9.10- Dihydroanthracene oxidation

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

Laudanosine contains four methoxyl groups. By exhaustive methyla-tion it yields trimethylamine and laudanosene (tetramethoxy-o-vinyl-stilbene), CH2=CH—C6H2(OCH3)2—CH=CH—C6H3(OCH3),. On oxidation with manganese dioxide and sulphuric acid it furnishes, in addition to the interesting by-product 2 3 6 7-tetramethoxy-9 10-dihydroanthracene, veratraldehyde and 4 5-dimethoxy-2 )3-methyl-... 

Howard and Ingold studied this equilibrium reaction in experiments on the oxidation of tetralin and 9,10—dihydroanthracene in the presence of specially added triphenylmethyl hydroperoxide[41]. They estimated the equilibrium constant K to be equal to 60 atm-1 (8 x 103 L mol-1, 303 K). This value is close to T=25atm-1 at 300 K (A/7=38kJ mol-1), which was found in the solid crystal lattice permeable to dioxygen [84], The reversible addition of dioxygen to the diphenylmethyl radical absorbed on MFI zeolite was evidenced and studied recently by the EPR technique [85],... 

Mahoney and DaRooge [57] studied the kinetics of oxidation of 9,10-dihydroanthracene at 333 K in the presence of some phenols and estimated the ratio of rate constants ks/k2. From these ratios, the rate constants ks were calculated for several para-substituent phenols 4-YC6H4OH using 2 = 850L mol-1 s 1. 

Reactions of phenoxyl and aminyl radicals with RH and ROOH are chain propagation steps in oxidation inhibited by phenols and amines (see Chapter 14). Both reactions become important when their rates are close to the initiation rate (see Chapter 14). Mahoney and DaRooge [57] studied the oxidation of 9,10-dihydroanthracene inhibited by different phenols. He went on to estimate the values of rate constants ratio of the reaction of ArO with RH and the reaction In + In (reactions (9) and (10), see Chapter 14) by the kinetic study. The values of kw for the reaction... 

Moro-Oka et al. (1976) have reported that the oxidation of 9,10-dihydroanthracene by K02 solubilized in DMSO by 18-crown-6 gives mainly the dehydrogenated product, anthracene. Under the same conditions, 1,4-hexadiene is dehydrogenated to benzene. The authors proposed a mechanism in which the superoxide ion acts as a hydrogen-abstracting agent only. The oxidations of anthrone (to anthraquinone), fluorene (to fluorenone), xanthene (to xanthone) and diphenylmethane (to benzophenone) are also initiated by hydrogen abstraction. 

KMn04 (2.37 g, 15 mmol), KOH (0.28 g), and TBA-HS04 (0.34 g, 1 mmol) in H20 (35 ml) are added to the benzylic compound (10 mmol) in CH2C12 (35 ml) and the mixture is stirred at room temperature until the purple colour disappears (orTLC analysis shows complete oxidation). The mixture is acidified with AcOH (5 ml) and Na2SO, is added until the brown colour disappears. The organic phase is separated, washed well with H20, dried (Na2S04), and evaporated to yield the ketone (e.g. PhCOPh, 80% 2-pyridylCOPh, 75% 9,10-anthraquinone from 9,10-dihydroanthracene, 90%). 

Benzophenones are produced by the oxidation of diarylmethanes under basic conditions [6-9], The initial step requires a strongly basic medium to ionize the methane and the more lipophilic quaternary ammonium catalysts are preferred (Aliquat and tetra-n-octylammonium bromide are better catalysts than tetra-n-butyl-ammonium bromide). The oxidation and oxidative dehydrogenation of partially reduced arenes to oxo derivatives in a manner similar to that used for the oxidation of diarylmethanes has been reported, e.g. fluorene is converted into fluorenone (100%), and 9,10-dihydroanthracene and l,4,4a,9a-tetrahydroanthraquinone into anthraquinone (75% and 100%, respectively) [6]. 

Non-catalytic intermolecular oxidation of C - H and O - H bonds, i.e. 9,10-dihydroanthracene or xanthene and 2,4-di-ferf-butylphenol, respectively, by molecularly defined Cu complexes was reported [118]. 

