Trityl ion

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). 

In contrast to the findings of van Tamelen et al., whose experiments employed carbonium ion concentrations of 10 M, irradiation of the trityl ion in somewhat lower concentrations ( 10 m) in 99% sulfuric acid in the absence of hydrogen donors with oxygen present yielded only two products, ions 109 and 110 (Allen and Owen, 1971). An excited triplet carbonium ion is again invoked, which is thought to give rise to the intermediate 111. This species can then lose an allylic hydride ion... 

The chemistry of the 1 1 and 1 2 complexes differs with respect to hydrogenation (84,89). The 1 2 derivatives are inert to hydrogenation, while the 1 1 compounds are smoothly transformed into an ethylidene complex (see Scheme 1). This difference in behavior may well reflect the cause of differences in behavior of olefins on metal surfaces toward hydrogenation. The ethylidene complex may be converted back to the olefin adduct by reaction with trityl ion. The ethylidene adduct was first obtained for ruthenium by interaction of ethylene with H RujfCO) (89), and is structurally related to the corresponding cobalt derivatives, Co3(CO)9RC. As discussed above, the structure has been established in detail and involves a capping of the metal triangle... 

A related dienyl species may be obtained by reaction of the diene complex with trityl ion, when hydride ion abstraction occurs from the diene to yield a dienyl group coordinated to one metal center, [(C6H7)Os3(CO)10]+ (153). 

Results obtained by ourselves on the ions 5, 7, 11 and 13 (see Table 1) in MSA and DCM, and by Volz and Lotsch [12] in CM, are listed in Table 3 and plotted in the Figure 1, normalised against EV2 of the trityl ion, as suggested in the previous section. A remarkable feature which emerges from this representation of the results is that Ej in... 

The complex formation with monomer also provides a simple and logical explanation for the decrease of the rate-constant for the attack of a cation, R+, such as a trityl ion from an initiator, on a monomer, with increasing m this can be shown thus ... 

Dirarylmethyl and triarylmethyl ions (trityl ions) are even more stable than the /er/-butyl ion which is impressively demonstrated by the commercial availability of solid [Ph3C] [BF4] and similar salts. Triphenylchloromethane dissociates in polar, inert solvents such as SO2, and therefore, it is not surprising that El spectra of triphenylmethyl compounds almost exclusively exhibit this ion together with some of its fragments, whereas the molecular ion peak is usually absent. Field desorption allows to circumvent this problem (Chap. 8.5). 

Example The extraordinary stable trityl ion, PhsC, m/z 243, tends to dominate mass spectra (Chap. 6.6.2). Thus, neither the El spectrum of chlorotriphenyl-methane nor that of its impurity triphenylmethanol show molecular ions (Fig. 8.11). An isobutane PICI spectrum also shows the trityl ion almost exclusively, although some hint is obtained from the Ph2COH ion, m/z 183, that cannot be explained as a fragment of a chlorotriphenylmethane ion. Only FD reveals the presence of the alcohol by its molecular ion at m/z 260 while that of the chloride is detected at m/z 278. Both molecular ions undergo some OH or Cl loss, respectively, to yield the Ph3C fragment ion of minor intensity. 

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. 

The dilemma presented by these conflicting results was resolved by TaShma and Rappoport.265 They pointed out that the apparent dependence of kAz/ knl0 upon the reactivity of the carbocation arose because even the most stable cation reacting with azide ion did so at the limit of diffusion control. Thus while kn2o remained dependent on the stability of the cation in the manner illustrated in Fig. 7 the rate constant for the azide ion remained unchanged. Thus the most stable cation formed in the solvolysis reactions was the trityl ion, for which direct measurements of kn2o = 1 -5 x 105 s 1 and kAz = 4.1 x 109 now show that even for this ion the reaction with azide ion is diffusion controlled.22... 

Trityl ion effectively initiates polymerization of olefins having strongly electron-releasing substituents, especially vinyl alkyl ethers (2, 16),... 

Total trityl ion]e = [Ph3C+]e + [Ph3C+SbCl6-] , values are corrected for density change and quoted as at 25 °C. The sealed system was cooled and reheated in the sequence shown descending the first column. b Before adding dioxane. 

This kind of conformational dependence has been observed for many polyaryl systems. However, no explicit consideration has been given to its implications for the interpretation of correlations even in a qualitative manner. For this type of analysis, the trityl ion series [3C ] involves less than a sufficient number of substituents. Although the benzhydryl solvolysis series has sufficient substituent sets, the change in rotation appears too small to estimate the effect quantitatively. In practice, the solvolyses of a-trifluoromethyl-diarylmethyl tosylates [29C (X,Y)J best illustrate this analysis. 

