Quinone, hydrogenation

The rate of quinone hydrogenation depends in a nonlinear way on the concentration of cuprous acetate. Calvin carried out a brief kinetic study in which he noted that the rate of reaction was somewhat more than pro-... 

Fig. 2. Rate of quinone hydrogenation vs. cuprous acetate concentration (5).

Sodium periodate specifically oxidizes guaiacyl groups to quinones. Hydrogen and sodium peroxides in alkali are also somewhat selective in oxidizing lignin and destroy chromophoric groups such as quinones and carbonyl functions while also degrading only aromatic units with free phenolic hydroxyls to dibasic acids. 

The keto-enol tautomerism of the dihydroxy perylenequinones 33a-d was studied by H, and "C NMR spectroscopy " (equation 13). The most important factors determining the tautomeric equilibrium in these helix-shaped systems are the substituent effects, the strength of intramolecular phenol-quinone hydrogen bonds, the distortion from planarity of the perylenequinone structure and solvation as well as aggregation effects. 

Phenols - p-quinones. Hydrogen peroxide in the presence of MeReOj is capable of the transformation. However, the yields vary from 4 to 74% in 20 examples studied. 

Frank et al. [168] reported the formation of / -quinone + hydrogen atom via a transition state with a barrier that was estimated at 90 kcal mof and in which the O—O bridge is in para-position (Figure 6.6). Hadad et al. [32] reported a higher barrier of 127 kcal mof for this para-position transition state adduct. From G2M calculations, Tokmakov et al. [180] report a high barrier for this pathway as well (123 kcal mol ). Hadad et al. evaluated the transition state structure and determined the barrier to be around 138 kcal mof. The O—O bridge was found in the meta-position to form Y(C50 ) + CO. The third isomer is the ortho-position transition state structure. This structure was calculated in this work by different computational methods and by Hadad et al. It was found to be at the lowest energy (around 81 kcal mol ). 

A. Derlinkiewicz, M. Hasik, and M. Kloe, Pd/polyanihne as the catalysts for 2-ethylantra-quinone hydrogenation. The effect of palladium dispersion. Catalysis Lett., 64, 41-47 (2000). 

Preceding cyclovoltammetric studies proved that on reduction in aprotic DMF solution (ch+ < 0.1 ppm), its second and reversible half-wave potential is lowered by 0.6 V( ) on addition of the soluble salt Li [B"(C6H5)4] and simultaneously becomes irreversible [13] (Fig. 14 CV). The presumed microscopic reduction pathway in the presence of excess lithium cation, which, owing to its small ionic radius (ru+ = 60 pm), possesses a high effective ionic charge, is supported by independent ESR/ENDOR measurements [13] in THF (Fig. 14 ESR) the solvated radical anion M ", with two equivalent quinone hydrogen atoms, can be detected by its (1 2 1) triplet. In the subsequently formed contact ion-pair radical [M Li ] , the Li countercation docks at the quinone radical anion. Due to reduced symmetry > Cs, a doublet of doublets ESR signal pattern results. 

There is a fair amount of work reported with films at the mercury-air interface. Rice and co-workers [107] used grazing incidence x-ray diffraction to determine that a crystalline stearic acid monolayer induces order in the Hg substrate. Quinone derivatives spread at the mercury-n-hexane interface form crystalline structures governed primarily by hydrogen bonding interactions [108]. 

As well as the cr-complexes discussed above, aromatic molecules combine with such compounds as quinones, polynitro-aromatics and tetra-cyanoethylene to give more loosely bound structures called charge-transfer complexes. Closely related to these, but usually known as Tt-complexes, are the associations formed by aromatic compounds and halogens, hydrogen halides, silver ions and other electrophiles. 

In addition to CuCfi, some other compounds such as Cu(OAc)2, Cu(N03)2-FeCl.i, dichromate, HNO3, potassium peroxodisulfate, and Mn02 are used as oxidants of Pd(0). Also heteropoly acid salts comtaining P, Mo, V, Si, and Ge are used with PdS04 as the redox system[2]. Organic oxidants such as benzo-quinone (BQ), hydrogen peroxide and some organic peroxides are used for oxidation. Alkyl nitrites are unique oxidants which are used in some industrial... 

