Semiquinones, radical ions

When catechol was oxidized with Mn04 under aprotic conditions, a semiquinone radical ion intermediate was involved. For autoxidations (i.e., with atmospheric oxygen) a free-radical mechanism is known to operate. 

Dj. Steelink. Our proposals for radical ions in humates are entirely consistent with the proposals for the formation of hydroxy semiquinone radical ions of Eigen. 

Electron transfer between semiquinone radical ions and quinones H20, 5M in 2-propanol ar acetone at pH 7 (22 l... 

The reaction of cobalt(II) salen complexes with -quinones results in the formation of a [cobalt(III)-(salen)-(t7-SQ)] complex, where ( >-SQ) represents the semiquinone radical ion. (Similar reactions are observed with iron(II) and manganese(II)-salen complexes, the reactions with these metal centers being more extensive than those for cobalt(II)). In the case of 3,5-di-rm-butyl-t>-benzoquinone, however, the cobalt(III) ligand radical complex has been isolated, and from hyperfine coupling constant studies which are assumed as diagnostic of the extent of electron transfer, the complex is described best as a low-spin cobalt(III) system with a coordinated semiquinone. 

Neither the catalysis by acids nor by bases is conclusive evidence of a non-radical mechanism however. For example, the semiquinone radical is more stable in basic solution where it exists as a negative ion-radical, and the related nitrogen radicals are more stable in acid solution where they exist as positive ion-radicals. In both cases the ion-radicals have symmetrical resonance not possessed by the neutral radical. 

Poly[(aniline-2-chloroaniline)-4-toluenesulfonic acid salt] was obtained by oxidative copolymerization of aniline with 2-chloroaniline in solutions containing 4-toluenesulfonic acid. The copolymer salt was subjected to heat treatment under nitrogen atmosphere at elevated (about 150°C) temperatures. The heat-treated samples acquired electric conductivity of 2.7 X 10 f2 cm . According to ESR spectra, the heated poly[(aniline-2-chloroaniline)-4-toluenesulfonic salt] exists as the poly(semiquinone imine ion-radical) in which unpaired electrons are localized on or near the nitrogen atoms (Palaniappan 1997). 

The basic forms of phenols (phenolate anions) are easily oxidized to semiquinone radicals through electron transfer. These radicals can then react with another radical to form an adduct through radical coupling or, in the case of o-diphenols, undergo a second oxidation step yielding o-quinones that are electrophiles as well as oxidants. Oxidation reactions are very slow in wine, due to the low proportion of phenolate ions at wine pH values, but take place extremely rapidly when oxidative enzymes are involved (see Section 5.5.2.2). 

The advent of ESR opened several classes of moderately stable radical ions to scrutiny, namely, ketyls, semidiones, semiquinones, and those of aromatic sys-... 

Anderson, R. F. Flavin-oxygen complex formed on the reaction of superoxide ions with flavo-semiquinone radicals. In Flavins and flavoproteins (Massey, V., Williams, C. H. eds.) pp. 278-283, New York, Elsevier North Holland 1982... 

The presence of metal ions in many flavoproteins suggested a direct association of metal ions and flavins. Although oxidized flavins do not readily bind most metal ions, they form red complexes with Ag+ and Cu+ with a loss of a proton from N-3.278 Flavin semiquinone radicals also form strong red complexes with many metals.264 If the complexed metal ion can exist in more than one oxidation state, electron transfer between the flavin and a substrate could take place through the metal atom. However, chelation by flavins in nature has not been observed. Metalloflavoproteins probably function by having the metal centers close enough to the... 

How is the reduced cofactor reoxidized Presumably the copper ion adjacent to the TPQ functions in this process, passing electrons one at a time to the next carrier in a chain. There is no copper in the TTQ-containing subunits. Electrons apparently must jump about 1.6 ran to the copper ion of amicyanin, then another 2.5 nm to the iron ion of the cytochrome c.472 Reoxidation of the aminoquinol formed in Eq. 15-53, step d, yields a Schiff base whose hydrolysis will release ammonia and regenerate the TTQ. Intermediate states with Cu+ and a TTQ semiquinone radical have been observed.4833... 

Intramolecular photoinduced electron transfer reactions of homonaphthoquinones are also made possible by the presence of Mg(C104)2 in MeCN [212]. As shown in Scheme 27, the photoexcitation of 10 in the presence of Mg2 + results in intramolecular electron transfer due to the complexation of Mg2 + with the semiquinone anion moiety, which can accelerate the photoinduced electron transfer and at the same time may retard the back electron transfer [212], No reaction occurred in the absence of Mg2+ or in the dark at ordinary temperature [212], The generated radical ion I undergoes ring... 

