Semiquinones radical anions

The semiquinone radical dissociates to a proton and semiquinone radical anion in the polar solvent (water). 

Self-reactions of semiquinone radical anions proceed via electron transfer reaction [2]. 

Interrupted oxidations of 9,10-dihydroanthracene or 9,10-dihydro-phenanthrene in DMSO (80% )-terf-butyl alcohol (20%) containing potassium ferf-butoxide produced the 9,10-semiquinone radical anions, apparently as a product of oxidation of the monoanion. 

The characterization of the semiquinone radical anion species of PQQ in aprotic solvents was undertaken to provide information about the electrochemistry of coenzyme PQQ and to give valuable insight into the redox function of this coenzyme in living systems <1998JA7271>. The trimethyl ester of PQQ and its 1-methylated derivative were examined in aprotic organic solvents by cyclic voltammetry, electron spin resonance (ESR), and thin-layer UV-Vis techniques. The polar solvent CH3CN was found to effectively solvate the radical anion species at the quinone moiety, where the spin is more localized, whereas the spin is delocalized into the whole molecule in the nonpolar solvent CH2CI2. 

Wurster in 1879 had already prepared crystalline salts containing radical cation 23 (equation 12). Subsequently, radical cations of many different structural types have been found, especially by E. Weitz and S. Hunig, and recently these include a cyclophane structure 24 containing two radical cations (Figure 3). Leonor Michaelis made extensive studies of oxidations in biological systems, " and reported in 1931 the formation of the radical cation species 25, which he designated as a semiquinone. Michaelis also studied the oxidation of quinones, and demonstrated the formation of semiquinone radical anions such as 26 (equation 13). Dimroth established quantitative linear free energy correlations of the effects of oxidants on the rates of formation of these species. ... 

As mentioned above, in an earlier work, Nohl et al. [9] suggested that neutral ubisemi-quinone reduced dioxygen to superoxide (this suggestion was dropped in subsequent studies of these authors). Although the participation of neutral semiquinone in the reduction of dioxygen is impossible, the observation of these authors might be interpreted as the support of a role of ubihydroquinone in mitochondrial superoxide production. If neutral semiquinone is indeed formed in mitochondria via the protonation of semiquinone radical anion (Reaction... 

A new class of heteroaromatic compound was introduced by the synthesis of a diphosphathienoquinone (20). It can be reduced to a semiquinone radical anion and dianion at lower potentials than phosphaalkenes. The ESR spectrum indicates that the two P atoms are not equivalent213. 2,4>6-Tricyano- 1,3,5-triazine undergoes dimerization to yield 4,4, 6,6 -tetracyano-2,2 -bitriazine214. 

The mechanism for the generation of quinone methide 58 by reductive elimination of 55 has been investigated.106 Single-electron reduction by 55 by pulse radiolysis in water gives the semiquinone radical anion 56, whose decay was monitored by transient absorption spectroscopy. This radical anion partitions between disproportionation to 60 and elimination to form the radical 58. Disproportionation dominates at pH 7, but as the pH is lowered to 3 the competing elimination reaction to form the quinone methide radical 58 is observed for X = -OMe and -OPh. It was proposed that the product yields are controlled by the position of the equilibrium for protonation of 56 and that 56 undergoes mainly disproportionation, while the semiquinone radical 57 - undergoes mainly elimination of HX (Scheme 28). The quinone methide 59 is then formed by the one-electron reduction of 58. 

Electron transfer between oxygen and semiquinone radical anions, duro-quinone and nitro-substituted radical anions, and two identical semiquinone radical anions H20 (22 -0)... 

Hydro quinone transforms in the presence of irradiated nitrite to yield ben-zoquinone and hydroxybenzoquinone [78,79]. At the irradiation wavelength adopted in the cited works (365 nm), hydroquinone direct photolysis should be limited and benzoquinone most likely forms upon reaction between hydroquinone and hydroxyl (reactions 44 and 45 hydroquinone absorbs radiation at A, < 320 nm). Hydroxybenzoquinone is likely to be a product of benzoquinone photolysis. No nitration or nitrosation intermediates of hydroquinone were observed in the presence of nitrite under irradiation, differently from the cases of resorcinol and catechol [78,79]. The reaction between hydroquinone and nitrogen dioxide is, however, quite rapid [106,115], as confirmed by the marked inhibition of phenol nitration upon nitrite photolysis by added hydroquinone [62], The point is that the reaction between hydroquinone and NO2 mainly yields benzoquinone [62], Another interesting feature in the case of hydroquinone is the formation of the fairly stable semiquinone radical anion upon reaction between benzoquinone and depro-tonated hydroquinone. The spectrum of the resulting solution shows the typical absorption bands of the semiquinone at 308, 315, 403, and 430 nm [79]. 

