Quinones from aromatic hydrocarbons

The most common triplet state electron acceptors are ketones and quinones, whereas aromatic hydrocarbons, often bearing one or more cyano groups, are the most frequently used singlet state electron acceptors. For the generation of radical cations from a given donor it is important that the exothermidty of the electron transfer reaction can be adjusted to fall within an appropriate range, typically... 

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. 

It is certain that, in the first reduction step in aprotic solvents, an electron is accepted by the LUMO of the organic compound. However, it was fortunate that this conclusion was deduced from studies that either ignored the influence of solvation energies or used the results in different solvents. Recently, Shalev and Evans [55] estimated the values of AG V(Q/Q ) for 22 substituted nitrobenzenes and nine quinones from the half-wave potentials measured by cyclic voltammetry. For quinones and some substituted nitrobenzenes, the values of AG V(Q/Q ) in a given solvent were almost independent of the EA values. Similar results had been observed for other aromatic hydrocarbons in AN (Section 8.3.2) [56]. If AG V(Q/ Q ) does not vary with EA, there should be a linear relation of unit slope between El/2 and EA. Shalev and Evans [55], moreover, obtained a near-linear relation between AG V(Q/Q ) and EA for some other substituted nitrobenzenes. Here again, the Ey2-EA relation should be linear, although the slope deviates from unity.8)... 

It is also possible to exploit quenching of ECL in the detection of various substances. Recently Richter and coworkers have shown that ECL from [(bpy)3Ru]2+, generated following oxidation in the presence of trialkylamines, is quenched by quinones and other aromatic hydrocarbons in nonaqueous solvents [60],... 

In the electroreduction of aromatic hydrocarbons, nitro compounds, and quinones in aptotic solvents, the first step is the transfer of an electron from the electrode to form a radical anion. Once the radical anion is formed, electron repulsion will decrease the facility with which a second electron transfer occurs. But solvation and ion pairing diminish the effect of electron repulsion and tend to shift the reduction potential for the addition of the second electron to more... 

Similar to ketones, quinones have also been known for a long time to undergo PET processes with several donors such as aromatic hydrocarbons, olefins and amines etc. [10b, 11]. It is pertinent here to illustrate one of the synthetically important quinone-olefin [186] reactions which has been utilized for the preparation of Benz (a) anthracene 7,12-dione derivatives (237). In the present example, the excited state of naphthaquinone (234) reacts with ethene (235) to give 236, provided the electron transfer is thermodynamically allowed (ca. AGet < 0). In the follow up processes, (236) is transformed to (237) in 30-50% yield. Another report from the same group has described [187] a novel method of photoallylation of naphthaquinone by allyl stannane. 

The synthesis of quinones from arenes is an area which demands further research, despite the number of reagents presently available for this transformation. This is highlighted by the synthesis of the naphthoquinone (3). Direct oxidation of the dibromoarene (1) was unsatisfactory, and therefore Bruce and coworkers had to resort to a multistep sequence involving nitration, reduction, diazotization, displacement by hydroxide and finally oxidation of the phenol (2) with Fremy s salt (Scheme 1). Although there are examples of the oxidation of polynuclear aromatic hydrocarbons to quinones, the direct oxidation of an arene to a quinone is a process not encountered in the synthesis of more complex mt ecules. 

Zinc dichromate tiihydrate, ZnCr207<3H20, is obtained as an orange-red solid by adding zinc carbonate to a cold solution of chromium trioxide in dilute sulfuric acid [660]. The applications are oxidations of acetylenes lo a-diketones, of aromatic hydrocarbons to quinones, of alcohols to aldehydes, and of ethers to esters and the oxidative regeneration of carbonyl compounds from their oximes [660]. 

The applications of ruthenium tetroxide range from the common types of oxidations, such as those of alkenes, alcohols, and aldehydes to carboxylic acids [701, 774, 939, 940] of secondary alcohols to ketones [701, 940, 941] of aldehydes to acids (in poor yields) [940] of aromatic hydrocarbons to quinones [942, 943] or acids [701, 774, 941] and of sulfides to sulfoxides and sulfones [942], to specific ones like the oxidation of acetylenes to vicinal dicarbonyl compounds [9JS], of ethers to esters [940], of cyclic imines to lactams [944], and of lactams to imides [940]. 

The oxidation of benzene to p-benzoquinone is impractical, because benzoquinone is obtained from other compounds [647. Condensed aromatic hydrocarbons are oxidized to quinones by many reagents [429, 758, 802], most frequently by the compounds of hexavalent chromium [1121] (equation 150). 

