Structural Studies of 1:1 Quinone-Hydroquinone Complexes
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Abstract The structures of the 1:1 quinone-hydroquinone complexes of 2-phenyl and of 2-(4′-chloro) phenylbenzoquinone have been studied by X-ray methods. A superficial study would indicate that the quinhydrones are centrosymmetric and belong to the space group P21/c. However, other evidence indicates that the true crystal structure may belong to either the space group P21 or to Pc, or that the crystal may contain regions that would coresspond to each of these two non-centrosymmetric space groups. Some possible consequences of such a structural arrangement are briefly discussed.Keywords:
Hydroquinone
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V. D. Parker, J. Chem. Soc. D, 1969, 716 DOI: 10.1039/C29690000716
Hydroquinone
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Additions of strong acids to a solution of quinone in dichloromethane lead to protolytic equilibria which have a marked influence on its electrochemical behaviour. In the presence of free protons, the electroactive species is the monoprotonated form of quinone, QH + , and at higher acid concentrations the species QH 2 2+ is formed with the acidity constant p K (QH + /QH 2 2+ ) = 1.2. The overall equilibrium EC reaction scheme necessarily involves also the protonated forms of hydroquinone, at least the species QH 3 + which is, in contrast to the inactive hydroquinone, electroactive on the dropping mercury electrode in the medium of dichloromethane.
Hydroquinone
Dichloromethane
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The quinone/hydroquinone sesquiterpenes of drimane or rearranged drimane skeletons constitute a wide and diverse group of secondary metabolites of mixed biogenesis. These compounds are mainly of marine origin and their interest is not only for the variety of isolated structure but for the interesting biological activities that they present. In this paper a series of quinone/hydroquinone sesquiterpenes of natural origin that have been reported to date is presented. The structures of these compounds are gathered into eight groups with reference to their biological activities and compounds synthesised. Keywords: Sesquiterpenes quinone/hydroquinone, drimane, rearranged drimane
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Abstract The mono(glucosylthio)hydroquinone 2 was prepared by S ‐glycosidation of 2‐mercaptobenzene‐1,4‐diol and by addition of the acetylated 1‐thioglucose 3 to benzo‐1,4‐quinone ( Scheme 1 ). The second, higher yielding procedure was adopted for the preparation of a range of (glucosylthio)hydroquinones. Addition of 3 to 2‐chlorobenzo‐1,4‐quinone, followed by oxidation gave the 1‐thioglucosides 7 and 12 (1.3:1), while addition of HCl to the (glucosylthio)quinone 4 and oxidation gave mainly 12 ( Scheme 1 ). Similarly, the bis(glucosylthio)hydroquinone 33 was obtained from 3 and 4 ( Scheme 4 ), and the (cellobiosylthio)hydroquinone 18 from the thiol 16 and benzo‐1,4‐quinone ( Scheme 2 ). Addition of the 4‐thioglucoside 21 to benzo‐1,4‐quinone (→ 22 ) and to 4 was followed by oxidation to yield the mono(glucosylthio)quinone 23 and the disubstituted quinones 24 and 25 , respectively ( Scheme 3 ). A mixture 24 / 25 was also obtained from the addition of 3 to 23 . The tris(glucosylthio)hydroquinone 36 was obtained by addition/elimination to the dichloroquinone 29 or the dimesylate 31 , which was prepared in a simplified way ( Scheme 4 ). The tetrakis(glucosylthio)hydroquinone 37 was obtained from 3 and chloranil, followed by reduction. The acylated hydroquinones were deprotected (→ 5, 9, 14, 19, 27, 34 , and 38 ), and oxidized to the corresponding quinones ( 6, 10, 15, 20, 28, 35 , and 40 ). The (glucosylthio)quinones 6, 15, 20, 28 , and 35 were tested as time‐dependent inactivators of a retaining β‐1,4‐glucosidase from Agrobacterium faecalis ( Abg ), which has a strong exo‐glucosidase action ( Table 1 ). Similarly, compounds 20, 28 , and 35 were tested with a cellulase from Cellulomonas fimi ( Cex ) which degrades cellulose and cellooligosaccharides by hydrolysis of a cellobiose unit from the nonreducing terminus. The most effective inactivators for Abg were 6, 15 , and 35 , which inactivated this enzyme with similar second‐order rate constants. (Glycosylthio)quinone 28 was the worst inactivator and did not show normal saturation behaviour. Inactivation of Cex by the (glycosylthio)quinones was 3–500 times slower than that of Abg . The three inactivators 20, 28 , and 35 had approximately the same efficacy with Cex , suggesting that they bind to this enzyme in a similar mode. Further, the K i values observed are very similar to K m values measured for aryl cellobiosides, implying that they bind at the active site.
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Chloranil
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Abstract The structures of the 1:1 quinone-hydroquinone complexes of 2-phenyl and of 2-(4′-chloro) phenylbenzoquinone have been studied by X-ray methods. A superficial study would indicate that the quinhydrones are centrosymmetric and belong to the space group P21/c. However, other evidence indicates that the true crystal structure may belong to either the space group P21 or to Pc, or that the crystal may contain regions that would coresspond to each of these two non-centrosymmetric space groups. Some possible consequences of such a structural arrangement are briefly discussed.
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Octafluoro[2.2]paracyclophane (AF4) has been oxidized by treatment with HIO 3 in CF 3 CO 2 H to form the corresponding p -quinone along with a unique triketone. This quinone undergoes reduction to the respective hydroquinone as well as a Diels-Alder reaction with 1,3-cyclohexadiene. Its reduction potential was obtained by cyclic voltammetry and is discussed in the context of other quinones.
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Cyclophane
Oxidation reduction
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Hydroquinone
Catechol
Catechol oxidase
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The kinetics of reduction of ferricytochrome c by hydroquinone have been studied. The reaction does not conform to a simple second-order rate equation and it is demonstrated that the deviations are brought about by the presence of p-quinone, one of the products of the reaction. The accelerating effect of p-quinone is explained tentatively on the basis of an involvement of the semi-quinone. The effects on the reaction of pH, ionic strength, and temperature are reported and used to suggest features of the reaction mechanism.
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Reaction rate
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Abstract The stable carotenoporphyrin hydroquinone (Ib) and the unstable quinone (Ic) are synthesized starting from the ketone (Ia) via subsequent coupling with the carotene and hydroquinone units.
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Triad (sociology)
Methyl Ketone
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Abstract Polypyridylruthenium(II) complexes with redox‐active, structurally isomeric catechol or hydroquinone units were newly prepared to examine the structure‐stability relationships in the redox‐active sites. A marked difference based on the structure of the active site was confirmed with respect to the formation of the oxidized forms, i. e. the quinone complexes: only the para ‐isomer was completely converted into the corresponding quinone form. The conversion of Ru II to Ru III was not observed. Redox‐mediated interconversion between the hydroquinone and para ‐quinone units is also reported.
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Catechol
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