Oxidation of substituted benzenes

Scheme 8-4. Oxidation of substituted benzene with Pseudomonas putida.

Radiation-Induced Oxidation of Substituted Benzenes Structure-Reactivity Relationship... 

Structure-reactivity relationship. This chapter is not a comprehensive review of the published work on radiation-induced chemical oxidation of benzene derivatives, nor does it cover redox properties and energetics of radical cations of substituted benzenes. The latter aspects have already been reviewed by Jonsson " earlier. In a series of papers,Jonsson and co-workers have clearly shown correlations between substituent pattern and redox properties of radical cations of substituted benzenes. Further, it has been shown by them that the product pattern is governed by the charge distribution on the radical cation and the electron density distribution on the corresponding substituted benzene. This chapter is an overview of the work carried out on radiation-induced oxidation of substituted benzenes with emphasis on the contribution to the area from our research group. 

The fundamental aspects of structure-reactivity relationships in radiation-induced oxidation of substituted benzenes, bimolecular free electron transfer on the femtosecond time scale, the chemistry of sulfur-centered radicals and the radiolysis of metalloproteins are discussed in succeeding chapters. The effects of the direct and indirect mechanisms of radiation-induced DNA damage are discussed individually in two complementary chapters. The last chapter highlights the application of radiation chemical techniques to antioxidant research. 

This mutant has been developed by Ley, Hudlicky and others into a practical method for the asymmetric oxidation of substituted benzenes to give the unstable diols best preserved as acetals. Even one substituent is enough to make the diol chiral and dihydroxylation normally occurs at... 

The fact that the values of a are smaller in alkaline medium than in acidic medium (table 1) indicates that higher EOI and EOD values are generally obtained in alkaline medium for the electrochemical oxidation of substituted benzene derivatives. 

However, if this same reaction is carried out in a venturi loop reactor, the order of magnitude large values of k a in the venturi section is likely to result in a situation such that Equation 2.6 is valid. Consequently, the mass transfer limitation can be eliminated when the venturi loop reactor replaces the stirred tank type. Thus, the reaction can achieve the maximum intrinsic rate or operate at the maximum possible capacity. This matter has been briefly discussed in Section 3.4.2.4 for Uquid-phase oxidation of substituted benzenes. The solved reactor design problem in Section 8.13 shows that this is indeed the case for catalytic hydrogenation of aniline to cyclohexylamine. 

Methods of synthesis for carboxylic acids include (1) oxidation of alkyl-benzenes, (2) oxidative cleavage of alkenes, (3) oxidation of primary alcohols or aldehydes, (4) hydrolysis of nitriles, and (5) reaction of Grignard reagents with CO2 (carboxylation). General reactions of carboxylic acids include (1) loss of the acidic proton, (2) nucleophilic acyl substitution at the carbonyl group, (3) substitution on the a carbon, and (4) reduction. 

The isomer distribution obtained from the oxidation of mesitylene in acetic acid, sodium acetate depends on the anode material. Graphite strongly favours nuclear substitution to side chain substitution in the ratio 23 1 while at platinum this ratio is 4 1. Oxidation of methyl benzenes in acetic acid containing tetrabutykmmonium fluoroborate and no acetate ion gives benzyl acetate as the major product since loss of a proton from the radical-cation is now faster than nuclear substitution by acetic acid as the only nucleophile present [39]. 

When benzyne is generated by aprotic diazotization of anthranilic acid in boiling 1,2-dimethoxyethane in the presence of substituted benzene-2-diazo 1-oxides (o-quinonediazides), dibenzofurans are obtained in moderate yields (Scheme 64). ... 

The oxidative coupling may be applied to a wide range of substituted benzenes with acrolein or methacrolein. Table 8.3 gives different substrates that can be coupled with acrolein or methacrolein by the Pd(OAc)2 / HPMoV /O2 system. 

