Quinoid stable radicals

The particle composition of PM is extremely changeable since it depends on many factors such as chmate variations, emission soiuces, and geographical position. Its main components are transition metals, ions, organic compounds, quinoid stable radicals of carbonaceous material, minerals, reactive gases and biomaterials (Valavanidis et al. 2008). 

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... 

Nowadays, a characteristically modified flavin, 5-deazaflavin (33,142), has gained much interest in flavin enzymology because it can replace natural flavin at many enzyme sites. This derivative, however, is no quinoid system and its radical has no thermodynamic stability. It is, therefore, a mutilated flavin retaining the 2e"-transfer (i.e. nicotinamidelike) properties only (cf. Table 2). The thia-analog, however, is an O2-stable dihydro derivative, which exhibits a stable radical, but no reversibly reducible oxidized form. Hence, it is also a mutilated flavin, but one which retains the e"-transferase activity only. 

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. 

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]. 

Carbazoles are oxidized at controlled potential at a platinum anode in acetonitrile to cation radicals that are stable when the 3-, 6-, and 9-positions are blocked [223]. The radical cation from carbazole dimerizes predominantly at the 3-position to 3,3-dicarbazyl, which is further oxidizable to the quinoidal dication at the potential used. In the presence of pyridine, which may cause a rapid deprotonation of the N-H proton 9,9 -dicarbazyl is the isolated product. 

A simplified scheme of the redox cycle of plastoquinone is shown in Figure 3.8. The quinoid structure and the isoprene side chain make it possible for plastoquinone to take up one electron at a time, producing rather stable semiquinone radicals (which is not shown in the figure), and there are probably at least two plastoquinones involved. 

This research was motivated by Gomberg s discovery of the stable triphenylmethyl radical which led to the question does 2 exist in in the quinoid form or as a biradical 3. Subsequently a number of other tetraphenylquinodimethanes and related compounds were isolated some of which are in Fig. 1... 

Unlike the peroxidation of the hydrocarbon polymers, the oxidation of lignin occurs by a stoichiometric process and not a chain reaction. Because phenols are antioxidants, the phenoxyl radicals formed are too stable to participate in a peroxidation chain reaction and the aromatic system is converted to quinoid compounds and ultimately humus. Both abiotic transition metal ion catalysed peroxidation and biological oxidation ire involved in the conversion of lignin to hiunus. 

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