Hydroquinone hydroxylation

Illustrated in Scheme 7.8 are the mechanisms that give rise to the products shown in Scheme 7.7. These mechanisms involve either electrophilic attack or an internal redox reaction. The internal redox reaction shown in Scheme 7.8 involves proton trapping from the solvent or from the hydroquinone hydroxyl group as shown. This process has been documented for the mitomycin system50 and also occurs in many quinone methide systems.25,30,31... 

Although stable as a solid, rifampin has certain predictable instabilities in solution. For example, in alkaline solution the hydroquinone hydroxyls (C-24 and 27) would be expected to readily oxidize.36 The acetates at C-l 1 and C-22 are likely to hydrolyze slowly with time. Most important, acidic solutions would hydrolyze the C-25 hydrazone, regenerating the inactive, and also unstable, rifaldehyde. 

During the 1980s few innovations were disclosed in the Hterature. The hydroxylation of phenol by hydrogen peroxide has been extensively studied in order to improve the catalytic system as well as to master the ratio of hydroquinone to catechol. Other routes, targeting a selective access to one of the dihydroxyben2enes, have appeared. World production capacities according to countries and process types are presented in Table 1. 

Starting from Benzene. In the direct oxidation of benzene [71-43-2] to phenol, formation of hydroquinone and catechol is observed (64). Ways to favor the formation of dihydroxybenzenes have been explored, hence CuCl in aqueous sulfuric acid medium catalyzes the hydroxylation of benzene to phenol (24%) and hydroquinone (8%) (65). The same effect can also be observed with Cu(II)—Cu(0) as a catalytic system (66). Efforts are now directed toward the use of Pd° on a support and Cu in aqueous acid and in the presence of a reducing agent such as CO, H2, or ethylene (67). Aromatic... 

Other Methods. A variety of other methods have been studied, including phenol hydroxylation by N2O with HZSM-5 as catalyst (69), selective access to resorcinol from 5-methyloxohexanoate in the presence of Pd/C (70), cyclotrimerization of carbon monoxide and ethylene to form hydroquinone in the presence of rhodium catalysts (71), the electrochemical oxidation of benzene to hydroquinone and -benzoquinone (72), the air oxidation of phenol to catechol in the presence of a stoichiometric CuCl and Cu(0) catalyst (73), and the isomerization of dihydroxybenzenes on HZSM-5 catalysts (74). 

Oxidation. Oxidation of hydroxybenzaldehydes can result in the formation of a variety of compounds, depending on the reagents and conditions used. Replacement of the aldehyde function by a hydroxyl group results when 2- or 4-hydroxybenzaldehydes are treated with hydrogen peroxide in acidic (42) or basic (43) media pyrocatechol or hydroquinone are obtained, respectively. 

Hydronium ion, 187 concentration calculation, 192 concentration and pH, 190 model, 186 Hydroquinone, 345 Hydrosphere, 437 composition, 439 Hydroxide ion, 106, 180 Hydroxides of lhird row, 371 Hydroxylamine, 251 Hydroxyl group, 329 Hypobromiie ion, 422 Hypochlorite ion, 361 Hypochlorous acid, structure, 359 Hypophosphorous acid, 372 Hypothesis, Avogadro s, 25, 52... 

Styrene C6H5CH=CH2. Alkali Peroxy compds Vap in Air 1.1 to 6.1% > Ambient > Ambient Inhibitor—Methyl Ether of Hydroquinone—10-15 ppm, Phenol Subst-Hydroxyl Amine, etc. Store below 70°F. Avoid sparks in. vap/air mixt 17.4-17.8 490 Soln polymerization catalyzed with w or Ti tetrachloride. Also self polymerization... 

The TS-1 catalysed hydroxylation of phenol to a 1 1 mixture of catechol and hydroquinone (Fig. 2.16) was commercialized by Enichem (Romano et ai, 1990). This process offers definite advantages, such as higher selectivities at higher phenol conversions, compared to other catalytic systems. It also illustrates another interesting development the use of solid, recyclable catalysts for liquid phase (oxidation) processes to minimize waste production even further. 

Magnesium-hydroxyl interactions have been used in the same way with a p-hydroquinone unit attached at the meso position of a magnesium porphyrin (103). Again, the evidence for oligomerization comes from 1H NMR spectroscopy, and the results can equally be interpreted as dimerization rather than polymerization. 

Recently, the mechanism of 6-nitro-BaP ring hydroxylation has been elucidated by using 3-deutero-6-nitro-BaP (144). When incubated with 3-methylcholanthrene-induced rat liver microsomes, this deuterated analogue yielded the same metabolite profile previously observed with 6-nitro-BaP. Spectroscopic analysis of 3-hydroxy-6-nitro-BaP and 6-nitro-BaP-3,9-hydroquinone indicated that 30% of the deuterium label had migrated to carbon 2, presumably via an NIH shift. Therefore, it appears that 6-nitro-BaP-2,3-oxide is a common intermediate for these two metabolites. 

DNA breaks in human MCF-7 cells [65], Damaging effect of menadione was probably mediated by hydroxyl radicals as it was demonstrated by ESR spin-trapping method. The analogs of menadione 2-methylmethoxynaphthoquinone and 2-chloromethylnaphtho-quinone also stimulated DNA damage through the formation of superoxide and other free radicals [66]. Similar effects have been shown for hydroquinone, catechol, benzoquinone, and benzenetriol [67,68]. 

A linear hydrogen-bonded structure VIII has also been proposed, but accounts less satisfactorily for the color. The surprising thing about these complexes is the lack of any measurable exchange of the hydroxyl hydrogen atoms from the hydroquinone moiety to the quinone moiety... 

An interesting observation reported in Table XLIX is the increase in the hydroquinone/catechol ratio from 1.44 to 1.99 when the dielectric constant of the medium is decreased from 58.9 to 39.2 by addition of methanol to water. A similar increase in the hydroquinone/catechol ratios was also observed in phenol hydroxylation catalyzed by TS-1 (266) in dioxane-water and tert-butyl alcohol-water mixtures. The para/ortho ratio increased nearly 10-fold when 10% dioxane was added to water. Similarly, the para/ortho ratio more than doubled (1.3-3.0) when 10% tert-butyl alcohol was added to water. An opposite trend, namely, a decrease in the para/ortho ratio from 1.4 to 0.6, was observed when 10% formamide (s = 108) was added to water. Because of geometric constraints in the MFI pores, catechol is expected to be formed more easily on the external surface of TS-1 crystallites than in the pores (91). Hydroquinone, less spatially demanding, can form in the TS-1 channels. A greater coverage of the hydrophobic... 

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