Interrupted oxidations of 9,10-dihydroanthracene or 9,10-dihydro-phenanthrene in DMSO (80% )-terf-butyl alcohol (20%) containing potassium ferf-butoxide produced the 9,10-semiquinone radical anions, apparently as a product of oxidation of the monoanion. 

The major oxidation product isolated was anthracene, perhaps formed in part from the hydroperoxide (I). However, significant amounts of potassium superoxide accompanied the anthracene. This result suggests that the major source of anthracene involved the oxidation of the dianion. In pure DMSO in the presence of excess potassium tert-butoxide, a trace of oxygen converts 9,10-dihydroanthracene, 9,10-dihy-drophenanthrene, or acenaphthene to the hydrocarbon radical anions. These products are apparently formed in the oxidation of the hydrocarbon dianions. 

Isolation of Oxidation Products. After oxygen absorption had ceased, or reached the desired value, the oxidates were poured into water. In many cases the reaction product could be removed by filtration in high yield. In this manner xanthone (m.p. 172-174°C.), was isolated from oxidations of xanthene or xanthen-9-ol thioxanthone (m.p. 208-210°C.), from thioxanthene acridine (m.p. 107-109°C.), from acridan anthracene (m.p. 216-217°C.), from 9,10-dihydroanthracene phenanthrene (m.p. 95-99°C.), from 9,10-dihydrophenanthrene pyrene (m.p. 151-152.5°C.) (recrystallized from benzene) from 1,2-dihydropyrene and 4-phenan-throic acid (m.p. 169-171 °C.) (recrystallized from ethanol) by chloroform extraction of the hydrolyzed and acidified oxidate of 4,5-methyl-enephenanthrene. 

Anthrone did not react with DMSO under the reaction conditions. However, 9,10-anthraquinone (2 mmoles) in 25 ml. of DMSO (80%)-terf-butyl alcohol (20% ) containing potassium tert-butoxide (4 mmoles) gave a deep red solution at 25°C., from which 60% of the adduct could be isolated after 1 hour and 88% after 3 hours. This adduct was isolated from the oxidate of 9,10-dihydroanthracene (after hydrolysis, acidification, and filtrations of anthracene) by extraction of the aqueous filtrate by chloroform. Xanthone and thioxanthone failed to form isoluble adducts with DMSO in basic solution. 

The autoxidation mechanism by which 9,10-dihydroanthra-cene is converted to anthraquinone and anthracene in a basic medium was studied. Pyridine was the solvent, and benzyl-trimethylammonium hydroxide was the catalyst. The effects of temperature, base concentration, solvent system, and oxygen concentration were determined. A carbanion-initi-ated free-radical chain mechanism that involves a singleelectron transfer from the carbanion to oxygen is outlined. An intramolecular hydrogen abstraction step is proposed that appears to be more consistent with experimental observations than previously reported mechanisms that had postulated anthrone as an intermediate in the oxidation. Oxidations of several other compounds that are structurally related to 9,10-dihydroanthracene are also reported. 

A homogeneous reaction system was used in which benzyltrimethyl-ammonium hydroxide was the base and pyridine the solvent. An oxidation mechanism is proposed that is consistent with observations on the reaction variables and possible oxidation intermediates of dihydroanthracene. 

Table I. Effect of Initial Reaction Temperature on Oxidation of Dihydroanthracene to Anthraquinone0...

Table III. Effect of Water Content in Pyridine on Oxidation of Dihydroanthracene to Anthraquinone"...

Table IV. Effect of Various Solvents on Oxidation of Dihydroanthracene to Anthraquinone...

Oxidation of Related Compounds. Several other compounds related to dihydroanthracene in structure were oxidized in pyridine solvent (Table VI). No attempt was made to optimize the yields in any instance except with dihydroanthracene. It was surprising that anthrone reacted much more slowly than dihydroanthracene and that only a 40% yield of anthraquinone was obtained. 

However, when anthrone was oxidized under the same conditions as the dihydroanthracene, the conversion to anthraquinone was estimated to be only 40%, and that value was probably high because of interference by unreacted anthrone during analysis. Anthrone, then, was not readily oxidized, contrary to expectation if it were the intermediate to the quinone. 

