Meta oxides with phenols

Dehydrative condensation of benzofurazan oxide with phenolic enolates affords phenazine di-N-oxides (Scheme 7). The condensation proceeds under mild conditions (NaOH/H20, H2O, MeOH/RNH2, Si02/CHs-CN at room temperature) and can be applied to a broad range of substituted nucleophiles of varying oxidation levels (phenolates, resorcinolates, hydro-quinones, and benzoquinones). A few limitations have been encountered for the phenol, and these include insufficient reactivity of dioxalane-protected p-formyl phenol and decarboxylation of free carboxylates at even mild reaction conditions (NaOH/H20, 60 Nucleophilic attack on the benzofurazan will occur from the para position in ortho- and meta-substituted phenolates, whereas para-substituted phenols will attack from the ortho position. Subsequently, elimination of H2O or ROH will take place if possible otherwise, elimination of H2 will give the phena-... 

A detailed study ot the oxidation of ortho-, meta-, and para-tnfluoromethyl phenols showed that oxidation with chloric acid at 5 to 10 C for 0 5 h gives a complex mixmre of products [59] with isolable 2-chloro-1 hydroxy-2-tnfluoro-methylcyclopent-4-en-3-one-1 -carboxylic acid... 

The oxidation of phenol, ortho/meta cresols and tyrosine with Oj over copper acetate-based catalysts at 298 K is shown in Table 3 [7]. In all the cases, the main product was the ortho hydroxylated diphenol product (and the corresponding orthoquinones). Again, the catalytic efficiency (turnover numbers) of the copper atoms are higher in the encapsulated state compared to that in the "neat" copper acetate. From a linear correlation observed [7] between the concentration of the copper acetate dimers in the molecular sieves (from ESR spectroscopic data) and the conversion of various phenols (Fig. 5), we had postulated [8] that dimeric copper atoms are the active sites in the activation of dioxygen in zeolite catalysts containing encapsulated copper acetate complexes. The high substratespecificity (for mono-... 

CHO COOH. Oxidation of the complement inhibitor K-76 (1) in 1 N NaOH with excess Ag20 cleanly affords a monocarboxylic acid, shown to be 2 by lactonization experiments. Thus the formyl group meta to the phenolic hydroxyl is the... 

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]. 

Oxidation with alkaline CuO gave large amounts of meta-hydroxy derivatives of benzoic acid and benzene dicarboxylic acids (Hayatsu et al., 1980a). This suggests the presence of phenol ethers in the polymer structure. Interestingly, terrestrial polymers such as lignin, humic acid, and coal yield mainly para-rather than meta-hydroxy derivatives by this method. 

A kinetic model is presented by Pohlman and McColl (1989) to describe the initial and rapid redox processes between polyhydroxyphenolic acid and soil or Mn oxide suspensions. The oxidation process of polyhydroxybenzoic acid by soil and Mn oxides follows second-order kinetics. The rate constants derived from the model are similar in magnitude in both suspensions for the organic reductants studied (Table 8-16). Polyhydroxyphenolic acids with para- and ortho-OH groups are rapidly oxidized by Mn oxides with spectral evidence suggesting that the reaction leads to polymeric humic products probably via semiquinone or benzoquinone derivatives. By contrast, polyhydroxyphenolic acids with meta-oriented phenolic-OH groups are not oxidized by soil or Mn oxide suspensions within a 120-min reaction period. Presumably these compounds are not capable of being oxidized to semiquinone or benzoquinone intermediates. The rapid disappearance of polyhydroxyphenolic acids is accompanied by formation of Mn and colored humic products in both soil and Mn oxide suspensions. These results provide further evidence that abiotic oxidation of certain organics can lead to the formation of humic substances and the mobilization of Mn in nature. 

Figure 6.16 Correlation of sigma values with rate of oxidation of phenolate anions by singlet oxygen. Open symbols indicate phenols with meta substituents and filled symbols, phenols with para substituents. Numbers indicate phenols listed in Table 6.13. [Reproduced with permission from P. G. Tratnyek and J. Hoigne, Environ. Set Technol. 25, 1596 (1991). Copyright 1991, American Chemical Society.]...

Synthetic phenol capacity in the United States was reported to be ca 1.6 x 10 t/yr in 1989 (206), almost completely based on the cumene process (see Cumene Phenol). Some synthetic phenol [108-95-2] is made from toluene by a process developed by The Dow Chemical Company (2,299—301). Toluene [108-88-3] is oxidized to benzoic acid in a conventional LPO process. Liquid-phase oxidative decarboxylation with a copper-containing catalyst gives phenol in high yield (2,299—304). The phenoHc hydroxyl group is located ortho to the position previously occupied by the carboxyl group of benzoic acid (2,299,301,305). This provides a means to produce meta-substituted phenols otherwise difficult to make (2,306). VPOs for the oxidative decarboxylation of benzoic acid have also been reported (2,307—309). Although the mechanism appears to be similar to the LPO scheme (309), the VPO reaction is reported not to work for toluic acids (310). 

Picric acid, the 2 4 6-trinitro derivative of phenol, cannot bo prepared in good yield by the action of nitric acid upon phenol since much of the latte-is destroyed by oxidation and resinous products are also formed. It is more convenient to heat the phenol with concentrated sulphuric acid whereby a mixture of o- and p-phenolsulphonic acids is obtained upon treatment of the mixture with concentrated nitric acid, nitration occurs at the two positions meta to the —SOjH group in each compound, and finally, since sulphonation is reversible, the acid groups are replaced by a third iiitro group yielding picric acid in both cases ... 

Chiral (salen)Mn(III)Cl complexes are useful catalysts for the asymmetric epoxidation of isolated bonds. Jacobsen et al. used these catalysts for the asymmetric oxidation of aryl alkyl sulfides with unbuffered 30% hydrogen peroxide in acetonitrile [74]. The catalytic activity of these complexes was high (2-3 mol %), but the maximum enantioselectivity achieved was rather modest (68% ee for methyl o-bromophenyl sulfoxide). The chiral salen ligands used for the catalysts were based on 23 (Scheme 6C.9) bearing substituents at the ortho and meta positions of the phenol moiety. Because the structures of these ligands can easily be modified, substantia] improvements may well be made by changing the steric and electronic properties of the substituents. Katsuki et al. reported that cationic chiral (salen)Mn(III) complexes 24 and 25 were excellent catalysts (1 mol %) for the oxidation of sulfides with iodosylbenzene, which achieved excellent enantioselectivity [75,76]. The best result in this catalyst system was given by complex 24 in the formation of orthonitrophenyl methyl sulfoxide that was isolated in 94% yield and 94% ee [76]. 

In a recent paper, the same authors showed that iron(III) chloride can mediate the oxidative coupling of substituted aryl ethers with an observed regioselectivity that depends on the substitution pattern [66] meta-substituted phenol ethers 77 led to polymers (Scheme 18a) whereas para-substituted phenol ether 79 gave predominantly biphenyl structures (Scheme 18b). ortho-Substituted phenol ether 81 provided a dimer with the Ar-Ar bond at a position para to one of the methoxy substituents (Scheme 18c). 

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