Triphenylmethyl radical, generation

Phenyl and triphenylmethyl radicals generated from 6 contribute to the initiation and the termination, respectively, resulting in polymer 18 because of the remarkably different reactivities of these radicals (Eq. 21). The co-chain end terminated with 1 thermally redissociates to induce further polymerization. Therefore, the polymerization proceeded via a mechanism close to the model in Eq. (18). The recombination product of methyl isobutyryl radical and 1 was reported to have a quinonoide structure [82], suggesting a similar structure of the chain end, 18b. 

Unsymmetrical azo compounds must be used to generate phenyl radicals because azobenzene is very stable thermally. Phenylazotiiphenylmethane decomposes readily because of the stabihty of the triphenylmethyl radical ... 

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. 

Recent reports on the g factor and the coupling constants a of moderately stable radicals are shown below. A triphenylmethyl radical, which is generated by the reaction... 

Photochemically generated radicals undergo a subsequent photoinduced or thermal oxidation to the corresponding carbocation. Faria and Steenken produced the triphenylmethyl radical through two-photon ionization and subsequent decarboxylation of the triphenylacetate ion. Laser excitation of the triphenylmethyl radical resulted in photoinjection of an electron to give the triphenylmethyl cation and a solvated electron [Eq. (8)] [91]. 

Gomberg was the first to characterize a free radical when, in 1900, he generated triphenylmethyl radical 5 by reacting chlorotriphenylmethane (4) with zinc metal.l Triphenylmethyl radical 5 is unusual in that it is quite stable and its formation is probably the first experimental verification of a free radical. Frankland, however, may have been the first to generate transient methyl and ethyl radicals in the reaction of iodomethane and iodoethane with zinc, in 1849.2 In the last 30 years, attention has been focused on radicals, their reactivity, and their applications to organic synthesis. Excellent monographs by Davies and Parrott, Lazar et al., Hay, ... 

What occurred was a surprising reaction forming a colored solution. Addition of iodine, for example, produced triphenylmethyl iodide and a colorless solution. Gomherg had generated a stable, yet reactive, free radical—triphenylmethyl radical ... 

Eree radicals had been of interest since the middle of the 19th century. Gomberg s observation of the persistent triphenylmethyl radical (see chapter 1) added support to the case for the intermediacy of free radicals. In the case of Cl radical (or atom), only a minute steady-state concentration needs to be generated by the photochemical chain-initiating step CI2 + hv —> 2 Cl. Eree radicals are generally extremely reactive and shortlived, like Cl, rather than persistent, like Gomberg s triphenylmethyl... 

The tetraphenylethanes described above are symmetrical compounds used to generate the same two radicals by dissociation, while pentaphenylethane (28) is an unsymmetrical derivative, giving two different radicals, triphenylmethyl and diphenylmethyl radicals [138]. The former cannot initiate radical polymerization, but the latter is available as an initiating radical to produce the polymer 28, which can function as the polymeric iniferter [106]. 

Most of the living radical polymerizations using organic radicals as regulating persistent species involved nitroxides. Exceptions are triphenylmethyl and other carbon-centered radicals in the early work of Otsu and Braun.24,25 More recently, Chung showed that borinate radicals 10 formed by the thermal cleavage of in situ generated alkyl boryl peroxides (Scheme 31) can be employed to control methacrylate... 

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. 

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