Both Russell (5) and Barton (I) have examined the oxidation of dihydroanthracene in a solvent system consisting of 80% dimethyl sulfoxide and 20% tert-butyl alcohol and with potassium terf-butoxide as the base. In both studies, a large excess of base was used, so that there is a possibility of dicarbanion formation. In the present investigation, only catalytic amounts of base were used, which makes it unlikely that a... 

Postulated Mechanism. The first phase of the oxidation of 9,10-dihydroanthracene involving a free-radical process would be the following chain initiation. 

Oxidation of the dihydroanthracene (50 mmoles) by oxygen at 4 atm. consumed 1.80 molecular equivalents (90 mmoles) of oxygen. This amount of oxygen corresponds to an 87% conversion to anthraquinone and a 13% conversion to anthracene. Analysis of the product gave corresponding values of 90 and 10%. The difference between calculated and experimental conversions may well be within experimental error. 

This study indicates that the oxidation of dihydroanthracene in a basic medium involves the formation of a monocarbanion, which is then converted to a free radical by a one-electron transfer step. It is postulated that the free radical reacts with oxygen to form a peroxy free radical, which then attacks a hydrogen atom at the 10-position by an intramolecular reaction. The reaction then proceeds by a free-radical chain mechanism. This mechanism has been used as a basis for optimizing the yield of anthraquinone and minimizing the formation of anthracene. 

K. A. Schowaiter Our data indicate that both the hydroperoxide and anthrone oxidize at a slower rate than dihydroanthracene in our... 

Dr. Schowalter It appears that this is another mechanism that could account for the oxidation of dihydroanthracene without intermediate anthrone formation. 

Mahoney studied this kinetics by the oxidation of 9,10-dihydroanthracene inhibited by several substituted phenols [23,31,32,37,38,49]. 9,10-Dihydroanthracene possesses weak C—H bonds that are easily attacked not only by peroxyl radicals but also by phenoxyl radicals as well (for the rate constants of reaction (10), see Chapter 15). 

Selective formation of ketones may be achieved through base-catalyzed oxidations.848 Transformation of 9,10-dihydroanthracene catalyzed by benzyltrimethy-lammonium hydroxide (Triton B)855 starts with proton abstraction ... 

Some 10-X-3 species behave as oxidizing agents. Chloroiodinane (21) oxidizes aqueous potassium iodide to iodine. Brominane (35) oxidizes I, Br- and aniline to I2, Br2 and azobenzene, respectively. 9,10-Dihydroanthracene is dehydrogenated by (35) to give anthracene in quantitative yield (80JA7382). 

The photo-oxidation of n-butane has been modelled by ab initio and DFT computational methods, in which the key role of 1- and 2-butoxyl radicals was confirmed.52 These radicals, formed from the reaction of the corresponding butyl radicals with molecular oxygen, account for the formation of the major oxidation products including hydrocarbons, peroxides, aldehydes, and peroxyaldehydes. The differing behaviour of n-pentane and cyclopentane towards autoignition at 873 K has been found to depend on the relative concentrations of resonance-stabilized radicals in the reaction medium.53 The manganese-mediated oxidation of dihydroanthracene to anthracene has been reported via hydrogen atom abstraction.54 The oxidation reactions of hydrocarbon radicals and their OH adducts are reported.55... 

The use of Mn-salen catalysts for asymmetric epoxidation has been reviewed.30 Oxo(salen)manganese(V) complexes, generated by the action of PhIO on the corresponding Mn(III) complexes, have been used to oxidize aryl methyl sulfides to sulfoxides.31 The first example of C—H bond oxidation by a (/i-oxo)mangancsc complex has been reported.32 The rate constants for the abstraction of H from dihydroanthracene correlate roughly with O—H bond strengths. 

Studies in this field are just beginning, and the number of publications hardly exceeds a dozen. The most interesting results were obtained by the research groups of Yamada [160-162], Neumann [163,164] and Kozhevnikov [165, 166], Using various type catalysts (Ru porphyrene complexes, polyoxometalates, supported metals), the authors conducted selective oxidations of various types. These include epoxidation of alkenes, oxidation of alcohols, oxidation of alkylaromatics, oxidation and aromatiza-tion of dihydroanthracenes, and some other reactions. The experiments were typically conducted at 373—423 K under 1.0 MPa pressure of nitrous oxide. 

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