Diphenylmethane oxidation

Pyronines. Pyronines are diphenylmethane derivatives synthesized by the condensation of y -dialkylarninophenols with formaldehyde, followed by oxidation of tiie xantiiene detivative (12) to the coiiesponding xanthydiol (13) which in the presence of acid forms the dye (14). If R is methyl, the dye produced is... 

Benzophenone is produced by the oxidation of diphenylmethane (350). This free from chlorine (FCC) route is favored for perfume uses. The Friedel-Crafts reaction of benzene and benzoyl chloride in the presence of aluminum chloride is also possible this reaction may proceed in the absence of catalyst at a temperature of 370°C and pressure of 1.4 MPa (351). 

Diphenylmethane Base Method. In this method, the central carbon atom is derived from formaldehyde, which condenses with two moles of an arylamine to give a substituted diphenylmethane derivative. The methane base is oxidized with lead dioxide or manganese dioxide to the benzhydrol derivative. The reactive hydrols condense fairly easily with arylamines, sulfonated arylamines, and sulfonated naphthalenes. The resulting leuco base is oxidized in the presence of acid (Fig. 4). 

In a variation of this method, isolation of the ben2hydrol derivative is not required. The methane base undergoes oxidative condensation in the presence of acid with the same or a different arylamine direcdy to the dye. New fuchsine [3248-91 -7] Cl Basic Violet 2 (16), is prepared by condensation of two moles of o-toluidine with formaldehyde in nitrobenzene in the presence of iron salts to give the corresponding substituted diphenylmethane base. This base is also not isolated, but undergoes an oxidative condensation with another mole of o-toluidine to produce the dye. 

Closely related to this work is that of Wiberg and Evans on the Cr(Vf) oxidation of diphenylmethane in 95 % acetic acid. This has the rate law... 

A quantity of earlier work exists on chromic acid oxidation of hydrocarbons. It was noted that diphenylmethane and other hydrocarbons in glacial acetic acid solution are oxidised rapidly during the initial stages but that reaction is auto-retarded The autoretardation is eliminated on adding 2.5 % of sulphuric acid. The orders of the reaction with respect to diphenylmethane and Cr(VI) are one and two respectively , the latter differing from that found by Wiberg and... 

The oxidations of toluene, diphenylmethane and triphenylmethane are second order , relative rates at 22 °C being... 

The effect of solvent upon k2 has been reported , and it was concluded that the activated complex is not sufficiently polar to be called ionic . The oxidations of toluene and triphenylmethane exhibit primary kinetic deuterium isotope effects of 2.4 and ca. 4 respectively. No isotopic mixing occurred during formation of the Etard complex from a mixture of normal and deuterated o-nitrotoluene . The chromyl chloride oxidation of a series of substituted diphenylmethanes revealed that electron-withdrawing substituents slow reaction while electronreleasing groups have the opposite effect, the values ofp andp being —2.28 + 0.08 and —2.20 + 0.07 respectively . ... 

Stairs comments that this p value is strongly dependent on the temperature but his data have been criticised by Duffin and Tucker ", who prefer their method of observing the rate of formation of the adduct to that of estimating total residual oxidising power employed by Stairs, and they find p (25°) to be —2.32+0.10 as compared to a value of —2.20+0.07 at 40 °C. These values are considerably more negative than those found for chromic acid oxidation of diphenylmethanes (— 1.17) and toluenes (-1.12). 

The relative rates of oxidation of phenylmethanes cover too small a range to be compatible with carbonium ion formation cf. the discussion on chromic acid oxidation of diphenylmethane, p. 295), and an initial reaction to give a radical plus Cr(V) followed by rapid transfer of a second electron to form Cr(IV) is more... 

The transition state does not involve a large degree of charge separation and hence the relative rates of oxidation of toluene, diphenylmethane and triphenylmethane may be explained in terms of an ionic mechanism. 

Such a pre-equilibrium closely parallels that suggested by Dewar et for the manganic acetate oxidations of several aromatic ethers and amines (p. 405). Other features of the reaction are a p value of —0.7 and identical activation energies of 25.3 kcal.mole for oxidation of toluene, ethylbenzene, cumene, diphenylmethane and triphenylmethane. 

Direct oxidation of diphenylmethanes is of little practical value as color formers. In liquid sulfur dioxide, leuco diphenylmethane 12 (Scheme 2) undergoes hydride abstraction by triphenylcarbenium perchlorate at the benzylic amine position to form immonium ion7 13, whereas in acetonitrile... 

The practical route for oxidizing leuco diphenylmethanes 15 demands inital conversion to an imine salt 16. The imine salt is obtained by heating a mixture of diphenylmethane, sulfur, ammonium chloride, and sodium chloride at 175°C in a current of ammonia or by heating a mixture of diphenylmethane, urea, sulfamic acid, sulfur, and ammonia at 175°C (Scheme 3). Dyes 16 can be represented as the quinonoid resonance structure 17. Dyes of this class, known as auramines, are all yellow, with the only commercial representative being auramine O 16a. Due to its poor lightfastness and instability to hot acids and bases, its use has been restricted to dyeing and printing cotton, paper, silk, leather, and jute. 

Reduction of Michler s ketone gives Michler s hydrol (6.159), which forms Michler s Hydrol Blue in the presence of acid (Scheme 6.27). Michler s hydrol is produced industrially by the oxidation of the diphenylmethane precursor (6.156) further oxidation to give Michler s ketone takes place readily. 

