Hydroquinone dehydrogenation

Hydropyridinyl radicals ESR, 2, 145 Hydroquinoline synthesis, 1, 441 Hydroquinones synthesis, 1, 417 of-Hydroxy acids dehydrogenation by flavins, 1, 253... 

Quinones, which become reduced to the corresponding hydroquinones. Two important quinones often used for aromatizations are chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone) and DDQ (2,3-dichloro-5,6-dicyano-l,4-ben-zoquinone). The latter is more reactive and can be used in cases where the substrate is difficult to dehydrogenate. It is likely that the mechanism involves a transfer of hydride to the quinone oxygen, followed by the transfer of a proton to the phenolate ion °... 

Remarkably, the same Shvo complex can be used for the catalytic transfer dehydrogenation of aromatic amines to give imines (Scheme 7.14) [80]. This reaction produces high yields when carried out for 2-6 h in refluxing toluene with 2 mol.% catalyst. A quinone is used as the hydrogen acceptor, giving the corresponding hydroquinone. 

Washing with chloroform is necessary to dissolve some of the stilbene which crystallizes out with the hydroquinone. In many reactions, washing the hydroquinone with hot benzene is sufficient, as the dehydrogenation products crystallize to a limited extent from dioxane. 

Several norcarene derivatives were shown to undergo dehydrogenation with ring opening to form the more extended % system of cycloheptatriene. Thus, irradiation of chloranil in the presence of tricyclo[4.4.1.016]undeca-3,8-diene (45) gave rise to bicyclo[4.4.1]undeca-l,3,6-8-tetraene (46), while the quinone was reduced to the corresponding hydroquinone [229, 230]. 

It is also noteworthy that, as implied by eqn. (29), adsorption from hydroquinone (HQ) solutions leads to double dehydrogenation of the adsorbate in either final orientation. 

Treatment of l-methyl-4-(l-methyl-2-indolyl)-3-piperideine (140) with maleic acid A-phenylimide in the presence of hydroquinone results in a Diels-Alder cycloaddition which is followed by dehydrogenation.121 The l-methyl-3-indolyl analog of 140 reacted similarly.121... 

Dehydrogenation of 1-hydroxyimidazoles and their 3-oxides with lead dioxide gives A/ -oxides and A(,Ar -dioxides of imidazolyls. These short-lived radicals are also formed when alkali salts of 1-hydroxyimidazole 3-oxides react with halogen in polar organic solvents. Hydroquinone converts the AT,AT -dioxides back into 1-hydroxy 3-oxides. 

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]. 

Dehydrogenation of cyclohexane can be achieved by heating with a metal catalyst such as platinum. Dehydrogenation may also be performed by quinone, which is reduced to hydroquinone in the process. 

Dihydrodiols are seldom observed, as are catechol metabolites produced by their dehydrogenation, catalyzed by dihydrodiol dehydrogenase. The further oxidation of phenols and phenolic metabolites to a catechol or hydro-quinone is also possible, the rate of reaction and the nature of products depending on the ring and on the nature and position of its substituents. In a few cases, catechols and hydroquinones have been found to undergo further oxidation to quinones by two single-electron steps. The intermediate in this reaction is a semiquinone. Both quinones and semiquinones are reactive, in particular toward biomolecules, and have been implicated in many toxitication reactions. For example, the high toxicity of benzene in bone marrow is believed to be due to the oxidation of catechol and hydroquinone catalyzed by myeloperoxidase. 

The absorption spectra of the unstable negative semiquinone radicals like (c) have been obtained in solutions in the presence of oxidizing agents with flow methods 124r-126) and during the flash dehydrogenation of hydroquinones 127). 

Dehydrogenation of 4,4 -dimethoxydibenzyl [1, 219, before references]. Findlay and Turner24 added 103 mg. of DDQ dissolved in 1.5 ml. of dioxane to a solution of 100 mg. of 4,4 -dimethoxydibenzyl and refluxed the mixture in an oil bath at 105° for 18 hrs. The initially deep green solution became pale yellow as the hydroquinone crystallized. After cooling, the solid was removed by filtration and washed with warm benzene (1 ml.) and warm chloroform (6 ml.) and dried to give 95 mg. of pure (4). A solution of the semisolid residue in 5 ml. of ethyl acetate was passed through a column of 2 g. of neutral alumina and the column was eluted with 100 ml. of ethyl... 

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