Hydroxylated benzaldehydes oxidation

Lindgren, B. O., Nilsson, T. Preparation of carboxylic acids from aldehydes (including hydroxylated benzaldehydes) by oxidation with chlorite. Acta Chemica Scandinavica (1947-1973) 1973, 27, 888-890. 

Pyrolytic Decomposition. The pyrolytic decomposition at 350—460°C of castor oil or the methyl ester of ricinoleic acid spHts the ricinoleate molecule at the hydroxyl group forming heptaldehyde and undecylenic acids. Heptaldehyde, used in the manufacture of synthetic flavors and fragrances (see Elavors and spices Perfumes) may also be converted to heptanoic acid by various oxidation techniques and to heptyl alcohol by catalytic hydrogenation. When heptaldehyde reacts with benzaldehyde, amyl cinnamic aldehyde is produced (see Cinnamic acid, cinnamaldehyde, and cinnamyl... 

Hydroxyl elimination is necessary for the formation of benzaldehyde and benzoic acid derivatives and, ultimately, benzene and toluene (Fig. 7.46).2 It is proposed that a cleavage between the hydroxyl group and aromatic ring leads to benzenoid species which undergo further cleavage coupled with oxidation to give various decomposition products. 

Typical non-enolising aldehydes are formaldehyde and benzaldehyde, which are oxidised by Co(III) Ce(IV) perchlorate and sulphate , and Mn(III) . The main kinetic features and the primary kinetic isotope effects are the same as for the analogous cyclohexanol oxidations (section 4.3.5) and it is highly probable that the same general mechanism operates. kif olko20 for Co(III) oxidation of formaldehyde is 1.81 (ref. 141), a value in agreement with the observed acid-retardation, i.e. not in accordance with abstraction of a hydroxylic hydrogen atom from H2C(OH)2-The V(V) perchlorate oxidations of formaldehyde and chloral hydrate display an unusual rate expression, viz. 

A principally different approach for the indirect electrochemical oxidation of aromatic compounds goes via the formation of hydroxyl radicals from cathodically generated hydrogen peroxide and from reductively formed iron(II) ions. The thus in situ formed Fenton reagent can lead to side-chain as well as nuclear oxidations of aromatic compounds. Side-chain oxidations to form benzaldehydes according to Eqs. (18)—(24) can also be initiated by the redox pairs and Cu instead of... 

Toluene and its substituted derivatives are oxidized by sodium dichromate or chromyl chloride mainly into the corresponding benzoic acids or benzaldehydes in good yields.270 No or very little ring hydroxylation is observed. 

It is noticeable that in the oxidation of toluene under oxygen (Fig. 5) o-, m-, and p-cresols resulting from nuclear hydroxylation of the substrate are produced in very low yields in the absence of water, but tend to increase with increasing concentration of water on the contrary, benzaldehyde and benzyl alcohol arising from oxidation of the side chain are produced in much higher yields than cresols in the absence of water, but their yields do not vary with water concentration. 

The classic C—C bond-forming processes of aldehydes and ketones are aldol reactions. In Scheme 8.7, an iron-catalyzed sequential methanol oxidation to formaldehyde and its aldol reaction with [i-oxo ester 24a is shown [30]. The oxidant is 30% aqueous H202. Curiously for an oxidation, the reaction has to be performed under an atmosphere of Ar in order to prevent a-hydroxylation of the [i-oxo ester [31], The role of benzaldehyde (4f) as substoichiometric additive is not completely clear. 

Several studies have tackled the structure of the diketopiperazine 1 in the solid state by spectroscopic and computational methods [38, 41, 42]. De Vries et al. studied the conformation of the diketopiperazine 1 by NMR in a mixture of benzene and mandelonitrile, thus mimicking reaction conditions [43]. North et al. observed that the diketopiperazine 1 catalyzes the air oxidation of benzaldehyde to benzoic acid in the presence of light [44]. In the latter study oxidation catalysis was interpreted to arise via a His-aldehyde aminol intermediate, common to both hydrocyanation and oxidation catalysis. It seems that the preferred conformation of 1 in the solid state resembles that of 1 in homogeneous solution, i.e. the phenyl substituent of Phe is folded over the diketopiperazine ring (H, Scheme 6.4). Several transition state models have been proposed. To date, it seems that the proposal by Hua et al. [45], modified by North [2a] (J, Scheme 6.4) best combines all the experimentally determined features. In this model, catalysis is effected by a diketopiperazine dimer and depends on the proton-relay properties of histidine (imidazole). R -OH represents the alcohol functionality of either a product cyanohydrin molecule or other hydroxylic components/additives. The close proximity of both R1-OH and the substrate aldehyde R2-CHO accounts for the stereochemical induction exerted by RfOH, and thus effects the asymmetric autocatalysis mentioned earlier. 

