Triphenylacetic acid

Considerable effort has been expended by electrochemists in the quest for radical intermediates in the Kolbe reaction, i.e. the anodic oxidation of carboxylate anions in solution. This reaction produces a variety of products dependent upon several factors such as solvent, pH, electrode material, etc. 

The possible involvement of R in the Kolbe reaction has been regularly investigated using ESR but, to date, positive evidence for radical intermediates has been lacking apart from two cases, firstly the photo-Kolbe reaction at irradiated semiconductor electrodes [76] and, secondly, the anodic oxidation of triphenylacetic acid at platinum electrodes in acetonitrile. The latter will be discussed in this section. 

Bard and co-workers, using the in-situ cell described in Sect. 4.1, obtained rather different results [80] from Kondrikov. Direct oxidation of triphenylacetic acid was found not to result in an ESR spectrum, but spectra attributed to triphenylmethyl radicals 3C were observed if the electrode was held at a potential corresponding to TPA oxidation (2.0 V vs. Ag pseudoreference electrode) and then stepped to a value negative of about 0.35 V. 

The clear discrepancies in the two studies of TPA oxidation led to a further study of this reaction by Day and co-workers [81] using the in-situ 

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Figure 2.101 (a) The spectrum obtained by oxidation of triphenylacetic acid at + 2.0 V before... 

Triphenylacetic acid is not reduced under ordinary conditions in the medium of ether. But it is converted into a primary alcohol in good yield either by reduction in higher boiling solvent like THF or by first converting it into an acid chloride which is then readily reduced in ether. 

Whereas 1,1,1-triphenylethane 551 gave only 2% of triphenylacetic acid 552 after treatment with lithium and a substoichiometric amount (50%) of biphenyl, followed by carbonation and acidic hydrolysis, the same process carried out starting from 1,1,1,2-tetraphenyl ethane 553 or 1,1,2,2-tetraphenylethane 554 gave tri- 552 or diphenylacetic acid 555, respectively, with excellent yields (Scheme 148). ... 

Figure 2.101 (a) The spectrum obtained by oxidation or triphenylacetic acid at + 2.0 V before stepping to —1.8 V in wet acetonitrile, (b) Spectrum obtained by the ox idation of triphenylacetic acid at + 2.0 V before stepping to — 1.9 V in slightly moist acetonitrile. From Compton et al (1986). Reprinted from Comprehensive Chemical Kinetics, 29(1989), 297. 

Cells such as those described in References 23, 24, and 29 are particularly suited to study of short-lived intermediates requiring in situ generation at accurately controlled potentials. When a conventional electrochemical cell was used to study the Kolbe synthesis oxidation of triphenylacetic acid [53], it was concluded that the initially formed radical was triphenylacetoxyl (3-CCOO ), based on the assignment of two para- and four ortho-proton splittings. A more careful study [54] using the cell described in Reference 23 showed that it is in fact the triphenylmethyl radical that is formed initially the identity of the other species was not established, although it is clearly not the acetoxyl radical. 

Oxidative decarboxylation.1 Sodium hypochlorite is known to effect oxidative decarboxylation of a-hydroxy carboxylic acids to form ketones and C02, but a hydroxyl group is not essential since trisubstituted acetic acids are also subject to this oxidation. Thus triphenylacetic acid is oxidized by NaOCl to triphenylmethanol and benzophenone. In the presence of a phase-transfer catalyst, the rate is enhanced... 

In the conversion of di- and triphenylacetic acid to the corresponding fluorides, laser flash photolytic investigations demonstrated that a carbocationic intermediate was trapped by fluoride in the key step. This interesting conversion was claimed to take place through a two-photon process in which the photocatalytically generated radical remains bound to the surface of the photocatalyst, then trapping a second photogenerated hole to produce a carbocation. 

The triphenylmethyl carbanion, the trityl anion, can be generated by the reaction of triphenylmethane with the very powerful base, n-butyllithium. The reaction generates the blood-red lithium triphenylmethide and butane. The triphenylmethyl anion reacts much as a Grignard reagent does. In the present experiment it reacts with carbon dioxide to give triphenylacetic acid after acidification. Avoid an excess of n-butyllithium on reaction with carbon dioxide, it gives the vile-smelling pentanoic acid. 

Cleavage with dilithium biphenylide in THF gave, on carbonation triphenylacetic acid in 93% yield and detectable phenylacetic add. ... 

Gilman and Gaj prepared this reagent conveniently by adding an ethereal solution of n-butyllithium to a solution of triphenylmethane in tetrahydrofurane carbona-tion after 15 min. at room temperature afforded triphenylacetic acid in good yield. 

The conversion of the potassium to potassium triphenylmethide is assumed to be quantitative, producing 0.2 mol of the reagent. Carbonation of the reagent gives a 91 yield (based on either triphenylmethane or potassium) of triphenylacetic acid, mp 263-265°. 

The reaction also proceeds for triphenylacetic acid and its methyl esters. The outcome and the proposed carbene... 

Another example of how PET can be modulated by complexation involves the use of bulky carboxylates as molecular curtains . Intramolecular PET occurs in the Zn complex of the poly amine (166) between the dimethylaniline donors and the anthracene acceptor, however when triphenylacetic acid is added a strong enhancement of fluorescence occurs. This effect is ascribed to complexation of the triphenylacetate at the Zn cation in such a way as to block PET between the other two units in the complex. The host-guest chemistry afforded by calixarenes has also been adapted to the construction of efficient ET systems. 

Dimerization of alkanes can be brought about by the irradiation of zeolite-Y with added silver atoms and silver clusters lead to dimerization on irradiation at 220-300 nm104. Selective fluorination of di- and triphenyl methane in acetonitrile as solvent and TiC>2 as catalyst can be effected using AgF. Di- and triphenylacetic acid as well as alkenes can be fluorinated in the same way. A single electron transfer reaction is thought to be involved, followed by attack of fluoride105. It could be that the reaction is one that arises on the TiC>2 surface and that the Ag+, to which the electron transfer could take place, is not involved106. 

Fig. 28. Spectrum obtained by oxidation of triphenylacetic acid at +2.0V before stepping to - 1.9 V in wet acetonitrile.

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