While the semiquinone radical anion was strongly polarized, the counter radical cation was never observed. Therefore, it remains a good probability that the primary process may be an abstraction... 

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... 

Formation of the 7t-dimer semiquinone radical anion complexes with Sc " " in the Sc " " promoted ET from Ir(ppy)3 to Q was confirmed by their detection using... 

The same mechanism as Scheme 25 can be applied to the disproportionation of semiquinone radical anion (Q ) by the imidazolate-bridged Cu —Zn complex (155), (Scheme 25). Since the one-electron oxidation potential (Lqx vs SCE) of Q (—0.51 V), which is equal to the one-electron reduction potential of Q (135), is more negative than the one-electron reduction potential ( red vs SCE) of the Cu(II)... 

Coenzyme Q is a quinone derivative with a long isoprenoid tail. The number of five-carbon isoprene units in coenzyme Q depends on the species. The most common form in mammals contains 10 isoprene units (coenzyme Qio) For simplicity, the subscript will be omitted from this abbreviation because all varieties function in an identical manner. Quinones can exist in three oxidation states (Figure 18.10). In the fully oxidized state (Q), coenzyme Q has two keto groups. The addition of one electron and one proton results in the semiquinone form (QH ). The semiquinone form is relatively easily deprotonated to form a semiquinone radical anion (Q ). The addition of a second electron and proton generates ubiquinol (QH2), the fiilly reduced form of coenzyme Q, which holds its protons more tightly. Thus, ybr quinones, electron-transfer reactions are coupled to proton binding and release, a property that is key to transmembrane proton transport. 

The electron transfer reactivity of Ceo has been compared with that of p-benzoquinone which has a slightly more negative one-electron reduction potential ( °red relative to the SCE = -0.50 V) [44] than Ceo (E°red —0.43 V). The rate constants of electron transfer from Cgo and Ceo to electron acceptors such as allyl halides and manganese(III) dodecaphenylporphyrin [45] correlate well with those from semiquinone radical anions and their derivatives. Linear correlations are obtained between logarithms of rate constants and the oxidation potentials of... 

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... 

OH- is not the electron donor, but instead 7-benzoquinone itself acts as the electron donor in the presence of OH. The addition of OH" to p-benzoquinone initiates an electron transfer from the OH adduct of p-benzoquinone to p-benzoquinone leading to the noble disproportionation. This yields ten equivalents of semiquinone radical anion and rhodizonate dianion [176],... 

The simplest substance which can act as a catalyst in the electron transfer reduction of an electron acceptor may be a proton (C = H" "), since the radical anion of an electron acceptor (A ) becomes a much stronger base as compared with the neutral form (A). The substrates first described here are / -benzoquinone derivatives (Q), since the redox and acid-base properties of Q and the reduced forms (Q and as the one-electron and two-electron reduced form, respectively) have well been established and they exhibit important thermodynamic parameters in biological redox systems [75, 76], The variations of the reduction potentials with pH are governed by the acid-base properties of the reduced species. Semiquinone radical anion (Q ) is not only singly protonated but also doubly protonated, as shown in Eqs. 2 and 3 [75, 76]. 

Transient electronic spectra of the 1 1 and 1 2 complexes are also observed in the electron transfer reduction of 2,5-dichloro-p-benzoquinone and 2,5-dimethyl-p-benzoquinone [132]. Although the formation constant K for the 1 1 complex is too large to be determined, the formation constant Kj for the 1 2 complex can be determined as 4.5 M and the Kj value decreases with a decrease in the electron-donating ability of X-substituted semiquinone radical anion (X = 2,5-Mc2 > H > 2,5-Cl2) [132]. Thus, Mg + acts as a Lewis acid which can bind with the radical anion base, although the radical anion-Mg + complexes are unstable owing to the facile disproportionation [132]. The formation of such complexes is also confirmed... 

Figure 6. Transient absorption spectra of semiquinone radical anion formed in electron transfer reduction of p-benzoquinone (Q 2.4 x 10 M) by [Fe(Me5C5)2] (2.4 x 10 M) in the presence of Mg(C104)2 [1.0 X 10-2 jyi 2.0 X 10- M 3] and by [Fe(MeC5H4)2l (2.4 x 10- M) in the presence of Mg(C104)2 [1.6 M ( )] in deaerated MeCN at 198 K [132]. The solid line spectrum without symbols shows the absorption spectrum of Q in the absence of Mg(C104)2, prepared by the reaction of Q (2.4 x lO- M) with Me4N OH (2.4 x lO- M) in deaerated MeCN at 298 K [132].

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