Oxidants para-benzaldehydes

Anisic aldehyde, CgHgOj, is a methyl ether of para-oxy-benzaldehyde, which is found to a small extent in the oils of fennel and aniseed. It is manufactured on an extensive scale artificially, and is the basis of all the perfumes of the hawthorn or May blossom type. It is known commercially as aubepine . A certain amount of anisic aldehyde is obtained as a by-product in the manufacture of coumarin, but the greater- part of it is obtained by very careful oxidation of anethol, the characteristic constituent of aniseed oil, which has the constitution—... 

Lucchi , who studied the oxidation of substituted benzaldehyde derivatives found that chlorine atoms in the meta and para position accelerate the reaction and alkyl groups retard the oxidation. A Hammett plot of Lucchi s data yields a good straight line with the slope p = 1.06. These data suggest that the reaction proceeds by way of the chromic ester of hydrated benzaldehyde as intermediate, viz. 

Raja and Perumal reported the synthesis of novel 2,6-diaryl-3-(arylthio)piperidin-4-ones via a four-component reaction consisting of arylthioacetones, 2-substituted aromatic aldehydes and methylamine or ammonium acetate <06CPB795>. Further elaboration of this four component reaction to a novel five component tandem Mannich-enamine-substitution sequence involving the reaction of ethyl 2-[(2-oxopropyl)sulfanyl]acetate, two equivalents of a substituted aromatic aldehyde, and two equivalents of ammonium acetate is shown below <06T4892>. When this five-component tandem reaction involves para-substituted benzaldehydes, the cis (193) and trans (194) diastereomers of thiazones are obtained. Alternatively, orf/zo-substituted benzaldehydes form only the trans (194) diastereomer along with an air-oxidized product 195. 

The kinetics of the oxidation of a series of para- and meto-substituted benzaldehydes by quinolinium chlorochromate are first order in substrate, oxidant, and hydronium ion the results were subjected to a Taft analysis. Oxidation of 2-pyridinecarboxaldehyde to the acid by dichromate follows an unusual mixed fourth-order rate law it is first order in hydronium ion and Cr(VI), and second order in aldehyde. 

Quinolinium dichromate (QDC) oxidations of primary and secondary alcohols both proceed via a cyclic chromate ester. Acrylonitrile polymerization was observed in the oxidation of several para- and meffl-substituted benzaldehydes to the corresponding benzoic acids by quinolinium chlorochromate (QCC). QCC oxidations of diphenacyl sulfide and of aromatic anils have been studied. 

Reaction Steps 3a and 3b also can be used to rationalize the observed para-substituent effects presented in Table III the more electron-releasing, para-substituted benzaldehydes retard the rate of oxidative addition (18) for RhCl(PPh3)3. Therefore, p-methyl- and p-methoxybenzaldehyde are expected to be decarbonylated slower than the unsubstituted benzaldehyde, as is observed in Table III. (This argument requires that Reaction 3a be saturated to the right, which is expected, in neat aldehyde solvent with electron-releasing, para-substituted benzaldehydes.) The unexpected slower rate for p-chloro-benzaldehyde could be accounted for ifK for this aldehyde is small and saturation of equilibrium in Equation 3a is not achieved. Note that fcobs is a function of K and k (see Equation 4b) under this condition. It is also possible that the rate-determining step is different for this aldehyde. Present research includes a careful kinetic analysis using several aldehydes so that K and k can be determined independently. 

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]. 

In 1983, Mimoun and co-workers reported that benzene can be oxidized to phenol stoichiometrically with hydrogen peroxide in 56% yield, using peroxo-vana-dium complex 1 (Eq. 2) [20]. Oxidation of toluene gave a mixture of ortho-, meta-and para-cresols with only traces of benzaldehyde. The catalytic version of the reaction was described by Shul pin[21] and Conte [22]. In both cases, conversion of benzene was low (0.3-2%) and catalyst turned over 200 and 25 times, respectively. The reaction is thought to proceed through a radical chain mechanism with an electrophilic oxygen-centered and vanadium-bound radical species [23]. 

The regiospecific oxidation of dimethylanisoles to methoxymethyl-benzaldehydes is accomplished with copper sulfate and potassium peroxy-disulfate, which oxidize selectively only the methyls in the ortho or para positions with respect to the methoxy group (equation 171) [35J]. 

Benzaldehydes with hydroxyl or methoxy substituents in ortho or para positions are oxidized to the corresponding phenols (carboxylic acids are formed as by-products) in good yields [2, 3]. The yield is temperature- and solvent-dependent. 

In a cresols complex producing para-cresol and some of the downstream derivatives such as p-anisic aldehyde or other oxidation products such as 3,4,5-trimethoxy benzaldehyde, etc., important inorganic chemicals can be recovered and sold not only towards improvement of the eco-system but also for betterment of the bottom-line from sales of these by-products from the waste streams. 

In the case of phenol, only the ortho and para isomers are formed. By contrast, all the isomers are observed in the case of toluene. Oxidation of the methyl group, i.e. formation of benzyl alcohol, benzaldehyde and/or benzoic acid has never been oberved. 

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

Oxidation of aldehydes with peroxy-acids is not so synthetically useful as oxidation of ketones and generally gives either carboxylic acids or formate esters. However, reaction of ortho- and para-hydroxy-benzaldehydes or -acetophenones with alkaline hydrogen peroxide (the Dakin reaction) is a useful method for making catechols and quinols. With benzaldehyde itself, only henzoic acid is formed, but orr/jo-hydroxy-benzaldehyde (salicylaldehyde) gives catechol almost quantitatively (6.65) and 3,4-dimethylcatechol was obtained by oxidation of 2-hydroxy-3,4-dimethylacetophenone. 

In early studies of this chemistry we had examined the ability of such thiazolium salts to catalyse the benzoin condensation, a process which also formally involves an acyl anion but which is really of course the anion in which the thiazolium salt has been added to the carbonyl group (Figure 2.10). In this sense the thiazolium anion is very much like cyanide anion, the normal catalyst for simple benzoin condensations. Benzaldehyde would be expected to bind into a j8-cyclodextrin cavity, so we attached a thiazolium salt to a primary carbon of j8-cyclodextrin and examined it as a catalyst. We found that this was not a better catalyst for the benzoin condensation, apparently because there was no room in the j8-cyclodextrin cavity for the binding of two benzaldehyde molecules. However it was clear that at least the reaction intermediate was being formed we got very rapid tritium exchange from the aldehyde by formation of the thiazolium adduct, and as well a very rapid oxidation of para-f-butyl benzaldehyde by ferricyanide ion since it was able to oxidize the reaction intermediate formed when the bound t-butyl benzaldehyde underwent addition of the thiazolium ring. 

Copper-iron-polyphthalocyanine [251,252] showed a specific catalysis for the oxidations of saturated aldehydes and substituted benzaldehydes with oxygen. The catalytic reaction was solvent dependent so that tetrahydrofuran, ethanol, acetonitrile, ethyl acetate and anisole inhibited benzaldehyde oxidation while oxidation occurred readily in benzene or acetone. Benzaldehyde was catalytically oxidized with copper-iron-polyphthalocyanine and oxygen to give a quantitative yield of a mixture of perbenzoic (61%) and benzoic (39%) acids. Reaction was carried out at 30 °C and atmospheric pressure of oxygen and exhibited no induction period. By contrast p-methyl and p-chlorobenzaldehyde had induction periods of 8 and 15 min respectively while no oxidation of p-substituted benzaldehydes was observed when the para-substituent was NO2, OH, OCH3, or N(CH3)2. 

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