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. 

Catalytic hydrogenolysis (continued) M0O3-AI2O3 catalyst, 29 258-269 relative reactivity, 29 255-257 schematic model, 29 254 diphenylmethane kinetics, 29 241-243 reaction mechanism, 29 267 Catalytic oxidation,... 

Electron-rich olefins with substituents Y = phenyl, vinyl, amino, or alkoxy can be coupled by anodic oxidation to tail-tail dimers being either deprotonated to dienes and/or substituted a to Y, depending on Y and the reaction conditions (Eq. 6). Alkyl substituted arenes can be dehydrodimer-ized to diphenyls or diphenylmethanes depending on the kind of substitution (Eq. 7). 

Stance, oxidation of diphenylmethane with 0.1 mol % 5a and 10 equiv. of 30% H2O2 under ambient conditions afforded a 1 1 mixture of diphenylmethanol and benzophenone in 34% yield after 16 h. When heated to 80 °C for 5 h, the reaction is driven to the formation of benzoquinone in 87% yield. Cu(OAc)2 and Cu (salen) also afforded benzoquinone under identical conditions, but were found to be less effective than 5a. 

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. 

Oxidation of Diphenylmethane in Basic Solution. Diphenylmethane reacts with an excess of oxygen in the presence of potassium ferf-butoxide in various solvents to produce nearly quantitative yields of benzophenone. In DMSO (80% )-tert-butyl alcohol (20% ) a 96% yield of the benzo-phenone-DMSO adduct [ l,l-diphenyl-2- (methylsulfinyl) ethanol] was isolated at complete reaction (17). 

If the oxidation of diphenylmethane in DMSO (80% )-tert-butyl alcohol (20% ) is interrupted after the absorption of one mole of oxygen per mole of diphenylmethane, one obtains an 86% yield of benzhydrol, 10% yield of unreacted diphenylmethane, and a few percent of the benzophenone-DMSO adduct. The over-all course of the reaction fol-... 

The oxidation of diphenylmethane in DMSO solution does not involve Reaction 6. The stoichiometry and product isolation as defined in Figure 1 exclude Reaction 6. 

In the absence of DMSO the conversion of diphenylmethyl hydroperoxide to benzophenone apparently does follow Reaction 6, at least in alcohol-containing solvents. The stoichiometry becomes nearly one mole of oxygen per mole of diphenylmethane, and the carbinol is eliminated as an intermediate. Table I lists the observed stoichiometries and initial rates of oxidation of diphenylmethane. In pyridine-, DMF-, or HMPA-containing solvents, a high yield of benzophenone was isolated upon hydrolysis after oxygen absorption had ceased. In pure HMPA there was considerable evolution of oxygen upon hydrolysis. 

This interpretation was proved correct by considering the oxidation of a sample of diphenylmethane that had an isotopic purity of 97.0% a,a-dideuterio and 2.7% a-deuterio by mass spectrometry. The oxidation rate observed after the initial 15-second period (see Figure 2), during which the undeuterated and monodeuterated material were destroyed, yielded a second-order rate constant, ki = 0.0148 mole"1 per second. There is thus an appreciable isotope effect ku/kD of about 6 in the ionization of diphenylmethane by potassium ferf-butoxide in DMSO(80%)-tert-butyl alcohol (20% ) at 25°C. This compares with a value of fcH/ D of 9.5 reported for the ionization of triphenylmethane (16). The observation of primary isotope effects of this magnitude requires that the protonation of the diphenylmethide ion by tert-butyl alcohol in DMSO solution does not proceed at the diffusion rate which would, by the principle of microscopic reversibility, require the absence of an isotope effect in the deprotonation step. 

When the oxidation of a,a-dideuteriodiphenylmethane was interrupted after the absorption of 1.0 equivalent of oxygen, the product was found to be only benzhydrol and a trace of diphenylmethane (by GLPC). Mass spectroscopic analysis of the benzhydrol indicated 98.5% mono-deuterated material. 

The oxidation of fluorene in basic solution is in sharp contrast to diphenylmethane. Figure 1 emphasizes the clean stoichiometry observed in the oxidation of fluorene (one mole of oxygen per mole of fluorene). The over-all reaction for fluorene apparently involves Reactions 1, 2, 6, and 5 (18). 

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. 

Treatment of diphenylmethane in basic solution with a trace of oxygen in DMSO solutions fails to produce significant amounts of a paramagnetic product detectable by ESR spectroscopy. On the other hand, treatment of benzhydrol with traces of oxygen in basic solution can produce significant amounts of the ketyl. Pyridylthiazolylcarbinols are readily converted to the ketyls by base in alcoholic solution. (24). In pure DMSO significant amounts of the ketyl are formed whereas in tert-butyl alcohol or DMSO (80% )-tert-butyl alcohol (20% ) only traces of the ketyl can be detected. These results are consistent with the formation of the ketyl under oxidative conditions by Reaction 31. Only under the most basic conditions (pure DMSO) is the dianion formed by... 

The product of the absorption of 3 mmoles of oxygen by a solution of 3 mmoles of diphenylmethane and 15 mmoles of potassium tert-butoxide in 20 ml. of DMSO (80% )-terf-butyl alcohol (20%) was poured into water. Upon standing, colorless crystals of benzhydrol (m.p. 64°C.) formed which could be recovered in 75% yield by filtration. Complete oxidation of the diphenylmethane gave an oxidate which after... 

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