Quinol formation

Under different reaction conditions, phenols can be oxidized to p-quinones (equations 272600-602 and 273603), but in the case of phenol itself, insufficient selectivity has prevented, as yet, the commercial application of this potentially important synthesis of p-benzoquinone and hydroquin-one. The selectivity of p-benzoquinone, or p-quinol formation can be increased at the expense of oxidative coupling products by using a large excess of the copper reagent [Cu4Cl402(MeCN)3 or CuCl + 02 in MeCN] with respect to the phenolic substrate.604 The suggested mechanism involves the oxidation of the phenoxide radical (189) by a copper(II)-hydroxo species to p-quinol (190) which can rearrange (for R2 = H) to hydroquinone (191 Scheme 14), which is readily oxidizable to p-quinone.6... 

H- -BINDING, SEMIQUINONE DISAPPEARANCE AND QUINOL FORMATION IN REACTION CENTERS OF RB. SPHAEROIDES... 

The absorption of light in the reaction center (RC) of photosynthetic bacteria induces electron transfer from the special bacteriochlorophyll pair (P) through a series of one-electron acceptors (bacteriopheophytin, and a primary quinone, Q ) to a two-electron acceptor quinone, Qg [1], In RCs from sphaeroides, both and Qg are ubiquinone-10. It is generally believed that the doubly reduced secondary quinone (hydroquinone dianion) will form quinol (hydroquinone) by taking up two protons before being released from the RC and replaced by another quinone from the quinone-pool. The rate of quinol formation can be limi ted by either of these processes the second electron transfer from Qb to Q/vQb the... 

The novelty of these measurements is the spectral and kinetic resolution of quinol formation. Although many species can contribute to the observed trace in the UV region, they can be spectrally and kinetically separated from that of the quinol signal (Fig.1). These include the light-induced P Qa /PQa spectrum ferrocene/ferrocinium, the exogeneous donor to P" " used, which has no ox-red difference bands... 

Kinetic Correlation Between H+-Binding Semiquinone Disappearance and Quinol Formation in reaction centers of Rb. sphaeroides 165... 

After the first flash, reduction of Qa and Qb to the semiquinone anion state induces pK changes in several ionizable amino acids in the vicinity, resulting in a non-stoichiometric proton binding. After the second flash, further proton uptake satisfies the stoichiometry of quinol formation at the Qb site. Because of the very high pKs of ubiquinol (> 12), two protons are expected to be taken up per QH2 produced and released. 

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

In equation 7, ttimer radical (4) is produced when (3) dissociates. Whenever (4) couples with the other product of equation 7, ie, the 2,6-dimethylphenoxy radical, the tetramer is produced as described. These redistribution reactions of oligomers that proceed by ketal formation and subsequent dissociation ultimately generate terminal quinol ethers which enolize to the more stable terminal phenol (eq. 8). 

Wang, J. Pettus, L. H. Pettus, T. R. R. Cycloadditions of o-quinone dimethides with p-quinol derivatives regiocontrolled formation of anthracyclic ring systems. Tetrahedron Lett. 2004, 45, 1793-1796. 

The great affinity of quinone for hydrogen leads to the formation of anilinoquinone which can now, in the same way, add a further equivalent of aniline to the half of the molecule not yet involved. The dianilino-quinol is converted into dianilinoquinone in the same way as the first reaction product was changed. (Write the equation.)... 

At this point the oxidation stage of quinonediimine has been fully reached its (very unstable) salts have scarcely any colour. The production of colour only takes place when quinonoid and benzenoid systems are present together. The molecular union of the two substances at different stages of oxidation produces the intense absorption which is a prerequisite for the formation of a dye (Willstatter and Piccard). This union need not take place in the proportion 1 1, which obtains in the present case. The relations between quinhydrone and quinone-quinol are quite similar (p. 314). 

Other methods which have proved of value include the formation of substituted methides by the action of silver oxide on phenols (70JOC3666). It is postulated that upon oxidation of the phenol a phenoxy radical is formed which dimerizes to the quinol ether. Disproportionation to the methide and the original phenol follows. 

The oxidation of ds-2-hexene (II) catalyzed by palladium acetate proceeds after an induction period of 2-4 hrs shown in Figure 4. Using 0.00163M palladium acetate total inhibition of reaction was observed with 1.08 X 10 5M quinol. 2,4,6-tri (tert-Butyl) phenol (5.60 X 10 5M) only showed the reaction without total inhibition. The formation of the allylic complex IVb proceeds in a similar way to the reaction of I, and it is the major species in the catalyst solutions. The formation of unsatu-... 

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