The reaction was also carried out on the laboratory scale by Bayer 182) (use of special electrolytes and collidine as an auxiliary base), Fuso 183> (use of phosphorus compounds as conductive salts) and UOPl84) (use of alcoholates as electrolytes). Under comparable conditions, p-cresol cannot be oxidized to the corresponding p-hydroxy-benzaldehyde derivatives. If the phenolic hydroxyl group is protected, it is also possible to obtain p-hydroxybenzaldehyde derivatives. 

Although significant improvements have been made in the synthesis of phenol from benzene, the practical utility of direct radical hydroxylation of substituted arenes remains very low. A mixture of ortho-, meta- and para-substituted phenols is typically formed. Alkyl substituents are subject to radical H-atom abstraction, giving benzyl alcohol, benzaldehyde, and benzoic acid in addition to the mixture of cresols. Hydroxylation of phenylacetic acid leads to decarboxylation and gives benzyl alcohol along with phenolic products [2], A mixture of naphthols is produced in radical oxidations of naphthalene, in addition to diols and hydroxyketones [19]. 

Luo and Ollis [204] compared the influence of water vapour on toluene with other compounds and found that the influence of water depended upon the characteristics of the contaminants. The research indicated that in a toluene-air mixture there was no toluene degradation in the absence of water, the toluene oxidation rate began to decrease when the water concentration started to reach saturation levels. Martra et al. [209] found that water vapour was needed to keep steady state toluene conversion to benzaldehyde and that in a dry toluene/air mixture an irreversible deactivation of the catalyst occurred. Their results further indicated that the produced benzaldehyde could undergo further oxidation but only in the presence of water. Kaneko and Okura [232] reported that the concentration of CO2 increased linearly with increases in the relative humidity and that the yield at 60% relative humidity was greater by one order than under dry conditions. The yield of benzaldehyde, however, decreased sharply with increased relative humidity and it was proposed that an increase in hydroxyl radicals may compete and/or hinder adsorption of toluene on the surface of Ti02 hence resulting in retardation of toluene oxidation. Kaneko and Okura [232], however, concluded that the effect of water vapour on the photocatalytic oxidation of toluene may depend on the initial pollutants concentration and its adsorptiv-ity. Pengyi et al. [234] observed that the effect of water vapour was two sided in that a little humidity can improve the decomposition of toluene whilst too much can suppress the decomposition. This was explained by the fact that hu-... 

Role of the p-Hydroxyl Group. Preliminary nitrobenzene oxidation experiments were conducted on several benzylic hydroxyl compounds, both with and without a p-hydroxyl group (5). Contrary to what was expected from the literature, some compounds without a p-hydroxyl group formed benzaldehyde products. 

Alkaline hydrolysis of lignin increases the number of reactive benzylic hydroxyl groups and may also be important in further depolymerizing the lignin once the oxidative-cleavage reaction has occurred. The formation of a p-electron-withdrawing -CHO substituent on aryl lignin units should increase the rate of hydrolysis of the ether bonds (26). Hydrolysis also forms p-phenylate ions, which then protect the benzaldehyde from further reaction via the Cannizzaro reaction, as mentioned earlier. 

W. Ando and Y. Moro-oka, Role of Oxygen in Chemistry and Biochemistry , Elsevier, Amsterdam, 1988. Includes chapters on (a) Cation-Radical Chain Catalyzed Oxygenation of Alkenes, by S. F. Nelsen, et al. (b) Photooxygenation of Organic Compounds via Thieir Radical Ions, by K. Mizono and Y. Otsuji (c) Triphenylpyrylium Sensitized Oxygenations, by K. Tokumaru a al. (d) Catalytic Oxidation of Metiiylarenes to Benzaldehydes, by R. A. Shddon and N. de Heij (e) Oxidation of Arylamines with Horseradish Peroxidase, 1 Fujimori et al. (f) Arene Hydroxylation by Cytochrome P-4S0, by M. Tsuda et al., and marry others. 

In 2001, Itoh and coworkers reexplored the mechanism of ortAo-phenol hydroxylations using [ Cu°(L ) 2(02)] " (7, Figure 10). This complex contains a deutero-benzyl-amine moiety, and can undergo hgand auto-oxidation, forming benzaldehyde, through a proton-coupled electron transfer (PCET) reaction with the peroxo ligand. 

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