Triphenylmethyl reaction with oxygen

Reaction with other radicals is characteristic of the triarylmethyls and the chain reaction with oxygen has already been discussed. The absorption of the radical nitric oxide is also used as an analytical tnethod for triphenylmethyl. 

The reaction of the triphenylmethyl radical with oxygen to form the peroxide discovered by Gomberg in 1900 (equation 1) was a strong piece of evidence for the radical structure 1, and the affinity of carbon centered radicals for oxygen remains one of their defining characteristics. Willstatter and Haber in 1931 also proposed a general role for free radicals in oxygen reactions in chemistry, which further stimulated interest in this field. 

Treatment of triphenylmethyl chloride with silver gave not the expected hydrocarbon but an oxygen-containing compound later found to be the peroxide. The reaction run in an inert atmosphere did give a hydrocarbon, but one with unusual properties. It reacted rapidly with oxygen, chlorine, and bromine, and was quite different from tetra-phenylmethane or what was expected of hexaphenylethane. Gomberg... 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

Dialkyl peroxides ROOR are often obtained as the principal product or as by-product of autoxidation of organic compounds, but almost always as a result of secondary reactions of alkyl hydroperoxides. Direct union of relatively long-lived radicals with oxygen is of minor importance and occurs only in exceptional cases, e.g., in the formation of bis(triphenylmethyl) peroxide from triphenylmethyl 343... 

When such a solution is brought into contact with oxygen, iodine, or certain other substances, the triphenylmethyl present reacts as a consequence the equilibrium is disturbed, more hexaphenylethane changes into triphenylmethyl, and, finally, all the hexaphenylethane enters into reaction. Measurements indicate that in a solution of the hydrocarbon there is present about 10 per cent of triphenylmethyl. 

A mixture of 4-triphenylmethyl-l,2-benzoquinone, dimethyl phosphite, and dry benzene refluxed 2 hrs. dimethyl (5,6-dihydroxy- ,-a, -triphenyl-m-tolyl)phos-phonate. Y 90%. F. e., also reaction at oxygen with trialkyl phosphites, s. M. M. Sidky and F. H. Osman, Tetrahedron 29, 1725 (1973). 

We have already seen that triphenylmethyl radical reacts both with itself to form Chichibabin s hydrocarbon (formula II, not the diradical) and with halogens and oxygen. The reactions of the stable radical are important because of the expectation that radicals too unstable to isolate will betray their presence by giving similar products. 

For example, in the decomposition of phenylazotriphenylmethane in benzene, triphenylmethyl peroxide is formed if oxygen is present, indicating a radical reaction. The unstable phenyl radical does not persist long enough to dimerize but reacts instead with the solvent. 

FIGURE 7.27 Ester (IR 1825-1815 cm4) and amide IV-oxide (amide 3-oxide) (IR 1750-1730 cm4) forms of benzotriazole adducts.94 R = R O and R 0C(=0)-Xaa. Compounds with R = tBuO and PhCH2 are amide forms.97 The product from reaction of Trt-methionine and HOBt is the amide 3-oxide96 (Trt = trityl = triphenylmethyl). Note that the atoms bearing the oxygen atoms are numbered differently in the two compounds. 

Triphenylmethyl acetate67-84 and benzoate75 are solvolyzed with alkyl-oxygen fission even under neutral conditions, and the reaction is strongly acid-catalyzed in each case. The hydrolysis of triphenylmethyl acetate in 80% dioxan-water has been studied by Bunton and Konasiewicz 7. The reaction involves near-quantitative alkyl-oxygen fission in both acidic and initially neutral solution, as shown by the incorporation of l80 from enriched H20 into the triphenylmethyl alcohol produced, and by the lack of incorporation into the acetic acid. The rate of the acid-catalyzed reaction at 25°C is about 40% slower in 90% dioxan than in the more aqueous solvent. ... 

No triphenylmethyl cation or radical was detected. Malachite green leuco-methoxide, 51, in cyclohexane behaved similarly. In contrast, 51 showed rapid growth of the triaryl cation from excited Si with a rise time of 1 ns in 9 1 acetonitrile/water, and an even faster rise time in methanol. No evidence for the triarylmethyl radical was obtained. Also, the dithioketal, 52, has been used to generate the corresponding relatively long-lived cations by photoheterolysis of the carbon-oxygen ether bond the half-life for cation decay in 1 1 ethanol/water is 0.17 s. Rate constants for the reactions of this cation with a number of amine and anionic nucleophiles were measured. 

Photooxidation of the triphenylmethyl cation (as its tetrafluoroborate salt) in the presence of an aromatic donor was similarly found to afford bis(triphenyl-methyl) peroxide [31]. The mechanism was proposed to proceed through initial electron transfer from the aromatic donor to the singlet-excited triphenylmethyl cation to give the triphenylmethyl radical. Reaction of the radical with triplet oxygen, and subsequent coupling with another triphenylmethyl radical gave the observed peroxide. It was noted that electron transfer from the tetrafluoroborate counterion to the excited state cation could not be completely excluded, because the cation was slowly photooxygenated in the absence of an electron donor. 

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