9- fluorene, ionization

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]. 

Oxidation of diphenylmethane in basic solutions involves a process where rate is limited by and equal to the rate of ionization of diphenylmethane. The diphenylmethide ion is trapped by oxygen more readily than it is protonated in dimethyl sulfoxide-text-butyl alcohol (4 to 1) solutions. Fluorene oxidizes by a process involving rapid and reversible ionization in text-butyl alcohol solutions. However, in the presence of m-trifluoromethylnitrobenzene, which readily accepts one electron from the carbanion, the rate of oxygen absorption can approach the rate of ionization. 9-Fluorenol oxidizes in basic solution by a process that appears to involve dianion or carbanion formation. Benzhydrol under similar conditions oxidizes to benzophenone by a process not involving carbanion or dianion formation. 

The present work demonstrates that the oxidation of diphenylmeth-ane in basic solution follows a pattern similar to triphenylmethane and not to fluorene. At high concentrations of good electron acceptors it is possible to realize a situation wherein the rate of oxidation of fluorene is limited by and equal to the rate of ionization. The oxidations of benzhydrol and 9-fluorenol in basic solution are considered the difference in acidity of the methine hydrogens has a pronounced effect on the course of these oxidations. 

Fluorene and 9,9-dideuteriofluorene oxidized at the same rate in DMSO and in tert-butyl alcohol solution. This observation is consistent with a rapid, reversible ionization step. In tert-butyl alcohol the exchange of alpha deuterium atoms of dideuteriofluorene was measured (see experimental section) and a second-order rate constant for ionization calculated to be 0.12 = = 0.01 mole"1 per second. Under the conditions of this experiment the rate of oxygen absorption of undeuterated fluorene was approximately 1/50 the rate of deuterium exchange from the 9,9-dideuteriofluorene. 

It is apparent from Table IV that with nitrobenzene as the oxidation catalyst the ionization-limited rate was not reached even at a nitrobenzene concentration of 2.7M (0.02M fluorene, 0.02M potassium terf-butoxide). The rate of oxidation at the low nitrobenzene concentrations is first-order in nitrobenzene, fluorene, and base. This is consistent with an oxidation rate determined by Reaction 12 and involving an equilibrium concentration of the fluorene anions. 

The catalyst in the absence of fluorene absorbed oxygen, and in Table V a correction for this process has been attempted. The corrected data indicate an ionization rate constant of fluorene of 1.2 0.2 mole"1 per second at 29.5°C. The isotope effect in ionization of fluorene and 9,9-dideuteriofluorene is thus about 10. 

Final confirmation of this interpretation was attempted by studying the electron transfer between fluorene and m-trifluoromethylnitrobenzene in basic solution monitored by ESR spectroscopy in the absence of oxygen. Table VI summarizes data yielding an ionization rate constant of 0.9... 

The data of Tables XII and XIII appear to demand an oxidation mechanism similar to that observed for fluorene itself and involving a carbanion intermediate. The greater acidity of fluorene and xanthene relative to diphenylmethane (approximately 10 pKa units) (28) apparently promotes ionization to yield a dianion which can react directly with oxygen or undergo a catalyzed reaction—e.g., by nitroaromatics or Fe111. 

Ritchie and Uschold [13(b)] have calculated forward and reverse rate coefficients (k and kT) for the ionization of substituted fluorenes and triphenylmethane in methanol (107)... 

Fig. 1 Gas chromatography-flame ionization detection chromatogram of a complex mixture of PAHs extracted by SFE from a contaminated soil. (1) naphthalene (2) 2-methylnaphthalene (3) 1-methylnaphthalene (4) acenaphthene (5) fluorene (6) dibenzothiophene (7) phenanthrene (8) anthracene (9) fluoranthene (10) pyrene (11) benzo(a)anthracene (12) chrysene (13) benzo(e)pyrene (14) benzo(a)pyrene (15) indeno(l,2,3-cd)pyrene (16) dibenzo(a,h)anthracene (17) benzo(g,h,i)perylene. (From Ref. [12].)...

The properties of ionized, or charged, oligofluorenes have been characterized and modeled [177-179]. One of these studies has found that charge carriers on oligo- or poly-fluorene backbones delocalize to a greater extent than a neutral exciton [177]. In addition, the fluorene backbone is more planar in the charged state than in the neutral state [178,179]. 

The reaction of benzene with cesium and cesium alloys to form cesium benzenide is remarkable. In contrast benzene in 0.01 M solution in 2 1 by volume of THF and 1,2-dimethoxyethane with Na-K alloy according to ESR analysis gave (59) concentrations of radical anion at equilibrium of 10 to 10" M as the temperature decreased from -20° to -83 . The superior reducing power of cesium and its alloys was perhaps to be anticipated in view of the superior reducing power of cesium over potassium in aqueous solution and the appreciably lower ionization potential of cesium compared to potassium in the gas phase. These properties will be influenced by differential solvation of potassium and cesium ions by tetrahydrofuran and by the nature of the ion pairs produced. For 9-fluorenyl salts the fraction of solvent-separated ion pairs has been shown (52) to decrease rapidly in the order Li > Na > K > Cs and is a sensitive function of the solvating power of the medium. The cesium salt of fluorene in THF at -70°C has been shown to exist essentially entirely as contact ion pairs whereas the sodium and lithium salts were completely solvent-separated. The reluctance of cesium cations to become solvent-separated from counteranions means that cesium ions are available for strong electrostatic interaction with anions. 

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