Recent studies have shown that the reaction, as described by Eq. (11), also requires Pt(II) as a necessary catalyst (29, 84). A range of substituted benzenes has been examined, and by studying concurrent hydrogen-deuterium exchange, it was concluded that the two reactions had common intermediates (29). In this work aqueous acetic acid was used as the solvent, and reactions were followed by measuring the concentration of the chlorobenzene product. Of the several possible mechanisms for the oxidation that have been given (29), only one will be considered here this is the one that has received substantial support from the most recent work (84). In this study, the loss of reactant benzene, the formation of product chlorobenzene, and the formation of platinum(II) were monitored as the reaction proceeded. Also aqueous trifluoroacetic acid was used as the solvent, as it is known that acetic acid is oxidized to chloroacetic acid by platinum(IV) (18). 

Rates of oxidation of isolated hydroxyl groups, 225 Rates of reduction, 14 Rates of reduction of substituted benzenes 14... 

It is of interest to review ideas as to the point of hydrogen abstraction. Through 1939 most investigators believed that attack of paraffins was at the primary C—H bonds at the end of a chain (75, 109, 167, 168, 217, 220, 224), Attack at the a-carbon atom of substituted benzenes (217) and at the end methyl of olefins (109) was proposed. Preferential attack at 1° C—H bonds fitted in with the comparative ease of oxidation of n-paraffins and their low knock ratings. 

The Hammett correlation for nitrobenzenes is almost identical to the correlation at pH 9 therefore, the degradation of substituted benzenes at pH 3 can also be described by the same hole oxidation mechanism. Figure 9.21 demonstrates the oxidation of nitrobenzene by positive hole. At pH 3, substituted benzenes are oxidized by the formation of a positive hole. An electron transfer from nitrobenzene to Ti02 creates this positive hole. 

Palmisano et al. [41] in a study on the selectivity of hydroxyl radical in the partial oxidation of different benzene derivatives have investigated how the substituent group affect the distribution of the hydroxylated compounds. The reported results show that the primary photocatalytic oxidation of compounds containing an electron donor group (phenol, phenylamine, etc.) leads to a selective substitution in ortho and para positions of aromatic molecules while in the presence of an electron-withdrawing group (nitrobenzene, benzoic acid, cyanobenzene, etc.) the attack of the OH radicals is nonselective, and a mixture of all the three possible isomers is obtained. 

Table 12.4 summarizes the voltammetric oxidation potentials and peak currents for l,4-(MeO)2Ph and other alkoxy-substituted benzenes, phenols, and benzyl alcohols. Only the 1,4-(MeO)2PhX members of the series exhibit an initial irreversible anodic cyclic voltammogram via the sequence of Eq. (12.37). These plus the l,2-(MeO)2Ph isomer yield a metastable product from the second oxidation [species A, Eq. (12.37)] that undergoes a reversible reduction. Thus, the two-electron oxidation of dimethoxy benzenes yields the corresponding quinone. 

Figure 12.6 Voltammetric oxidation potentials ( p a) of substituted benzenes versus their substituent constants (apara, Ref. 16) slope, 2.43 V a. Conditions 1 mM substrate in MeCN [0.1 M (Et4N)C104] scan rate 0.1 V s 1 GCE (0.09 cm2) (Ref. 18).

We finish Chapter 18 by learning some additional reactions of substituted benzenes that greatly expand the ability to synthesize benzene derivatives. These reactions do not involve the benzene ring itself, so they are not further examples of electrophilic aromatic substitution. In Seaion 18.13 we return to radical halogenation, and in Section 18.14 we examine useful oxidation and reduction reactions. 

The oxidation of substituted aromatic amines with silver(l) carbonate on Celite has been shown to yield symmetrically substituted phenazines, albeit in poor yields. 2,7-Dimethoxy-hexafluorophenazine has been obtained upon electrolysis of solutions of 4-methoxytetrafluoro-aniline, and oxidative coupling of benzene-1,4-diamine with aniline or substituted anilines also gives phenazine derivatives. ... 

Evidence for the direct oxidation of benzene (or substituted benzenes) to benzene oxide (or substituted benzene oxides) by enzymatic (Refs. 5 and 35 and references therein) and chemical (Refs, 35 and 36 and references therein) methods is available both from the observation of the migration and retention of ring substituents during aromatic hydroxylation (NIH shift),and from the nature of the isolated products (phenols, rans-dihydrodiols). As a direct consequence of its thermal instability and high reactivity, benzene oxide 1 has not yet been isolated as an oxidation product of benzene. 

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