Disproportionation, semiquinone

Hence, the HMO method is inherently incapable of giving any picture for disproportionation equilibria, the reason being the neglect of electronic repulsion. In the SCF treatment, it is convenient to assume that all three systems in the redox equilibrium are built up from the same MO s, the only difference being the number of n electrons. Adopting this convention and denoting the oxidized form, radical, and reduced form as Ox, Sem (semiquinone), and Red, we can write... 

Nevertheless, the further mechanistic steps leading to indole dimerization is not defined and a computational investigation could suggest feasible reaction pathways, providing important anticipation about IR and UV absorption spectra, which could be very useful for the assignment of the intermediates involved. It has been experimentally proposed that the semiquinone 1-SQ may decay via disproportionation to... 

This dependence is shown in Fig. 3.10. The usual dependence for n = 2 is obtained for complete disproportionation, that is K 1. With decreasing K> the E-f3 curve changes and at K = assumes the usual shape for n — 1. For small K values three inflection points appear on the curve. This situation often occurs for organic quinones. Form B is then termed the semiquinone. 

Semiquinone radicals disproportionate with the formation of hydroquinone and quinone [2],... 

Disproportionation of sterically nonhindered semiquinone radicals occurs with diffusion rate constants. On the other hand, sterically hindered semiquinone radicals react by several orders of magnitude more slowly (see Table 15.11). 

Rate Constants of Disproportionation of Semiquinone Radicals at 298 K—continued... 

In Table 3 is a compilation of some of the rate constants which have been determined for various flavin semiquinone reactions. Note that whereas many of these reactions are quite rapid (at or near diffusion control), others are relatively slow. Of particular interest (see below) is the reaction of Oj with FH-, which is too slow to measure (due to the competing disproportionation). Note also that side chain and electrostatic repulsion effects can be seen in some of these reactions. 

Exercise 26-18 Acidification of a solution containing semiquinone radicals such as 1 tends to cause the radicals to disproportionate to the arenediol and arenedione. Why should acid cause changes in the relative stabilities of the semiquinones and the corresponding diol-dione pairs ... 

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. 

Figure 7. Structure and decay of 5-deazaflavin radicals. The 5-protonated tautomer, which would be analogous to the biologically essential blue flavo-semiquinone, is not formed, and disproportionation, which is characteristic for the flavin system, does not happen.

The lack of reactivity of the semiquinone per se with either thioredoxin or NADPH shows that it cannot be involved in catalysis. The rapid production of semiquinone by irradiation of partially reduced enzyme is a light-activated disproportionation since it is totally dependent upon the presence of some oxidized enzyme. Enzyme fully reduced by dithionite forms no semiquinone, while enzyme partially reduced by dithionite rapidly forms semiquinone upon irradiation. Furthermore, the light-activated disproportionation of enzyme first reduced with NADPH results in the reduction of NADP. Thus, FAD catalyzes the disproportionation in keeping with the known photosensitizing nature of free flavins. This reaction is reversed slowly (half-time ca. 150 min 25°) in the dark. The semiquinone is rapidly reoxidized by oxygen to yield an enzyme with unaltered spectral and catalytic properties (58). Similar reactions have been very briefly reported for lipoamide dehydrogenase the dark reverse reaction is comparatively rapid, being complete in 30 min (16S). 

A systematic survey of the influence exerted by the substituents upon the stability of semiquinone forms was undertaken by Tozer and Dallas Tuck, in order to investigate the effect of the substituent upon the biological activity of phenothiazine derivatives, an activity correlated by many authors with in vivo formation of radical species. If the disproportionation reaction shown in Eq. (3) (Section IV, B, 1) were to reach equilibrium, the value K defined in Eq. (5) would express the stability of substituted S+ species. 

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)... 

The one-electron reduced species (semiquinone intermediates) undergo disproportionation reaction [Eq. (17)]. 

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],... 

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... 

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],... 

An investigation of the oxidation of melanin precursors in the presence of azide radicals using pulse radiolysis has been reported (2J9. Thus, dopa and cysteinyldopa yielded first the unstable semiquinones that disproportionated to a quinone-quinol complex. The quinones decayed to more stable products dopaquinone produced dopachrome while cystei-nyldopa-quinones rearranged to benzothiazine isomers. 

Then the semiquinone disproportionates into the previous quinone and the hydroquinone ... 

It is also important to note a posteriori that the semiquinone generated just after the end of the pulse (reactions 3 and 4) did not react with any of the silver species and that it disappears totally through disproportionation. The situation concerning the size distribution of surrounding clusters just after the pulse is indeed quite different from that after tc. At microsecond range, the atoms Agi, complexes Ag2, and dimers Ag2 are predominant. But their redox potentials are all more negative than that of Q-Q . The semiquinone was unable to transfer any electron, and the only possible reaction was disproportionation (5). 

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