Hydroquinones silver® oxide

In small-scale syntheses, a wide variety of oxidants have been employed in the preparation of quinones from phenols. Of these reagents, chromic acid, ferric ion, and silver oxide show outstanding usefulness in the oxidation of hydroquinones. Thallium (ITT) triduoroacetate converts 4-halo- or 4-/ f2 -butylphenols to l,4-ben2oquinones in high yield (110). For example, 2-bromo-3-methyl-5-/-butyl-l,4-ben2oquinone [25441-20-3] (107) has been made by this route. 

There are indications that another type of catalysis is present in the reaction between hydroquinone and silver ions in alkaline solution. The increase of rate with increasing hydroquinone concentration is greater than direct proportionality. This situation is similar to that observed in the oxygen oxidation of durohydroquinone (tetramethylhydroquinone) (James and Weissberger, 16) where the quinone formed in the reaction catalyzes subsequent oxidation. A direct check on quinone catalysis of the hydroquinone-silver ion reaction was not made, since quinone is unstable in alkaline solution, particularly in the presence of sulfite which reacts with it. Experiments were made, however, on the reaction between durohydroquinone and silver ion. This reaction shows the same dependence of rate upon the square root of the silver ion concentration as the hydroquinone reaction does. Addition of duroquinone to the reaction mixture produces a definite acceleration, as shown in Table II. 

A dye developer is a compound composed of a silver halide developer fragment linked via an insulating group (Z) to a dye moiety. The compounds are intrinsically soluble in alkali because of acid functions contained in the developer fragment, normally a hydroquin-one. On interaction with exposed silver halide the hydroquinone becomes oxidized to the corresponding quinone. This renders the compound insoluble in alkali (Scheme 7). The reaction can be enhanced by the use of a mobile auxiliary developer such as p-methylphenyl-hydroquinone. In the areas of the emulsion layer where there has been no exposure of silver halide, the dye developer cannot be oxidized. It therefore remains soluble and free to transfer to the mordant layer. Thus, the transfer of dye is inversely proportional to the amount of exposed silver halide and the overall result is a positive of an original scene on the receiver. 

Phenols are more easily oxidized than alcohols, and a large number of inorganic oxidizing agents have been used for this purpose. The phenol oxidations that are of the most use to the organic chemist are those involving derivatives of 1,2-benzenediol (pyrocate-chol) and 1,4-benzenediol (hydroquinone). Oxidation of compounds of this type with silver oxide or with chromic acid yields conjugated dicarbonyl compounds called quinones. 

Complex polyfunctional molecules can often be assembled efficiently by short, spectacular sequences of reactions, an example of which is the preparation of the pentasubstituted benzofuran 1. Thus, addition of l-lithio-l-methoxy-3-(trimethylsilyl)-l,2-hexadiene to 3,4-dimethoxycyclobut-3-ene-l,2-dione gave the expected keto alcohol in 70% yield. This alcohol was heated at reflux temperature in toluene for 4 hours to give a 2,3,5,6-tetrasubstituted hydroquinone in 90% yield. Oxidation of the hydroquinone with silver oxide and potassium carbonate in anhydrous benzene (90%) followed by reaction of the quinone thus obtained with TFA in methylene chloride at 0°C then at room temperature for two days gave 1 in 75% yield. 

A-Benzoquinones.2 Manganese dioxide is often as efficient as the conventional, expensive oxidant silver oxide for conversion of 1,4-hydroquinones to the quinones, although it is ineffective for certain hydroquinones (2,3-dicyano- and 2,5-diformyl-hydroquinone, quinizarin). Activated MnOz is not necessary. 

Dithioacetic acid derivatives add to 1,4-benzo- or 1,4-naphthoquinones to give, after oxidation of the adduct with silver oxide or chloranil, the quinones 217 and 218 (69LA103). Quinones 218 were prepared also from 2,3-dichloro-1,4-naphthoquinone and salts of dithiocarbamic acids (51JA3459) and those of type 219 by oxidation of the corresponding hydroquinones. From reduction potentials and the semiquinone formation constants, it was concluded that their anion radicals are thermodynamically stable (86CC1489). 

Quinone 239 adds hydrogen chloride to give the 6-chloro compound after subsequent oxidation the 7-chloro isomer is obtained as a by-product (870PP249). Reactions with pyridine have been described (58MI3 71JMC1029). With dienes, adducts are formed that can be isomerized with acid into hydroquinones, which can be reoxidized with silver oxide to quinones (67JHC133 73JCS(P 1)2374). 

The absence of silver oxidation and/or reduction peaks is evidence for the electrochemical inactivity of the silver deposited on this carbon (in the form of metallic crystallites). The cyclic voltammogram recorded for the D—Ox carbon (Fig. 50, curve 2) exhibits two anodic peaks (fp., = +0.27 V, p,a = +0.77 V) due to the oxidation of adsorbed silver and surface hydroquinone-like groups, respectively. A single cathodic peak (Ep,) = +0.16 V) is due to the reduction of quinone-like surface groups according to Scheme 19. The large cathodic reduction wave confirms the presence of adsorbed silver cations and their reduction... 

