Phenol-quinone tautomerism

Enol imine-enaminone and phenol—quinone tautomerism in (arylazo) naphthols and in analogous Schiff bases were studied by Fabian et al. [92, 93]. In all these molecules there is a favorable N- -H- -O intramolecular hydrogen bond. Depending on the X-H sigma bond (X = N, O), there are two possible tautomers in solution. The solvent effect was calculated on the equilibrium [92], and a combined effect of the solvent and the benzene substituent was studied in [93]. While the FEP/MC simulations provided consistent organic solvent effects in accord with the experimental results [92], the wide spectrum of the solvent-effect calculation methods could predict rather diverse results for several groups of systems in [93]. 

The keto-enol tautomerism of the dihydroxy perylenequinones 33a-d was studied by H, and "C NMR spectroscopy " (equation 13). The most important factors determining the tautomeric equilibrium in these helix-shaped systems are the substituent effects, the strength of intramolecular phenol-quinone hydrogen bonds, the distortion from planarity of the perylenequinone structure and solvation as well as aggregation effects. 

Coelenterazine (A) is oxidized into dehydrocoelenterazine (D) by MnC>2 in a mixed solvent of ethanol and ether (Inoue et al., 1977b). Dehydrocoelenterazine (C26H19O3N3) can be obtained as dark red crystals. It does not have the capability of chemiluminescence. The ultraviolet absorption spectrum (Fig. 5.6) shows its absorption maxima at 425 nm (e 24,400) and 536 nm (g 12,600) in ethanol. An addition of NaOH significantly increases the 536 nm peak at the expense of the 425 nm peak. Dehydrocoelenterazine can take a tautomeric structure of quinone type (not shown), in which the phenolic proton on the 2-substituent is shifted onto the N(7) of the imida-zopyrazinone ring. Dehydrocoelenterazine can be readily reduced to... 

Of the methods for determining lignin in solution based on a specific chemical reaction, that involving nitrosation, the so-called Pearl-Benson method, has found the widest application. In this procedure, reaction of the phenolic units in lignin with acidified sodium nitrite leads to the formation of a nitrosophenol which, upon addition of alkali, is tautomerized to an intensely colored quinone mono-oxime. The absorbance of the latter structure is measured at 430 nm and related to lignin concentration by calibration with a standard lignin. The procedure described below is essentially that developed by Barnes et al. (1963), who modified the original Pearl-Benson method (Pearl and Benson 1940) to improve its sensitivity. 

Nitration takes place easily and again there is a tendency for polysubstitution. Phenols are readily nitrosated by nitrous acid. The resultant nitrosophenols are tautomeric with the corresponding quinone monoximes (Scheme 4.16). 

Very interesting tautomeric properties are inherent in polycyclic systems that contain annulated phenol and quinone rings. The simplest model for these compounds is napht-hazarin 32 which can exist, both in solution and in the solid state, as a fast equilibrium mixture of several tautomers (32a-32c) where forms 32a and 32b (i.e. a degenerate tautomeric pair of identical 1,4-diones) predominate (equation 12). 

The introduction of a second hydroxy group into an orf/zo-position of the phenolic fragment in the Schiff base influences significantly the tautomeric equilibrium. Thus, in the series of iV-(2,3-dihydroxybenzylidene)amine derivatives 47a-e all the compounds are characterized by the presence of a strong intramolecular O—H N bond which determines the formation of a six-membered pseudocycle. Except for compound 47b, all the molecules are associated as dimers with two intermolecular O—H---OH bonds which are included into a ten-membered pseudocycle . In contrast to the Af-(2-hydroxybenzy-lidene)amines for which the phenolic tautomer prevails considerably, in compounds 47 the quinonic form is present in significant amounts and is dominant even for compound 47d. 

The hydroformylation of the orf/zo-prop-2-enylphenol 81 which contains no benzylic hydroxy group gives a mixture of the open-chain aldehyde 82 and the seven-membered cyclic hemiacetal 83 in a 82 83 ratio of approximately 40 60 (equation 34) °. The ben-zofuran epoxide 85 and its valence-isomeric quinone methide 86, both readily obtainable from benzofuran 84, rearrange thermally above —20°C to form the allylic alcohol 87 and the tautomeric phenol 88 (equation 35) . ... 

The mushroom tyrosinase-catalyzed oxidative decarboxylation of 3,4-dihydroxyphenyl mandelic acid (111, R = H) and a-(3,4-dihydroxyphenyl) lactic acid (111, R = Me) proceeds via the quinone methide intermediate 112. The coupled dienone-phenol rearrangement and keto-enol tautomerism transforms the quinone methide 112 into 1-acyl-3,4-dihydroxyphenyl compounds 113 (equation 48) . ... 

The structures and properties of quinone methides were recently reviewed . Inter alia, the microbial tyrosine phenol lyase (TPL) catalyzes the a, /3-elimination of L-tyrosine to phenol and ammonium pyruvate. It is assumed that the process includes three steps, the second of which is tautomerization of the aromatic moiety which converts it into a good leaving group (equation 49) °. 

Aromatic thiolcarbonates, e.g. benzo- or naphtho[d]oxathiol-2-ones, behave like aromatic thiol esters on irradiation in undergoing S-acyl bond cleavage followed by CO elimination. If a phenolic OH group is located in para position to the C-O bond, a tautomeric equilibrium hydroxy-keto-thione / 2-mercapto-p-quinone is established [82]. This mercaptoquinone can be trapped with alkenes... 

Note that the phenoxide radical has several resonance structures including those which contain the free radical inside the benzene ring. This radical may attack phenol to form a dimer which quickly tautomerizes to give 4,4 -di-hydroxybiphenyl, which in turn will repeat the previous sequence to form a more complex quinoid structure or it may react with the peroxide catalyst to form a two ringed quinone. Summarizing ... 

Nitrovinyl-substituted quinone derivatives 17 react regioselectively with indoles 16 to form the bis-phenol products 19 upon tautomerization of the cycloadducts 18, where the most electron-deficient part of the diene (C-3) forms a bond with the most electron-rich (C-3) carbon atom of the indole (Scheme 6). In some cases, the oxidation products of the bis-phenols (i.e., quinones 20) were also isolated as minor products [19]. 

Besides their function as efficient H-donors, phenols are able to participate at other reaaions which depends on the substitution pattern phenols with 2-, 4-, and 6-tert-alkyl substitution where no tautomeric benzyl radical can be formed can react stoichiometrically with peroxy radicals. Phenols with methylene or methyl substitution at least in 2-, 4-, or 6-position are able to form corresponding quinone methides. Subsequent inter- and intramolecular recombinations often lead to generally irreversibly formed C-C coupling produas (5). 

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