assumed to occur to account for formation of hexanal and other aldehydes from hnoleic acid. The further oxidation reactions of monohydroperoxides and carbonyl compounds are among the possibilities. 

Reaction products of reactive aldehydes derived from oxidised lipids, such as acrolein, ( )-4-hydroxynon-2-enal and malondi-aldehyde, with lysine, arginine and other amino acids are described as examples in Section 4.7.5.6. These products, ALE (advanced lipoxidation end products), formed in vivo are markers of oxidative stress in the organism. Reaction mechanisms are discussed in Section 3.8.1.12.1. As the final reaction products, proteins and oxidised lipids also form dark insoluble macromolecular products that contain variable proportions of lipid and protein fractions. In particular, such products include protein oligomers, proteins with oxidised sulfur amino acids, proteins containing imine bonds (C=N) formed by reaction with aldehydes or hydroperoxides (they mostly arise from the -amino group of bound lysine) and... 

Overall reaction of enzyme system consisting of lipoxygenase and hydroperoxide lyase in tea chloroplasts was investigated. As described earlier, tea chloroplast thylakoids contained lipoxygenase and hydroperoxide lyase. ( ) This enzyme system catalyzes formation of C -aldehydes including hexanal, (3Z)-hexenal and (2 )-hexenal from linoleic acid and linolenic acid but not C -aldehydes from these fatty acids. 

In contrast to oxidation in water, it has been found that 1-alkenes are directly oxidized with molecular oxygen in anhydrous, aprotic solvents, when a catalyst system of PdCl2(MeCN)2 and CuCl is used together with HMPA. In the absence of HMPA, no reaction takes place(100]. In the oxidation of 1-decene, the Oj uptake correlates with the amount of 2-decanone formed, and up to 0.5 mol of O2 is consumed for the production of 1 mol of the ketone. This result shows that both O atoms of molecular oxygen are incorporated into the product, and a bimetallic Pd(II) hydroperoxide coupled with a Cu salt is involved in oxidation of this type, and that the well known redox catalysis of PdXi and CuX is not always operalive[10 ]. The oxidation under anhydrous conditions is unique in terms of the regioselective formation of aldehyde 59 from X-allyl-A -methylbenzamide (58), whereas the use of aqueous DME results in the predominant formation of the methyl ketone 60. Similar results are obtained with allylic acetates and allylic carbonates[102]. The complete reversal of the regioselectivity in PdCli-catalyzed oxidation of alkenes is remarkable. 

Oxidation. Olefins in general can be oxidized by a variety of reagents ranging from oxygen itself to ozone (qv), hydroperoxides, nitric acid (qv), etc. In some sequences, oxidation is carried out to create a stable product such as 1,2-diols or glycols, aldehydes, ketones, or carboxyUc acids. In other... 

Hydroxyall l Hydroperoxides. These compounds, represented by (1, X = OH, R = H), may be isolated as discreet compounds only with certain stmctural restrictions, eg, that one or both of R and R are hydrogen, ie, they are derived from aldehydes, or that R or R contain electron-withdrawing substituents, ie, they are derived from ketones bearing a-halogen substituents. Other hydroxyalkyl hydroperoxides may exist in equihbrium mixtures of ketone and hydrogen peroxide. 

Hydroxyalkyl hydroperoxides having at least one a-hydrogen ie, (7, X = OH, R = alkyll, R = R = H), ie, those derived from aldehydes, lose hydrogen peroxide and form dialkyl peroxides (2, X = Y = OH), especially in the presence of water ... 

Acidic hydrolysis of these hydroxyaLkyl hydroperoxides yields carboxyUc acids, whereas basic hydrolysis regenerates the parent aldehyde, hydrogen peroxide, and often other products. When derived from either aldehydes or cycHc ketones, peroxides (1, X = OH, = H, R, = alkylene or... 

Polymeric OC-Oxygen-Substituted Peroxides. Polymeric peroxides (3) are formed from the following reactions ketone and aldehydes with hydrogen peroxide, ozonization of unsaturated compounds, and dehydration of a-hydroxyalkyl hydroperoxides consequendy, a variety of polymeric peroxides of this type exist. Polymeric peroxides are generally viscous Hquids or amorphous soHds, are difficult to characterize, and are prone to explosive decomp o sition. 

Safety. Since organic peroxides can be initiated by heat, mechanical shock, friction or contamination, an enormous problem in safety presents itself. Numerous examples of this problem have already been shown in this article. Additional examples include the foilowing methyl and ethyl hydroperoxides expld violently on heating or jarring, and their Ba salts also are extremely expl the alkylidene peroxides derived from low mw aldehydes and ketones are very sensitive and expld with considerable force polymeric peroxides of dimethyl ketene, -K>-0-C(CH3)2C(0)j-n, expld in the dry state by rubbing even at —80° peroxy acids, especially those of low mw, and diacetyl, dimethyl, dipropkmyl and methyl ethyl peroxides, when pure, must be handled only in small amts and... 

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