The cyclic voltammograms of all the carbons carrying preadsorbed silver, recorded in dilute nitric acid solution (Fig. 51), exhibit a Ag"/Ag" couple (cathodic wave < +0.4 V and an anodic response in the +0.40-0.60 V potential range), as well as the electroactive quinone/hydroquinone-like surface system ( p, s +0.50 V p.a2 = +0.90 V). The presence of distinctly shaped anodic silver oxidation peaks indicates the partial solution of sorbed (deposited) metal. An almost sixfold higher anodic peak for D—Ox carbon confirms the partially ionic form of the adsorbed silver. 

Mercuric oxide, HgO (yellow modification or the less reactive red modification), resembles silver oxide in its oxidizing properties. This reagent transforms phenols and hydroquinones into quinones [383, 384] and is used especially for the conversion of hydrazones into diazo compounds [355, 386, 387, 388, 389, 390, 391, 392]. Dihydrazones of a-diketones furnish acetylenes [393, 394, 395, 396], A -Aminopiperidines are dehydrogenated to tetrazenes [397] or converted into hydrocarbons [395]. 

Miscellaneous oxidations2 Some aromatic amines (e.g., aniline, p-toluidine, p-chloroaniline) are oxidized by silver(II) oxide to the corresponding azo derivative in moderate yield. Hydroquinone is oxidized by AgO rapidly to p-benzoquinone in quantitative yield. Oxidation of benzil dihydrazone with AgO gives diphenylacetylene in 95% yield. 

Silver oxide is the reagent of choice for oxidation of vitamin K, hydroquinone, because this substance is readily soluble in ether and because the conversion is quantitative evaporation of the filtered solution gives analytically pure K, a yellow oil. Tn this case commercial reagent is satisfactory. 

Oxidation of hydroquinones. o-Benzoquinone, a highly reactive red compound sensitive even to water, eluded all altempis to obtain it by oxidation of catechol until Willstatter and Pfannenstiel found that the reaction cun be effected with silver oxide prepared by precipitation from aqueous solution and washed extensively with several portions of distilled water, then with ncetone, and Anally with absolute ether, A solution of catechol in absolute ether was (rented with anhvdroui sodium... 

Dehydrogenation of pyrocatechol by silver oxide gives 0-benzoquinone.469 For preparation of amino-0-benzoquinones see Homer and Lang.470 Hydro-quinones are also dehydrogenated with success by lead tetraacetate.222 Further, 2,3-dichloro-5,6-dicyano-l,2-benzoquinone is obtained from the hydroquinone in 83% yield when a solution of the latter in 5% ethanolic hydrogen chloride is shaken with a mixture of Pb02 and benzene.471... 

The phenols 1,2- and 1,4-benzenediol (for which the respective common names catechol and hydroquinone are retained by lUPAC) are oxidized to the corresponding diketones, ortho- and parfl-benzoquinone, by a variety of oxidizing agents, such as sodium dichromate or silver oxide. Yields can be variable when the resulting diones are reactive, as in the case of o-benzoquinone, which partly decomposes under the conditions of its formation. 

Another positive-working release by cyclization, illustrated by equation 5, starts with an immobile hydroquinone dye releaser (8), where R = alkyl and X is an immobilizing group. Cyclization and dye release take place in alkaU in areas where silver haUde is not undergoing development. In areas where silver haUde is being developed, the oxidized form of the mobile developing agent oxidizes the hydroquinone to its quinone (9), which does not release the... 

Figure 6.5 Outline of the process of instant3 photography. A water-soluble hydroquinone (HQ-C) linked to a dye (D) is oxidized to an insoluble quinone (Q-C) by metallic silver. The soluble and insoluble parts of the emulsion are separated through contact with an alkaline paste...

Historically, the first supported oxidizing reagent, reported by Fdtizon and Golfier, was silver carbonate on celite (another diatomaceous earth). This was obtained by precipitation of the reagent onto its support. Ag2C03 on celite smoothly oxidizes primary and secondary alcohols, a,(o-diols, hydroquinones and amines. The main practical asset of the reagent is that it avoids the need to filter off finely divided silver salts after reaction. 

Oxidative demethylation. Both silver(II) oxide (4,431-432) and CAN (7,55) have been used for oxidative demethylation of dimethyl ethers of 1,4-hydroquinones to give p-quinones. A direct comparison of the two reagents indicates that yields are higher with CAN. With either reagent, yields are improved when the N-oxide of pyridine-2,6-dicarboxylic acid is added as catalyst. The oxide is prepared by oxidation of pyridine-2,6-dicarboxylic acid with H2O2 and Na2S04. ... 

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