Weak to moderate chemiluminescence has been reported from a large number of other Hquid-phase oxidation reactions (1,128,136). The Hst includes reactions of carbenes with oxygen (137), phenanthrene quinone with oxygen in alkaline ethanol (138), coumarin derivatives with hydrogen peroxide in acetic acid (139), nitriles with alkaline hydrogen peroxide (140), and reactions that produce electron-accepting radicals such as HO in the presence of carbonate ions (141). In the latter, exemplified by the reaction of h on(II) with H2O2 and KHCO, the carbonate radical anion is probably a key intermediate and may account for many observations of weak chemiluminescence in oxidation reactions. 

Rate studies show that base-cataly2ed reactions are second order and depend on the phenolate and methylene glycol concentrations. The most likely path involves a nucleophilic displacement by the phenoxide on the methylene glycol (1), with the hydroxyl as the leaving group. In alkaline media, the methylolated quinone intermediate is readily converted to the phenoxide by hydrogen-ion abstraction (21). 

An example of the Michael chemistry, typical of all quinones bearing a replaceable hydrogen, is the preparation of a sulfone (6) (in 55% yield), which was ultimately converted to a polystyrene redox polymer (11). 

Dehydrogenation. The oldest and stiH important synthetic use of quinones is in the removal of hydrogen, especially for aromati2ation. This... 

Photochemical Reactions. Increased knowledge of the centraUty of quinone chemistry in photosynthesis has stimulated renewed interest in their photochemical behavior. Synthetically interesting work has centered on the 1,4-quinones and the two reaction types most frequentiy observed, ie [2 A 2] cycloaddition and hydrogen abstraction. Excellent reviews of these reactions, along with mechanistic discussion, are available (34,35). 

Hydrogen bromide adds to acetylene to form vinyl bromide or ethyHdene bromide, depending on stoichiometry. The acid cleaves acycHc and cycHc ethers. It adds to the cyclopropane group by ring-opening. Additions to quinones afford bromohydroquinones. Hydrobromic acid and aldehydes can be used to introduce bromoalkyl groups into various molecules. For example, reaction with formaldehyde and an alcohol produces a bromomethyl ether. Bromomethylation of aromatic nuclei can be carried out with formaldehyde and hydrobromic acid (6). 

Tetracyanobenzoquinone [4032-03-5] 3,6-dioxo-l,4-cyclohexadiene-l,2,4,5-tetracarbonitrile, is a remarkably strong oxidizing agent for a quinone it abstracts hydrogen from tetralin or ethanol even at room temperature (50). It is a stronger TT-acid than TCNE because it forms more deeply colored TT-complexes with aromatic hydrocarbons. 

Silylated cyanohydrins have also been prepared via silylation of cyanohydrins themselves and by the addition of hydrogen cyanide to silyl enol ethers. Silylated cyanohydrins have proved to be quite useful in a variety of synthetic transformations, including the regiospecific protection of p-quinones, as intermediates in an efficient synthesis of a-aminomethyl alcohols, and for the preparation of ketone cyanohydrins themselves.The silylated cyanohydrins of heteroaromatic aldehydes have found extensive use as... 

Other mild oxidising agents which abstract the terminal hydrogen atoms and thus facilitate disulphide formation may be used as vulcanising agents. They include benzoyl peroxide, p-nitrosobenzene and p-quinone dioxime. 

Since the Claus process by itself removes only about 90% of the hydrogen sulfide in the gas stream, the Beaven, SCOT, or Wellman-Lord processes are often used to further recover sulfur. In the Beaven process, the hydrogen sulfide in the relatively low concentration gas stream from the Claus process can be almost completely removed by absorption in a quinone solution. 

The dissolved hydrogen sulfide is oxidized to form a mixture of elemental sulfur and hydro-quinone. The solution is injected with air or oxygen to oxidize the hydro-quinone back to quinone. The solution is then filtered or centrifuged to remove the sulfur and the quinone is then reused. 

---