Quinone, reduction

In the benzene and naphthalene series there are few examples of quinone reductions other than that of hydroquinone itself. There are, however, many intermediate reaction sequences in the anthraquinone series that depend on the generation, usually by employing aqueous "hydros" (sodium dithionite) of the so-called leuco compound. The reaction with leuco quinizarin [122308-59-2] is shown because this provides the key route to the important 1,4-diaminoanthtaquinones. 

The bioreductive alkylating agents developed in this laboratory did not afford observable quinone methide species upon quinone reduction and leaving group... 

The results presented above indicate that the previously unknown head-to-tail polymerization is the major reaction product of the iminium methide species. To investigate the generality of this reaction, we next studied a neutral ene-imine species shown in Scheme 7.9.48 As illustrated in this scheme, the generation of this reactive species requires quinone reduction followed by elimination of acetic acid. The ene-imine is structurally related to the methyleneindolenine reactive species that is a metabolic oxidation product of 3-methylindole (Scheme 7.9).57 59... 

We studied the reactions shown in Scheme 7.18 using the global fitting methodology described in Section 7.2.1. The quinone methide species rapidly built up in solution upon quinone reduction and trapped by either water or a proton to afford the final products shown in Scheme 7.18. global fitting provided the rates of quinone methide... 

Of more intrinsic interest are processes involving two electrons, since these constitute the great bulk of organic reductive processes, especially in protonic solvents. The reason for this is that the first electron transfer will generate an unstable radical species, whereas the second can regenerate a stable closed-shell product. One example that we have already encountered is C02 reduction but a case that has been well studied is that of quinone reduction as ... 

Figure 4.2. Monophenolase and diphenolase activities of polyphenoloxidase and stabilization of the formed quinones. (A) Quinone reduction. (B) Descarboxylation. (C) Modification by an exomolecular nucleophile. (D) Modification by an endomolecular nucleophile.

Most quinone reductions go through an intermediate radical or semiquinone stage, usually revealed by a one-electron step in the redox potential.100 The radical formed by the reduction of compound VI is especially stable, probably because of the additional involvement of the benzoyl group.101 The ordinary semiquinones are more stable in basic solution since some of the resonance structures of the neutral radical involve separation of charges. 

Reductions Epoxide hydroplase Azo and nitro reduction Carbonyl reductase Disulfide reduction Sulfoxide reduction Quinone reduction Reductive dehalogenation Microsomes, cytosol Gut microflora Cytosol Cytosol Cytosol Cytosol, microsomes Microsomes... 

Cummings, J., Willmott, N., Hoey, B.M., Marley, E.S., and Smyth, J.F., 1992, The consequences ofdoxomhicin quinone reduction in vivo in tumor tissue. Biochem. Pharmacol. 44 2165-2174... 

Quinone Reduction This is a reversible, one-electron transfer reaction to the semi-quinone radical, followed by a second, reversible electron transfer that results in the formation of hydroquinone, as shown in Fig. 13.2. 

In this scheme veratryl alcohol was viewed to be metabolized by the combined action of oxidative systems (the lignin peroxidase and possibly other active oxygen species) and reductive conversions (aldehyde and quinone reductions). A possible route via veratric acid was discounted because both veratraldehyde and veratric acid were not substrates for the lignin peroxidase under the condition studied. However, both veratraldehyde and veratric acid were rapidly and quantitatively reduced by ligninolytic cultures of P. chrysosporium (21). 

Polarograms for the quinone-hydroquinone couple are shown in Figure 3.28. The electrode mechanism for quinone reduction is... 

Fig. 19. Dependence of (1) hydroquinone oxidation and (2) quinone reduction peak current on the square root of potential scan rate. Solution of 0.01 M hydroquinone (or quinone) +0.5 M H2SO4. Polycrystalline electrode (data of Yu. V. Pleskov and Yu. E. Evstefeeva).

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

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