Much of the early work on these substances was carried out before the advent of modern physical methods of structure determination. Typical experiments involved reduction and deoxygenation with zinc dust to form identifiable aromatic hydrocarbons, e.g. 2-methylanthracene from a C-2 substituted anthra-quinone. The oxygen functions were then placed on this carbon skeleton by unambiguous synthesis. 

Quinone—C,H,(OOy —is tlie representative of a number of similar compounds, denvable from the aromatic hydrocarbons. It is produced by the oxidizing action of MnO + H SO, or of dilute chromic acid, upon quite a number of para-benzene derivatives but best by the limited oxidation of quinic acid. 

In the early days of the studies on the specific tumorigenicity of various classes of compounds to mouse skin, investigators were intrigued by the activities exhibited by aromatic hydrocarbons, their dihydric phenols, and the quinones corresponding to the dihydric phenols. The results of mouse skin-painting bioassays with various aromatic hydrocarbons ranging in complexity from monocyclic to hexacyclic, their dihydric phenols, and the corresponding quinones are summarized in Table IX.B-1. 

From the studies on the chemical relationship between aromatic hydrocarbons and their quinones, the theory of the oxidation-reduction potential of quinones was proposed. 

Soon observed was the pronounced contrast between the gradation in specihc tumorigenicities in the mouse skinpainting bioassay from the nontumorigenicity of the mono-and bicyclic aromatic hydrocarbons benzene and naphthalene, respectively, to the potent tumorigenicities of the pentacyclic aromatic hydrocarbons DB[a,/ ]A and B[a]P vs. the tumorigenicities of the quinones 2,5-cyclohexadien-l,4-dione (p-ben-zoquinone), 1,2-naphthalenedione (1,2-naphthoquinone), and... 

Photolytic decomposition of peroxides is not v y efBcient in crosslinking. An enhancement effect on the extent of photocrosslinking of polyolefins in the presence of peroxides is displayed by aromatic hydrocarbons such as naphthalene. These transfer the exdtation energy absorbed to a peroxide. This procedure, however, does not represent an important improvement when compared with that refored to earlier, namely the photoreduction of pdyethylene with aromatic ketones and quinones [84. From aromatic ketones and quinones, particulariy bena>phenone [32], chlorinated benzophenones, benzoyl-l-( dohexanol [82], a, -dimethot - hen acdr henone, 2,4,6-trimethyl benzoyl phenyl phosphinic ethyl ester [85], anthrone [86], anthraqui-none [87], naphthoquinone, benzoquinone, and their d vatives have all been examined. 

Polycyclic organic matter, derived from the total exhaust emission, is an extremely complex mixture. It includes a large number of compounds such as polynuclear aromatic hydrocarbons (PAH), derivations of PAH such as nitro-PAH and amino-PAH, oxygenated PAH such as phenols and quinones, and heterocyclic aromatic compounds containing sulfur and oxygen. In order to assist in the identification of classes of toxic compounds it is possible to fractionate the exhaust emissions into vapor and... 

The V-Mo-O oxides are well-known industrial catalysts for the synthesis of acrylic acid from acrolein and maleic anhydride from benzene more recently, V-P-0 systems are being utilized for maleic anhydride production from -butane. The V20s/Ti02 combination was employed for phthalic acid production from o-xylene. V-Fe-O catalyzes oxidation of polycyclic aromatic hydrocarbons to dicarboxylic acids and quinones. Methyl formate is produced by the oxidation of methanol over V-Ti-0 catalysts [58]. For many of these processes, it has been experimentally proved that the catalytic reaction follows a Mars-van Krevelen mechanism. The surface coverage with active oxygen 0 in the steady state of the redox reaction following Mars-van Krevelen mechanism is given by... 

The zinc chloride-catalyzed condensation of polycyclic aromatic hydrocarbons with pyromellitic dianhydride at temperatures of 250°-300°C yielded dark, insoluble, infusible polymers. Their structure was not unequivocally determined. Judging from what is known in the literature, they could possess either a quinone [39] or a lactone [40] type structure, with the former predominating (22, 30). 

Between 1961 and 1967 the electrochemical generation method has been used to obtain, identify, and investigate free radicals derived from about 400 different organic compounds, such as unsaturated acyclic and alicyclic hydrocarbons, condensed and noncondensed polynuclear aromatic hydrocarbons, heterocyclic compounds, quinones, carbonyl compounds, nitriles, nitroso and nitro derivatives, and carboxylic esters. 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

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