Quinones and Quinoid Compounds

Substitution 1 if R/R = t-C 4 6 alkyl/ Free radicals formed more stable and 

Molecular Design of Chemicals of Low Carcinogenic and Mutagenic Potential 

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Quinones and quinoid compounds (e.g., 1,4-quinone, 1,2-quinone, 1,4-quinonedii-mine, 1,4-quinonemethide) (Figure 14.24) are electrophiles capable of reacting with... 

L Hemra, K Lundquist. Acidolytic formation of methanol from quinones and quinoid compounds related to lignin. Acta Chem Scand 27 365-366, 1973. 

NAD-dependent glucose dehydrogenase, NAD-GDH, generally has good substrate specificity, but its cofactor is not bound and must be included as part of the strip reagent matrix as an additional component. Also, the number of effective mediators for NAD-GDH-based systems is more limited due the specific requirements for rapid reaction of the mediator with NADH (e.g., quinones and quinoid compounds). 

The chemistry of quinone dyes has been discussed in a series of books entitled The Chemistry of Synthetic Dyes, edited by Venkataraman.1 The general chemistry of quinoid compounds has been discussed by Patai.2 There have been many books that cover quinoid compounds as dyes and pigments but very few discuss the chemistry of the corresponding leuco dyes. Traditional vat dyes are applied to cellulosic fiber in the leuco form. The chemistry of the leuco form of vat dyes is rather simple. Some leuco quinones are quite stable in the solid state and can be stored for a year. Other leuco dyes are unstable in solution and gradually undergo aerial oxidation. 

Some quinones, having the ability to form intra- and/or intermolecular hydrogen bonds, exhibit high molecular hyperpolarizability and are third-order nonlinear optical (NLO) materials. Compound 39 has a %(3) of 5 x 10 11 esu at 1.9 pm, and is a third-order NLO material.23 The optoelectric properties of quinoid compounds correlate with their structures in crystals or on thin films.23... 

To understand the overall biological activity of quinoid compounds, it is necessary to study the chemical properties of these substances. Inasmuch as the cellular damage that is induced by quinones resembles that seen after radiolysis, the most prominent reactions involving quinones are probably DNA damage and generation of oxygen free radicals. 

N. P. Gritsan and L. S. Khmenko, Photochromism of quinoid compounds properties of photoinduced ana-quinones, J. Photochem. Photobiol. A. Chem. 70, 103-117 (1993). 

Quinoid Structure of Dyes.— This is known as the quinone or quinoid structure and, according to theories regarding the relation between constitution and color in compounds possessing properties of dyestuffs, it is the presence of a group with this structure which endows the dyes of this and related classes with color. 

The anodic oxidation of phenylenediamines parallels that of aminophenols (see Sec. III.A.l) and has been reviewed by Adams [108]. If unsubstituted at the nitrogens, the two-electron oxidation leads to the quinone dimine. This compound either undergoes hydrolysis to the quinone inline and benzoquinone, or a 1,4-addition of a nucleophile, for example, the parent phenylenediamine itself, to the quinoidal systems occurs. Further oxidation of the products may take place. In acetonitrile, the one-electron oxidation to the cation radical predominates [109]. Under these conditions,/7-phenylenediamine also leads to 1,4-coupling products [110,111]. A-Substituted phenylenediamines are forming more stable cation radicals. For example, tetrakis(/7-bromophenyl)/7-phenylenediamine ( °= 0.91V vs. NHE) and tetrakis(2,4-dibromophenyl)-/7-phenylenediamine E° = 0.94 V vs. NHE) in acetonitrile even show reversible behavior for the second oxidation step to the dication [78]. 

R Foster and MI Foreman. Quinone Complexes. In S Patai, ed. The Chemistry of the Quinoid Compounds, I. London John Wiley and Sons Ltd., 1974, pp. 257-333. 

Quinoid compounds are excellent acceptors of electrons and form electron donor-acceptor (EDA) complexes as a consequence of low-lying unoccupied electronic energy levels205. The EDA complexes may be easily formed in interactions with phenolic or amine components of a stabilizing mixture, with other additives which have reactive H atoms, with RO 2 radicals, or with some metallic impurities in polymers via rr-orbital interactions. Quinones efficiently participate in oxidation of polymers by virtue of these processes. 

With respect to the presence of carbonyl group and the system of conjugated double bonds, participation of quinoid compounds in light-induced reactions, and, consequently, their active role in weathering of polymers have to be considered. Great attention was paid to the light-induced reactions of quinones generally223 ... 

Brunmark and Cadenas (27A15) reviewed the major mechanisms that are involved in quinone-induced cytotoxicity in 1989. The redox chemistry of quinoid compounds was surveyed in terms of (1) reactions involving only electron transfers, such as those accomplished during the enzymatic reduction of quinones and nonenzymatic interaction with redox couples generating semiquinones, and (2) nucleophilic addition reactions. In their explanation of the mechanisms involved, quinone is reduced to the hydroquinone or semiqui-none radical by cellular reductase. The semiquinone radical then undergoes rapid autooxidation with the generation of the parent quinone and concomitant formation of superoxide. The hydroquinone reacts rapidly with superoxide to form H2O2 and the semiquinone. 

The quinone structure is incorporated in a wide variety of naturally occurring compounds called quinoids. The most common quinoids are the 1,4-quinones (p-quinones, 124) and the 1,2-quinones (o-quinones, 125).Quinoids can be prepared by many methods, including oxidation of non-quinoid precursors, cycli-zation methods, condensation methods, and annulation methods. This section will focus only on oxidation reactions, which constitute the only completely general methodology. ... 

One example is the formation by phenol oxidases of quinoid compounds from phloridzin in Malus sp. after microbial infections. The quinones built are able to bind to proteins and may inactivate in this way exoenzymes of the invading microorganisms. A similar function may be attributed to hydrocyanic acid and mustard oils set free by degrading enzymes from cyanogenic glycosides and glucosinolates after the infection of several other plant species. 

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