The mechanism of the catalytic reaction proved indeed to be very different from that found for [Cu2([22]py4pz)( r-0H)](C104)3 H20. Thus, in the first step of the reaction, a stoichiometric oxidation of catechol by the dicopper(II) complex takes place however, only one electron is transferred in this stoichiometric reaction, resulting in the formation of a semiquinone radical and a mixed-valence Cu"Cu species. Interestingly, the dicopper(II) complex was found to be essentially dinuclear in solution nevertheless, only one of the two copper(II) ions was found to participate in the redox process, whereas the second one played a purely structural... 

If the original semiquinone radical QH is polarized, the semi-quinone radical anion derived from eq. 52 can be expected to retain much of the initial polarization. Thus in the CIDEP studies of the photoreduct ion of quinones in triethylamine solution, the primary photochemical process was thought to involve the possible exciplexes (42) ... 

In the presence of Y(OTf>3 (1.0 x lO M), photoexcitation of the Fc AQ system using a femtosecond laser also results in efficient ET from Fc to AQ within 500 fs (53). However, the transient absorption band is observed at 700 nm in the presence of Y(OTf>3 and is significantly red-shifted as compared with that observed at 600 nm in the absence of Y(OTf>3, as shown in Fig. 15(a) (53). Such a red-shift was reported for the complex formation of semiquinone radical anions with metal ions (54). The decay of absorbance at 420 and 700 nm due to the Fc + —AQ" Y(OTf)3 complex obeys first-order kinetics to afford... 

Ceo is known to undergo reduction to Ceo by addition of a MeO solution. This reduction is, nevertheless, accompanied by the formation of the adduct anions, C6o(OMe) (n = 1, 3, 5, 7) [174]. Electron transfer from MeO to Ceo is thought to result in the formation of Ceo and the corresponding adduct anions [174]. p-Benzoquinone is also known to be reduced to semiquinone radical anion in a reaction with OH in acetonitrile [175, 176]. The hydroxide ion is a much stronger electron donor in aprotic solvents such as acetonitrile than in water, since the solvation energy for OH is less in aprotic solvents than in water [175]. However, no oxidized products of OH were found in the reaction of p-benzoquinone with OH [176]. The only oxidized product detected evolved from p-benzoquinone itself, namely, the rhodiz-onate dianion that is the ten-electron oxidized product of p-benzoquinone [176]. Thus, the one-electron reduction of ten equivalents of / -benzoquinone is accompanied by the ten electron oxidation of one equivalent of / -benzoquinone. In this case... 

Silverman s studies on mechanism based MAO inactivation have provided overwhelming support for the role of electron transfer in the MAO catalyzed dealkylation of amines. It must be mentioned however that spectroscopic attempts for detecting the radical ion intermediates have hitherto been unsuccessful. Yasanobu and coworkers could not find EPR spectral evidence for radical intermediates in MAO-catalyzed oxidation of benzylamine [205]. Miller et al. failed to observe the flavin semiquinone or an amine-flavin adduct in rapid-scan-stopped flow spectroscopy [206]. The only time-dependent absorption change observed in this study was the bleaching of the oxidized flavin. Furthermore, no influence of a magnetic field up to 6500 G was observed on the rate of MAO B reduction. The reaction rates of systems with kinetically significant radical pair intermediates are known to be altered... 

Reactions of /)-benzoquinone and its derivatives with alkoxide ions (RO R = H, Me, Et, i-Pr, PhCH2) in MeCN also result in formation of the corresponding semiquinone radical anions accompanied by the formation of RO-substituted p-benzoquinones, which are the oxidized products of p-benzoquinones [360], Detailed product and kinetic analyses of the reactions indicate that RO-adduct anion of /)-benzoquinone is an actual electron donor and that RO is acting as a very strong base or nucleophile rather than a one-electron reductant in an aprotic solvent, such as MeCN [360], Similarly, the reaction of C o with methoxide anion (MeO ) in benzonitrile (PhCN) results in the disproportionation of Cgo to yield both C(,o and the methoxy adduct [360]. Spectroscopic and kinetic studies also indicate that a methoxy adduct anion of Ceo is a real electron donor and that MeO is acting as a very strong base or nucleophile rather than an electron donor in PhCN [360],... 

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