Oxidation of 9,10-dihydroanthracene

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. 

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. 

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. 

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

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 ... 

Oxidation of the methylene bridge of heterocyclic derivatives of 9,10-dihydroanthracene have also been successful ... 

Table 7.3 Oxidative dehydrogenation of 9,10-dihydroanthracene ito anthracene in the liquid phase over carbon-based catalysts.

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). 

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],... 

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]. 

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. 

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. 

The oxidative properties of this stable compound have not been explored in detail but it appears that they are more pronounced than those of BTI. For example, 9,10-dihydroanthracene was oxidized to anthraquinone (68%) at room temperature, whereas the use of BTI required heating and the product was anthracene in low yield. 

Anthracene hydride (the anion derived from 9,10-dihydroanthracene) reacts rapidly with chalcone to form an anionic Michael adduct along with a chalcone dimerization product (Scheme 83). Prolonged reaction in the presence of anthracene hydride cleaves the Michael adduct into anthracene and the enol-ate of the saturated ketone. The partial structure RCCCO is essential for this fragmentation, as mesityl oxide, for example, gave only the Michael adduct. 

When Nu is electron donating the product is as a rule more easily oxidized than the starting material, resulting in further oxidation under the reaction conditions and, frequently, complex reaction mixtures. The anodic methoxylation of naphthalene, which results in 1-methoxy-, 1,2-dimethoxy-, and 1,4-dimethoxynaphthalene, approximately in a 1 2 1 ratio, serves as an illustration of this problem [67]. However, in other cases, a single major product is obtained after a sequence of reactions, such as the oxidation of mesitylene in MeCN-diluted H2SO4 to 2,4,6-trimethyl-4-hydroxycyclohexa-2,5-dien-l-one in a substitution-elimination reaction [68] or the oxidation of anthracene in MeOH to 9,9,10,l0-tetramethoxy-9,10-dihydroanthracene in a substitution-addition reaction [Eq. (28)] [69]. 

The results of the inhibited oxidation experiments with polypropylene are summarized in Tables I and II. A few experiments where 9,10-dihydroanthracene was used as the substrate are summarized in Table III. 

Two high-yield three-step syntheses of (195) from anthraquinone have been developed via the bis-epoxide (194) (67% and 89% overall). Compound (195) was obtained from (194) either by conversion into lO-hydroxymethyl-9-anthraldehyde with LiBr, followed by oxidation, or by conversion into 9,10-dihydroanthracene-9,10-dicarboxaldehyde with BF3, followed by dehydrogenation. 

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