Triphenylmethyl structure

The ortho-ring junction that converts the triphenylmethyl structure into that of the ion LX increases the stability of the carbanion but decreases that of the carbonium ion. It will be recalled that this structural modification of the triphenylcarbonium ion had about the same effect as the introduction of one to two nitro groups. 

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. 

AgN03, EtOH, Pyr, 90°, 1.5 h H2S, 47% yield. An 5-triphenylmethyl thioether can be selectively cleaved in the presence of an 5-diphenylmethyl thioether by acidic hydrolysis or by heavy-metal ions. As a result of the structure of the substrate, the relative yields of cleavage by AgN03 and Hg(OAc)2 can be reversed. 

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). 

Okamoto and his colleagues60) described the interesting polymerization of tri-phenylmethyl methacrylate. The bulkiness of this group affects the reactivity and the mode of placement of this monomer. The anionic polymerization yields a highly isotactic polymer, whether the reaction proceeds in toluene or in THF. In fact, even radical polymerization of this monomer yields polymers of relatively high isotacticity. Anionic polymerization of triphenylmethyl methacrylate initiated by optically active initiators e.g. PhN(CH2Ph)Li, or the sparteine-BuLi complex, produces an optically active polymer 60). Its optical activity is attributed to the chirality of the helix structure maintained in solution. 

An interesting X-ray structure of the highly strained, sp hybridized vinyl cation 8 was recently reported by Muller et al. The synthesis of 8 was accomplished by the reaction of alkynylsilane 7 and triphenylmethyl (trityl) cation. 

Polymerization of triphenylmethyl methacrylate in the presence of a chiral anion catalyst results in a polymer with a helical structure that can be coated onto macroporous silica [742,804). Enantioselectivity in this case results from insertion and fitting of the analyte into the helical cavity. Aromatic compounds and molecules with a rigid nonplanar structure are often well resolved on this phase. The triphenylmethyl methacrylate polymers are normally used with eluents containing methanol or mixtures of hexane and 2-propanol. The polymers are soluble in aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran which, therefore, are not suitable eluents. 

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 ... 

In the paper published in 1900, he reported that hexaphenylethane (2) existed in an equilibrium mixture with 1. In 1968, the structure of the dimer of 1 was corrected to be l-diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene 3, not 2 [38]. Since Gomberg s discovery, a number of stable radicals have been synthesized and characterized, e.g., triarylmethyls, phenoxyls, diphenylpicryl-hydrazyl and its analogs, and nitroxides [39-43]. The radical 1 is stable, if oxygen, iodine, and other materials which react easily with it are absent. Such stable radicals scarcely initiate vinyl polymerization, but they easily combine with reactive (short-lived) propagating radicals to form non-paramagnetic compounds. Thus, these stable radicals have been used as radical scavengers or polymerization inhibitors in radical polymerization. 

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. 

A structure that prevents coplanarity prevents resonance stabilization to a corresponding degree. Thus the triptycyl radical has none of the stability of triphenylmethyl and the benzene rings are tied back in a position with the very maximum deviation from coplanarity.16... 

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. 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

The optically active poly(TrMA) shows a large optical activity and intense circular dichroism (CD) due both to the triphenylmethyl group, indicating that this group has a chiral propeller structure, and to the helicity. Poly(TrMA) of degree of polymerization (DP) over 80 is insoluble in common organic solvents. 

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