Quinone reaction

Benzo[b]thiophene-2 -indole indigo, 4, 910 Benzo[b]thiophene-2,3-quinone reactions, 4, 824... 

With trisubstituted benzoquinones and use of the cationic oxazaborolidinium catalyst B, 2-[tra-(isopropyl)silyloxy]-l,3-butadiene reacts at the monosubstituted quinone double bond. The reactions exhibit high regioselectivity and more than 95% e.e. With 2-mono- and 2,3-disubstimted quinones, reaction occurs at the unsubstituted double bond. The regiochemistry is directed by coordination to the catalyst at the more basic carbonyl oxygen. 

As in the case of phenoxyl and nitroxyl radical reactions, the value of Ee0 for the quinone reaction with phenol (AriOH) is much lower than that for the reaction of Q with R1 H (AEdJ 23 kj mol ). Such a difference is the result of the high triplet repulsion in TS of the type C H and low in the TS of the type H O, as in the reactions of the nitroxyl radical. The very low value of Ee0 for the reaction Q with aromatic amine is due to a high difference in electron affinity of N and atoms in TS of the type H N. The values of rate constants of p-benzoquinone with several inhibitors were calculated by the IPM method. The parameters of the IPM model are collected in Table 18.9. 

The three-dimensional structures of the reaction centers of purple bacteria (Rhodopseudomonas viridis and Rhodobacter sphaeroides), deduced from x-ray crystallography, shed light on how phototransduction takes place in a pheophytin-quinone reaction center. The R. viridis reaction center (Fig. 19-48a) is a large protein complex containing four polypeptide subunits and 13 cofactors two pairs of bacterial chlorophylls, a pair of pheophytins, two quinones, a nonheme iron, and four hemes in the associated c-type cytochrome. 

While this type of reaction has been demonstrated with most of the group VIII noble metals, palladium has the highest activity and, therefore, is normally used. Regenerating agents (co-catalysts) are most frequently CuCl2, FeCl3, and quinone. Reaction details are complex (6). 

The quinones have excellent redox properties and are thus important oxidants in laboratoty and biological symlions. The presence of an extensive array of conjugated systems, especially the a, /(-unsaturated ketone arrangement, allows the quinones to participate in a variety of reactions. Characteristics of quinone reactions include nucleophilic substitution electrophilic, radical, and cycloaddition reactions photochemistry and normal and unusual carbonyl chemistry. 

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.

Other examples of electron polarization transfer in photochemical systems include the reaction of polarized isopropanol radicals with a ground-state quinone (reaction 55) and the polarization transfer from the primary amine radical to biacetyl (97). These examples serve to emphasize how CIDEP and polarization transfer can be used to follow complex photochemical reaction mechanisms. 

QUINONE REACTIONS AND ELECTRON TRANSEER BY ftcj COMPLEX... 

Theis, E. R. (1945). The collagen-quinone reaction. I. Fixation and thermolability as a function of pH values. J. Biol. Chem. 157, 23-33. 

Photochemical activity in the ubiquinone-depleted reaction centers may also be restored by readdition of other types of quinones. Reaction centers reconstituted with quinones other than ubiquinone have a different recombination times between P870 and Q in the dark, and also a different spectral widths for the broad EPR signal. Such differences are reasonable, as these parameters are expected to be sensitive to the nature of the acceptor quinone molecule. 

Fig. 3. Absorbance-change patterns due to quinone reaction in the bacterial reaction center of Rb. sphaeroides excited by four consecutive flashes. Quinone compositions examined were (A) only Qa present (B) only Qa and Qb present (C) Qa, Qb and quinone pool all present. Figure adapted from Clayton (1980) Photosynthesis Physical Mechanisms and Chemical Patterns, pp 215, 216. Cambridge University Press.

The diones and aldehydes whose photoreactions have been reported are summarized in Table 5 where it can be seen that this system has been investigated almost to excess. Most of the quinone reactions were performed in Cairo sunlight using benzene solutions of quinone containing a moderate excess of aldehyde. The adducts crystallized from solution usually in good to excellent yield no attempts were made to analyze the total reaction product. With one exception, all quinone adducts are products of 1,4-addition both cyclic 146> and open-chain 83> tautomeric structures have been proposed for these enediol monoesters. Infrared... 

In a few cases, catechols and hydroquinones have been found to undergo further oxidation to quinones (reactions 1-G and l-I). Such reactions occur by two single-electron steps and can be either enzymatic or nonenzy-matic (i.e., resulting from autoxidation and yielding as by-product the superoxide anion-radical 0, -). The intermediate in this reaction is a semiquinone. Both quinones and semiquinones are reactive, particularly to-... 

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