Radical triphenylmethyl

Von Baeyer (Nobel Prize, 1905) should be credited for having recognized in 1902 the saltlike character of the compounds formed. He then suggested a correlation between the appearance of color and salt formation—the so-called halochromy. Gomberg (who had just shortly before discovered the related stable triphenylmethyl radical), as well as Walden, contributed to the evolving understanding of the structure of related cationic dyes such as malachite green. 

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 ... 

The combination of carbon-centered radicals usually involves head-to-head (a,a ) coupling. Exceptions to this general rule occur where the free spin can be delocalized into a n-system. The classic example involves the triphenylmethyl radical (13) which combines to give exclusively the a-para coupling product (26), Scheme I.8).27 This chemistry is also seen in cross reactions of 13 with other tertiary radicals.146... 

Stable carbon-centered radicals, in particular, substituted diphenylmethyl and triphenylmethyl radicals, couple reversibly with propagating radicals (Scheme 9.11). With, the carbon-centered radical-mediated polymerization systems described to dale, the propagating radical should be tertiary (e.g. methacrylate ester) to give reasonable rates of activation. 

Otsu and Tazaki90 have reported on the use of triphenylmethylazobenzene (39) as an initiator. In this case, phenyl radical initiates polymerization and the triphenylmethyl radical reacts mainly by primary radical termination to form a macroinitiator. The early report91 that triphenylmethyl radical does not initiate MMA polymerization may only indicate a very low rate of polymerization. The addition of triphenylmethyl radical to MMA has been demonstrated in radical... 

It is of interest to speculate on the precise structure of the macroinitiator species in these polymerizations. The work of Engel et a .94 suggests the likelihood of a quinonoid intermediate (e.g. 45, Scheme 9.13), at least for the polymerizations involving triphenylmethyl radical (44). 

The first conclusive qualitative evidence for the relative stability of aryldiazenyl radicals was the isolation of phenylazotriphenylmethane (8.50) in the thermolysis of a-(phenylazo)cumene in the presence of triphenylmethyl radicals by Porter et al. (1978), as shown in Scheme 8-32. 

The radical dissociation of the Gomberg dimer , [3-(diphenyl-methylidene)-6-(triphenylmethyl)-l,4-cyclohexadiene] [48], is familiar to organic chemists as the original observation of carbon-carbon a bond dissociation in a solution (Gomberg, 1900 Lankamp et al., 1968). The yellow colour of the triphenylmethyl radical in the benzene solution should have been an observation convincing synthetic organic chemists of the stable existence of the triphenylmethyl radical [8-j. 

Carbon-centered organic radicals are highly reactive trivalent species with only one nonbonding electron. While most known radicals have their unpaired electron in a pure p- or a delocalized Ji-orbital, there are also examples of radicals centered in s/t" hybrid o-orbitals, such as the well known phenyl and cyclopropyl radicals. The first radical reported in the literature is credited to Gomberg s landmark paper in 1900 when he postulated the formation of triphenylmethyl radical 36, also known as tri-fyj 99,100 jj-jjyj j-adical is an example of a persistent radical that exists in equilibrium... 

Sodium triphenylmethane treatment of triphenylmethyl chloride with an excess of sodium in benzene leads, via the triphenylmethyl radical (Gomberg, 1900), to triphenylmethyl sodium ... 

The importance of establishing the correct structure of the reaction product is best illustrated by the confusion that can result when this has been assumed, wrongly, as self-evident, or established erroneously. Thus the yellow triphenylmethyl radical (3, cf. p. 300), obtained from the action of silver on triphenylmethyl chloride in 1900, readily forms a colourless dimer (m.w. = 486) which was—reasonably enough—assumed to be hexaphenylethane (4) with thirty aromatic ... 

The decomposition of hexaphenylethane to triphenylmethyl radicals in liquid chloroform has been studied at 0 °C. 

The situation with polyarylmethanes is very similar. Due to the stabilization of free valence in arylmethyl radicals, the bond dissociation energy (BDE) of the bond C—02 for example, in triphenylmethyl radical is sufficiently lower than in alkylperoxyl radicals. This radical is decomposed under oxidation conditions (room temperature), and the reaction of Ph3C with dioxygen is reversible ... 

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. 

Here the radical 1 acts as a strong terminator to prevent the formation of oligomers and polymers. On the other hand, it is expected that the substituted diphenylmethyl radicals which are less stable than 1 serve as both initiators and primary radical terminators. In fact, it was reported [84] that the apparent polymerization reactivities decreased in the following order diphenylmethyl, phenylmethyl, and triphenylmethyl radicals, which were derived from the initiator systems consisting of arylmethyl halides and silver. 

The dissociation is the slow step that fixes the rate of the over-all oxidation reaction, but only if an inhibitor is added to remove the intermediate triphenylmethylperoxy radicals. Otherwise they contribute to the disappearance of the ethane by attacking it directly rather than waiting for additional free triphenylmethyl radicals. 

Since triphenylmethane is not stabilized by any resonance not already present in hexaphenylethane, the difference between the two heats of hydrogenation, or 22 kcal., might be a measure of the steric effect alone. The difference in the heats of dissociation into radicals when ethane and hexaphenylethane are compared is 62 kcal. This leaves about 40 kcal. to be accounted for as resonance stabilization of the radical.16 This degree of resonance stabilization for the triphenylmethyl radical does not violate quantum mechanical expectations. 

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. 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

However, when m-DNB was added to a solution of triphenylchloromethane and potassium tcrt-butylate in 2,2-dimethoxypropane, the yield of the substitution product and dimer of the triphenylmethyl radical markedly increased and decreased, respectively (Simig and Lempert 1979). Therefore, the main pathway of the reaction does not involve the ion-radical step. These authors suggested an alternative pathway, which is conformed by a thorough structural analysis of the secondary products formed along with tert-butyl ester of triphenylcarbinole (Huszthy et al. 1982a, 1982b) (Scheme 4.21). 

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