Hydroquinone Complexes

The aromatic compound hydroquinone is well known to dermatologists as a drug to treat skin pigmentation, and has been used clinically [84-86]. However, hydroquinone is very easily discoloured to black [87]. This is because hydroquinone [CeH4(OH)2] is easily oxidized to quinone (C6H4O2) as follows  

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An electrocyclic ring closure then leads to a cyclohexadienone complex 7, which upon migration of a proton, yields the chromium tricarbonyl-hydroquinone complex 3. 

The aldehyde/ketone 1 (4.00 mmol), or the ketone 7 (2.00 mmol), or the isothio-cyanate 5 (2.00 mmol) was ball-milled with the solid hydrazine-hydroquinone complex 2 (2.00 mmol) at 25-30 °C for lh (3 h in the case of Id). The yield was quantitative in all cases, as spectroscopically pure mixtures of 3, 6, 8 with 4 were obtained. Hydroquinone 4 was removed by 5 min trituration with 20 mL of water, filtration and three washings with 2 mL of water, each. The residue was dried in a vacuum to obtain the pure products. 4 was recycled from the aqueous washings by evaporation, addition of 1.0 g of 80% hydrazine hydrate in water per 4 mmol of initially reacted 2 and recrystallization to give 520 mg (91%) of pure 2 after filtration, washing with water and drying. 

The importance of non - covalent interactions in biological systems has motivated much of the current interest in supramolecular assemblies [1]. A classical example of a supermolecule has been provided by the rotaxanes [2,3], in which a molecular rotor is threaded by a threaded by a linear axle . Another examples have been previously included as cyclic crown ethers threaded by polymers, paraquat -hydroquinone complexes [4] and cyclodextrin complexes [5,6],... 

Little et al. presented results of their investigation of phenolic particulate-phase free radicals (e.g., benzoquinone-hydroquinone radical) in MSS measured by direct ESR measurements at low temperatures (2379) at the 58th TSRC. Variable temperature ESR studies were performed on the particulate-phase free radicals in MSS. Different particulate-phase free radicals showed temperature dependence. Isolated particulate-phase free radicals began to form at about 450°C. It should be noted that stable benzoquinone-hydroquinone complexes (CAS No. 106-34-3) had been identified in tobacco smoke and Cytrel smoke in 1969 by Green et al. (1378) and by Newell et al. (2767) in 1974. In neither study was any special procedure used to deal with free radicals. 

Photoisomerization of tranj-13-demethylretinal (115) gave a mixture of C-7, C-9, C-11, and C-13 stereoisomers which were separated by h.p.I.c. and identified by 400 MHz n.m.r. The photochemistry of (115) and of 14-methylretinal (116) in polar and non-polar solvents has been examined. Conditions for the photoisomerization of methyl retinoate (117) to various ci5-isomers have been studied.Photoisomerization studies of the all-tran -isomers of 3,4-dehydroretinal (118), 10-fluororetinal (119), and 14-fluororetinal (120) suggest Zwitterion intermediates, e.g. (121), the stability of which is influenced by the fluorine substituent or the extra double bond. Conditions have been optimized for reduction of the retinal-hydroquinone complex to retinol by aluminium isopropylate. The electrodimerization of retinal in the presence of electron donors has been studied. ... 

Kaupp and Schmeyers subjected to ball milling the solid aldehydes and ketones with the hydrazine-hydroquinone complex 83 in a 2 1 ratio to obtain azines 84 at room temperature (Scheme 3.22) [19]. Experiments were carried out in Retsch MM2(X)... 

Kaupp G, Schmeyers J. Solid-state reactivity of the hydrazine-hydroquinone complex. J Phys Org Ghem 2000 13 388-94. 

These studies differ in that, whereas Wells and co-workers have spectrally characterised an intermediate and suggested from the data that the substitution rate for Mn" is 5 x 10 1 mol s" (c/. the recent study by Diebler on the formation of fluoride species), since it should lie between that for the chromium(ii) species and the oxidation rate for the Mn" -hydroquinone complex, Davies and Kustin postulate in the acid range 0-6—3-60 mol 1 an inner-sphere mechanism with hydrogen atom transfer as the predominant mode of reaction for MnOH +, with rates not too different from those derived from studies on other systems (Table 1). The reaction is first-order in each reactant and the observed second-order rate constant is invariant with [Mn" ]o, [H2Q], and [Mn"]. A mechanism consistent with intermediate formation may be written... 

Cannon, A. S. Foxman, B. M. Guarrera, D. J. Van Engen, D. Warner, J. C. Noncovalent Derivatives of Hydroquinone Complexes with Trigonal Planar Tris(N,N-dialkyl)trimesamides. Cryst. Growth Des. 2005, 5, 407. 

Figure 4.59 The L value measurements on emollient creams with BCDAC-hydroquinone complex (a) and hydroquinone only (b). Reprinted from [88] with permission from the Japanese Cosmetic Science Society. Copyright 2005...

Table 4.7 Evaluation for hydroquinone only and for the BCDAC-hydroquinone complex...

Finally, the BCDAC-hydroquinone complex mixed with cream was applied to a patient who suffered from chloasma. The complex with cream was applied to the skin once a day for 1 month. Photographs of the patient s face before and after the treatment are compared in Figure 4.60, from which it is clear that the chloasma disappeared. The above results suggest the BCDAC-hydroquinone complex should be a very effective whitening agent. 

These findings were registered at the Japanese Patent Office (Patent No. 3712066). The whitening creams, including the BCDAC-hydroquinone complex, have been commercialized by several companies. 

Electronic properties of hydroquinone-containing ruthenium polypyridyl complexes Ruthenium polypyridyl complexes bound to hydroquinone/ quinone moieties are expected to yield information on the behaviour of hydroquinone-type compounds in biological processes. Furthermore, ruthenium(II)-hydroquinone complexes involving O and N bonds are likely to absorb well into the visible region and therefore have potential as dyes in sensitized solar cells. A recent example in the application of spectroelectrochemistry to the study of hydroquinone-containing ruthenium polypyridyl complexes is the oxidation of [Ru(bipy)2(HL )]+ (H2L0 = 1,4-dihy-droxy-2,3-bis(pyrazol-l-yl)benzene) (Figure 9). 

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