Radical Quinoidal

Redox reaction two redox processes indole cation radical quinoid structure or dication [699],... 

Occasionally, equilibria between a quinoid and a diradicaloid form of tetraazafulvaleiies of type 77 have been discussed (66AG303 72NKK100 79JOC1241). Based on ESR measurements, only traces of radicals (0.1% at 200°C) could be observed and therefore 77 (Ar = Ph) exists at room temperature predominately in the quinoid structure. Other authors stated that the thermochromism of 77 mainly results from a change in intermolecular interaction, not from biradical formation (84MI1030). 

One of the most important properties of quinoid compounds is the two step redox reaction. Quinoid compounds undergo one electron reduction to so-called semiquinone anion radicals, and further one electron reduction of semiquinone anion radicals gives dianions (Scheme 14). 

Perhaps due to oxidizing quinoid type electronic structure of benzotriazol-2-yl derivatives, some of their properties are completely different from those of isomeric benzotriazol-l-yl derivatives. Thus, anions derived from 2-alkylben-zotriazoles 388 are rapidly converted to appropriate radicals that undergo coupling to form dimers as mixtures of racemic 289 and meso 390 forms <1996LA745>. When the reaction mixture is kept for an extended period of time at —78 °C, (Z)- 391 and (E)- 392 alkenes are formed. When benzophenone is added to the reaction mixture, alcohols 387 are obtained in good yields however, benzaldehyde does not react under these conditions (Scheme 63). 

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. 

The initial removal of electrons (following the oxidation, p-doping process) leads to the formation of a positive charge localised in the polymer chain (radical cation), accompanied by a lattice distortion which is associated with a relaxation of the aromatic structural geometry of the polymer chain towards a quinoid form. This form extends over four pyrrolic rings ... 

Acceptor substituents in nitrophenyl anion-radical frequently form quinoid structures. Donor substituents cannot participate in the formation of such structures. As to electron spectroscopy, it is very sensitive to changes in the electron structure of a chromophore system. The influence of acceptor groups is, therefore, stronger than that of donor groups. If changes in chromophore systems are absent, the method of spectrophotometry remains relatively less informative. 

For instance, poly-p-phenylenes in their doped states manifest high electric conductivity (Shacklette et al. 1980). Banerjee et al. (2007) isolated the hexachloroantimonate of 4" -di(tert-butyl)-p-quaterphenyl cation-radical and studied its x-ray crystal structure. In this cation-radical, 0.8 part of spin density falls to the share of the two central phenyl rings, whereas the two terminal phenyl rings bear only 0.2 part of spin density. Consequently, there is some quinoidal stabilization of the cationic charge or polaron, which is responsible for the high conductivity. As it follows from the theoretical consideration by Bredas et al. (1982), the electronic structure of a lithium-doped quaterphenyl anion-radical also differs in a similar quinoidal distortion. With respect to conformational transition, this means less freedom for rotation of the rings in the ion-radicals of quaterphenyl. This effect was also observed for poly-p-phenylene cation-radical (Sun et al. 2007) and anion-radical of quaterphenyl p-quinone whose C—O bonds were screened by o,o-tert-hutyl groups (Nelsen et al. 2007). 

A possible reductive role for veratryl alcohol oxidase is proposed in Figure 5. Laccases from C. versicolor can produce both polymerization and depolymerization of lignin (29). In phenolic lignin model dimers, laccase can perform the same electron abstraction and subsequent bond cleavage as found for lignin peroxidase (30). The phenolic radical is however likely to polymerize unless the quinoid-type intermediates can be removed, for example by reduction back to the phenol. Veratryl alcohol oxidase, in... 

Some bipyridinium salts are remarkable herbicides. They rapidly desiccate all green plant tissue with which they come into contact, and they are inactivated by adsorption on to clay minerals in the soil. This potent herbicidal activity is found only in quaternary salts, e.g. diquat (254) and paraquat (255), with redox potentials for the first reduction step between -300 and -500 mV (equations 158 and 159) (B-80MI20504). The first reduction step, which is involved in herbicidal activity, involves a completely reversible, pH independent, one-electron transfer to yield the resonance stabilized radicals (256) and (257). The second reduction step, (256 -> 258) and (257 -> 259), is pH dependent and the p-quinoid species formed are good reducing agents that may readily be oxidized to diquatemary salts. 

This interpretation agreed with the chemical behavior of triphenylmethyl , with the free radical as the reactive species present in a low concentration and the dimer as a reservoir for it. However, most chemists at this time preferred to leave out the free radical and instead defend the notion of an unusually reactive dimer, such as for example the quinoid structure 1 or its symmetrical analogue 2 or even hexaphenylethane. 

In the light of the more complete study of ring-halogenated triphenylchloro-methanes in this paper, the free radical hypothesis was back - if it ever was excluded in the previous paper - in the final discussion of the constitution of triphenylmethyl , now with two tautomeric triphenylmethyl radical structures in equilibrium with each other and the Jacobson dimer 1 (Scheme 2). Note that the radical was symbolized by an open valence (a thick line is used here for clarity). The strong results obtained with 3 (Scheme 1) were explained by removal of the quinoid bromine atom from 4 giving a radical 6 which tautomerized to the triphenylmethyl analogue 7. By analogy with the... 

Many enzyme-catalysed redox processes include the transfer of the equivalent of two electrons by one two-electron step or two one-electron steps. The latter is considered as a radical process involving the use of cofactors like flavin, quinoid coenzymes or transition metals. 

A reasonable model has been proposed to accommodate these results (2/y 23). The presence of quinoid functions in lignin would give rise to electron donor-acceptor complexes with existing phenolic groups. These complexes, like quinhydrone, would form stable radical anions (semiquinone anions) on basification, according to the scheme shown below. Both biological and chemical oxidation would create more quinone moieties, which in turn would increase the contribution of Reactions 1 and 2. Alternately, enzymatic (< ) and/or alkaline demethylation 16) would produce... 

Naturally, the product of two-electron reduction of the para derivative can be depicted (Scheme 3-54) as an anionic diborataquinoid system. In contrast with the para-dib-orataquinoid dianion, an anionic meta-quinoid system is impossible. Indeed, the meta-sub-stituted isomer depicted in Scheme 3-54 has been characterized as a spin-unpaired triplet species with boron-centered spins (Rajca et al. 1995). This dianion diradical can be viewed as two stable borane anion radicals linked with a ferromagnetic coupling unit, i.e., 1,3-phenylene see Chapter 1. 

Metodiewa D, Jaiswal AK, Cenas N, Dickancaite E, Segura-Aguilar J. 1999. Quinone may act as a cytotoxic prooxidant after its metabolic activation to semiquinone and quinoidal products. Free Radic Biol Med 26 107-116. 

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