Quinone, electrochemical reduction

It is known that the reduction potentials of quinones are related to the aromatic stabilization of the parent conjugated systems. In an attempt to relate the annulenediones 171,177, and 178 to the tetradehydro[18]annuIene system Breslow and coworkers63 have studied their electrochemical reduction by cyclic voltammetry. These diones can easily be reduced to the corresponding dianions, e.g. 171 - 179. These... 

Smertenko et al. (2000) have proposed a special index for this purpose based on differences in redox potentials between a quinone and oxygen. This index takes into account the peak merging for electrochemical reductions of the quinone and oxygen and, according to first estimations, works well. 

Quinone Imines. - These display similar chemical properties to quinones, differing slightly due to the difference in electronegativity between the -C = O and -C=NH functions. Alberti et al. have examined the redox properties of the anti-cancer agent 5//-pyridophenoxazin-5-one. 18 Electrochemical reduction under anaerobic conditions gave the spectrum of the radical anion 02 was trapped in the presence of 02. 

Fig. 17.3. Reactions catalyzed by PPO (17.5) Hydroxylation of monophenol to o-diphenol and (17.6) Dehydrogenation of o-diphenol to o-quinone. Reaction (17.7) is the electrochemical reduction of o-quinone to o-diphenol.

A simple and effective chemical method was developed for quantitatively reducing quinones, based on their reaction with metallic zinc and zinc ions [248]. Comparison of this method with conventional electrochemical reduction [249-252] revealed the chemical method to be considerably superior. A reduction reaction of vitamin Kj and other quinones in the presence of Zn° and Zn2+ eliminates the need to apply large negative potentials and may also be performed in the absence of any applied electrochemical potential. Some quinones used, such as UQ-10, menadione, and vitamin K, of the menaquinone series (MKs 4-10) could all be reduced to their corresponding hydroquinones in these conditions. 

Recently, some tetrakisdehydro[18]annulenediones (201-204) have been synthesized --The electrochemical reduction of 201, 203 and 204 clearly indicates that these annulenediones are indeed quinones derived from the aromatic tetra-kisdehydro[18]annulene . 

Proton-coupled electron transfer is a prominent theme in biological redox systems. There are three basic mechanisms for these processes (Figure 18). In the first mechanism (path A), electron transfer occurs prior to proton transfer. This mechanism is commonly observed for the electrochemical reduction and oxidation of quinones and flavins in protic media [52], In this interfacial environment, proton transfer is manifested as an ECE (E represents an electron transfer at the electrode surface and C represents a homogeneous chemical reaction) two-electron reduction of these systems to their fully reduced states (Figure 19). As electron transfer occurs prior to the proton transfer event, proton transfer does not affect either the redox potential or the electron transfer rate to or from the cofactor. 

Mitomycin C and certain mitosene derivatives were reduced catalytically or by electrochemical reduction (86JA4158 87JA1833 87T255). These reductions do not affect the quinone part. 

C-NMR spectra of several isoquinoline-5,8- and -7,8-diones were recorded (85CPB823). A comparative study of electrochemical reduction of isoquinoline-5,8-dione and other heterocyclic quinones revealed that all compounds show two well-defined reduction peaks and that these reductions are reversible (72MI2). 

This method was applied to assemble integrated electrically-contacted NAD(P)-dcpcndcnt enzyme electrodes. The direct electrochemical reduction of NAD(l ) cofactors or the electrochemical oxidation of NAD(P)H cofactors are kineticaUy unfavored. Different diffusional redox mediators such as quinones, phenazine, phenoxazine, ferrocene or Os-complexes were employed as electrocatalysts for the oxidation of NAD(P)H cofactors An effective electrocatalyst for the oxidation of the NAD(P)H is pyrroloquinoline quinone, PQQ, (7), and its immobilization on electrode surfaces led to efficient electrocatalytic interfaces (particularly in the presence of Ca ions) for the oxidation of the NAD(P)H cofactors. This observation led to the organization of integrated electrically contacted enzyme-electrodes as depicted in Fig. 3-20 for the organization of a lactate dehydrogenase electrode. 

The first mechanism is often found in the electrochemical reduction and oxidation of quinones and flavins in aprotic media.Using cyclic voltammetry (CV) and simultaneous electrochemistry and electron paramagnetic resonance (SEEPR),... 

Alternatively, one may polarize the platinum disk to the quinone-hydroquinone equilibrium potential with the help of a current supplied by an auxiliary circuit. Then one may determine the required current 7, and the rate of consumption of quinone or the rate of formation of hydroquinone. At the quinone-hydroquinone equilibrium potential the electrochemical reduction of hydroquinone vanishes. Consequently, a finite rate of the formation of hydroquinone at the quinone-hydroquinone equilibrium potential equals the partial rate due to the nonelectrochemical mechanism according to Eqs. (VIII.lla)-(VIII.11c). 

Marcus and Hawley have also studied the electrochemical reduction of a-tocopherylquinone and 2,3j5-trimethyl-6-(3 -methyl-3 -hydroxy-butyl)quinone (2,3>5-TMHQ, 15) in nonaqueous solvents. The electrochemical... 

The electrochemical reduction of (XXIX) [101], a 2,ll-bisdehydro[18]-annulene-1,10-dione [102], and of 2,4,11,13-tetrakisdehydro[18]-annulenc-1,6-diones and 2,4,11,13-tetrakisdehydro[18]annulene-l,10--diones [103,104] is consistent with the reduction behaviour of quinones. They are more easily reduced than is benzoquinone, possibly because there is less electrostatic repulsion between the resultant negatively charged oxygen atoms in the products, since they are further apart than in a six-membered ring. The [16]dione (XXX) is less easily reduced than the [18]diones this could either reflect that reduction in this case leads to a 4u TT-electron system rather than a (4n+2) system, or it may be due to the greater proximity of the oxygen atoms in (XXX) [104]. 

Kim, J., T.D. Chung, and H. Kim (2001). Determination of biologically active acids based on the electrochemical reduction of quinone in acetonitrile -F water mixed solvent. J. Electroanal. Chem. 499, 78-84. 

In an alternative approach to mimic tyrosinase activity a copper(I)-copper(n) redox couple and a hydroquinone-quinone redox couple were incorporated in one complex (scheme 17). The hydroquinone moiety should act as an electron shunt between an external reducing agent, i.e. ascorbic acid, zinc or electrochemical reduction, and the copper ions. Catalytic oxygenation by monooxygenases is usually accompanied by the formation of water, with the aid of an external electron and proton source.35 46 Activation of O2 by dinuclear copper(I) complex 58 results in superoxo- or p-peroxo-dicopper(II) complex 59, which oxygenates an external substrate molecule. Internal electron transfer to quinone dicopper(II) complex 60 is followed by quinone to hydroquinone reduction. The electron transfer system shown here is reminiscent of the quinone based systems found in the primary photochemical step of bacterial photosynthesis, and in (metallo)porph3nin-quinone electron transfer systems.In contrast to expectation, the hydroquinone dinuclear copper(II) complex 60 (L = (2-pyridylethyl)formidoyl, scheme 17), designed to mimic step c in this cycle, is a stable system in which the hydroquinone moiety is not oxidized to a quinone structure 61. 

---