Hydroquinones chromium oxide

Encouraged by the short synthesis of K vitamins, the chromium-mediated benzannulation was extended to the synthesis of vitamin E 68 [59]. The problem of imperfect regioselectivity of alkyne incorporation - which did not hamper the approach to vitamin K due to the final oxidation to the quinone - was tackled by demethylation of both regioisomeric hydroquinone monomethyl ethers 67 to give the unprotected hydroquinone. Subsequent ring closure yielded a-tocopherol (vitamin E) 68 (Scheme 39). 

The tetrafunctional alcohol pentaerythritol is a popular core in dendrimer chemistry it has been modified into a tetrakis chromium phenylcarbene 85, which underwent a quadruple benzannulation upon reaction with 3-hexyne. The reaction proceeded with only moderate diastereoselectivity in terms of the planes of chirality formed demetalation by mild oxidative work-up gave the tetrakis-hydroquinone derivative 86 (Scheme 33) [76]. 

Chromium(VI) oxide will ako oxidize hydroquinones to quinones, and thiols to disulfides. ... 

Copper(II) and cerium(IV) have been studied as oxidants in acetonitrile. The copper(II)-copper(I) couple has an estimated electrode potential of 0.68 V relative to the silver reference electrode. It has been studied as an oxidant for substances such as iodide, hydroquinone, thiourea, potassium ethyl xanthate, diphenylbenzidine, and ferrocene. Cerium(IV) reactions are catalyzed by acetate ion. Copper(I) is a suitable reductant for chromium(VI), vanadium(V), cerium(IV), and manganese(VII) in the presence of iron(III). For details on many studies of redox reactions in nonaqueous solvents, the reader is referred to the summary by Kratochvil. ... 

Much more work was done on extended quinones. Syntheses of six parent compounds (159-164) has been accomplished by two methods. In the first approach, a Friedel-Crafts reaction between thiophene-3,4-dicarboxylic acid and benzene or thiophenes was used, and in the second one chromium trioxide oxidation of the corresponding acetoxy derivatives was applied (72JOC1712). These systems behave differently in the presence of excess lithium aluminum hydride and aluminum hydride. Compounds 161 and 162 were reduced to hydroquinones, compounds 160 and 163 were deoxygenated... 

Moreno-Castilla and coworkers [139,140] did clarify the relationship between carbon surface chemistry and chromium removal. Table 3 summarizes some of the key results. Upon oxidation of carbon M in nitric acid (sample MO), the surface has become much more hydrophilic and more acidic, and the uptakes increased despite a decrease in total surface area. The enhancement in Cr(III) uptake was attributed to electrostatic attraction between the cations and the negatively charged surface. The enhancement in Cr(VI) uptake (at both levels of salt concentration) was attributed to its partial reduction on the surface of carbon MO (perhaps due to the presence of phenolic or hydroquinone groups), which is favored by the lower pH. The increase in uptake on carbon MO with increasing NaCl concentration is consistent with this explanation, from a straightforward analysis of the Debye-Hvickel and Nemst equations the decrease in uptake on carbon M was attributed to the competition of specifically adsorbed Cl and CrOj- ions on the positively charged surface. 

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... 

The classical method for preparing hydroquinone is based on the oxidation of aniline with manganese dioxide or sodium dichromate in sulfuric acid. The qui-none which is obtained as an intermediate is reduced to hydroquinone with iron filings in dilute hydrochloric acid. This process, which is still used e.g. in India and China, is characterized by the simultaneous production of large amounts of manganese or chromium and iron salts, together with ammonium sulfate. For this reason, oxidative cleavage of p-diisopropylbenzene similarly to phenol synthesis was developed for the production of hydroquinone. 

Preparation by Fries rearrangement of hydroquinone dibenzoate with aluminium chloride [249,250], (good OH yield) [251], at 140° for 1 h (33%) [252], at 190-200° for 1 h 30 min (50%) [253] or at 200° for 20 min [20]. Preparation by oxidation of 5-(benzoyloxy)-2,3-diphenyl-benzofuran with chromium trioxide in boiling acetic acid for 2 h, followed by saponification of the keto ester formed with boiling 8% sodium hydroxide in ethanol [254]. 

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