Oxidation of benzhydrols

N-Hydroxyphthalimide A-Hydroxyphth-alimide (NHPI) was shown to be a mediator for the electrochemical oxidation of (1) to (2) (Eq. 6) [57, 58], although the yields of (2) were not always satisfactory. A large deuterium isotope effect ( h/ d = 10.6) was observed in the oxidation of benzhydrol [59]. Recently, tetrafluoro-NHPI was found to be efficient for the oxidation of borneol [60]. 

Some lactol-to-lactone oxidations were effected by TPAP/NMO/PMS/CH Clj [498, 499], or TPAP/NMO/PMS/CH3CN [159]. The system RUCI3 or RuO / Na(Br03)/aq. M Na3(C03) generates [RuO ]" in aqueous solution and oxidised secondary alcohols to ketones in high yield (Table 2.2) [213]. Kinetics of the oxidation of benzhydrol and 9-fluorenol by TPAP/NMO/CH3CN/30°C were measured. 

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. 

Oxidation of Benzhydrol in Basic Solution. Reaction of benzhydrol with oxygen in basic solution results in the formation of benzophenone, or in DMSO solutions the benzophenone—DMSO adduct. Table VIII summarizes data on the oxidation of benzhydrol in three solvents and in the presence of various concentrations of potassium ferf-butoxide. The rates are the maximum oxidation rates, often observed after an inductive period (Figure 3). 

Table VIII. Oxidation of Benzhydrol in Basic Solutions at 27 = = 2°C.

Ferric chloride (0.002M) reduced the rate of oxidation of benzhydrol (0.15M) in the presence of 0.39M potassium ferf-butoxide in tert-butyl alcohol to a rate of 0.001 mole of oxygen per mole of benzhydrol per minute, while arsenic trioxide (0.01M) reduced the rate of oxidation of 0.12M benzhydrol and 0.36M potassium ferf-butoxide to 0.0001 mole of oxygen per mole of benzhydrol per minute for a 4-hour period, after which the oxidation occurred at the uninhibited rate (Figure 3). Table IX summarizes some observed stoichiometries in the oxidation of benzhydrol. 

To ascertain the possibility of the intervention of benzophenone ketyl, (C6H5)2CO ", in the oxidation of benzhydrol, the oxidation of... 

The data apparently require a free radical chain mechanism for the oxidation of benzhydrol. Potassium superoxide filtered from a completed oxidation completely removed the induction period for a fresh oxidation. Thus, potassium superoxide must either serve as an initiation of oxidation... 

The oxidation of xanthenol and fluorenol showed a number of differences from the oxidation of benzhydrol. No induction period was observed (Figure 5). The rate was enhanced by ferric ion, and the stoichiometry was altered to 0.5 mole of oxygen per mole of fluorenol (Figure 5), apparently because of Reaction 25. 

The oxidation of benzhydrol and 9-fluorenol in basic solution again shows a difference in regard to mechanism that can be primarily attributed to a difference in acidity as carbon acids. In tert-butyl alcohol benzhydrol enters into an oxidation scheme as the mono (oxy) anion. The data strongly suggest a free radical chain. Under these conditions the more acidic fluorenol or xanthenol oxidizes via carbanions or dianions. These oxidations can be catalyzed to occur via a free radical chain process by one-electron acceptors, such as nitrobenzene, and a free radical chain process may well be involved in the absence of the catalyst. 

Oxidation of Potassium Peroxide. Determination of Potassium Superoxide. Potassium peroxide was prepared by the addition of a tert-butyl alcohol solution of 90% hydrogen peroxide to potassium tert-butoxide in DMSO or tert-butyl alcohol. Oxygen absorption was followed in the standard manner (20). Analysis of solid precipitates for potassium superoxide followed exactly the method of Seyb and Kleinberg (23). Potassium superoxide formed in the oxidation of benzhydrol was determined in a 15-ml. aliquot of the oxidation solution. To this aliquot 10 ml. of diethyl phthlate was added to prevent freezing of the solution. The mixture was cooled to 0°C., and 10 ml. of acetic acid-diethyl phthlate (4 to 1) added over a period of 30 minutes with stirring. The volume of the evolved oxygen was measured. 

Barium manganate, prepared from potassium manganate and barium chloride [5JJ] or by the reduction of potassium permanganate with potassium iodide in the presence of barium chloride and sodium hydfoxide [5J2], is used for the quantitative oxidation of benzhydrol to benzophenone. The reaction mixture is refluxed in benzene for 0.5-2 h [SJ5]. The result is comparable with and even better than that of oxidation with manganese dioxide [250, 525]. 

The oxidation of benzhydrols by tributylammonium chlorochromate (TBACC) is first order each in TBACC, benzhydrol, and H+ ions. The reaction exhibits a primary kinetic isotope effect and the rate increases with [AcOH] in the aqueous mixture a mechanism involving hydride ion transfer is proposed. ... 

In general, use of Ru(TDCPPXO)2 instead of Ru(TMP)(0)2 for the aerobic oxidations made little difference, i.e. the activities were similar over 24 h, but the TDCPP analogue was more susceptible to deactivation via a decarbonylation process that generated the Ru(CO) derivative. For die oxidation of benzhydrols, the perchloro derivative Ru(TDCPP-Clg)(0)i exhibited activity similar to that of Ru(TMP)(0)2. ... 

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