Macroscopic solvent effects can be described by the dielectric constant of a medium, whereas the effects of polarization, induced dipoles, and specific solvation are examples of microscopic solvent effects. Carbenium ions are very strong electrophiles that interact reversibly with several components of the reaction mixture in addition to undergoing initiation, propagation, transfer, and termination. These interactions may be relatively weak as in dispersive interactions, which last less than it takes for a bond vibration (<10 14 sec), and are thus considered to involve "sticky collisions. Stronger interactions lead to long-lived intermediates and/or complex formation, often with a change of hybridization. For example, onium ions are formed with -donors. Even stable trityl ions react very rapidly with amines to form ammonium ions [41], and with water, alcohol, ethers, and esters to form oxonium ions. Onium ion formation is reversible, with the equilibrium constant depending on the nucleophile, cation, solvent, and temperature (cf., Section IV.C.3). 

There is some spectroscopic evidence that aromatic compounds complex carbenium ions [42]. For example, the complexation equilibrium constant between trityl ions and hexamethylbenzene is K = 68 mol-1 L at 0° C [43]. Complexation should be stronger with more electrophilic carbenium ions such as those derived from styrene and a-methylstyrene. On the other hand, the monoalkyl-substituted phenyl rings attached to the polymer chain are weaker nucleophiles than hexamethylbenzene. A complexation constant K = 4 mol 1 L was reported for trityl cation and styrene [43]. Similar complexes have been proposed to explain the red color observed in inifer systems based on l,4-bis(I-chIoro-l-methyl-ethyl)benzene and BCI3 in CH2C12 at low temperature [44],... 

Trityl (triphenylmethylium) and tropylium cations are two commercially available carbenium ions. They react with alkenes by either direct addition or by hydride abstraction tropylium ions are usually less reactive than trityl. As shown in Eq. (39), trityl ions initiate styrene polymerization by direct addition. 

Before discussing the "trityl" ion it is important to note that the methylpentane isomerization is slow in fresh acid (there s an induction period as with 2,3,4-trimethylpentane), but may be accelerated by the addition of small amounts of an olefin like 2-methyl-l-butene. However, the reaction initiated by the olefin is rapidly quenched as the acid reduces a momentarily high intermediate ion concentration to a steady state level. The reduced rate is now a measure of the hydride transfer rate in the acid. The data to be discussed were obtained at 23°C with emulsions containing equal volumes of methylpentane and 95 percent H2S0. ... 

Fig. 15 Non-linear substituent effects on the pi R+ for triarylmethanols. The non-linearity is shown schematically as the plots against sum of Y-T a with r = 0.76., trisaryl (X = Y = Z) carbocations (as reference) O, bisaryl (X = Y Z = H) , monoaryl (X Y = Z = H) A, bis-(p-MeO)-trityl ions (X Y = Z = p-MeO) series. Data taken from Deno and Schriesheim (1955), Deno et al. (1955, 1959), Deno and...

After Dauben s observation that trityl perchlorate produced 9-phenylfluorene upon day-li t exposure van Tamelen and Cole systematically studied the photolysis of the trityl ion in various acidic media with visible light from a medium-pressure mercury... 

The higher stability of these ionic species compared to trityl ions is directly proved by the fact that tropylium salts can by prepared by hydrogen abstraction from cyclohep-tatriene induced by trityl salts An excellent account of the reactions of the tropylium ion has been published by Harmcm ... 

Within the context of their invest ation Vairon and Villesai e observed that the interaction of cyclopentadiene with trityl hexachloroantimonate in carbon tetrachloride or benzene produced the rapid disappearance of the characteristic yellow colour of the trityl ion but no polymerisation. This interesting phenomenon of false initiation has also been reported in some systems involving styrene and will be disaissed when we deal with this monomer. 

The polymerisation of this monomer by stable carbenium salts was first reported by Sauvet et who only remarked that at —70°C in methylene chloride, about 30% yield was obtained in two hours with a 10 M concentration of trityl hexachloroanti-monate. In the same year, Higashimura et al. measured the rate of interaction between trityl pentachlorostannate and a-methylstyrene at 30-60°C in ethylene chloride and mixtures of this solvent with benzene. They monitored the disqipearance of the characteristic visible bands of the trityl ion as the reaction proceeded. Good first order plots were obtained and the external order in monomer was found to be unity so that kinetically the reaction was bimolecular with a kj of 1.3 min at 30°C in pure eth-... 

Reports on the possibility of inducing the polymerisation of styrene by trityl salts are among the oldest in this field . However, surprisingly enou these processes have not yet been fully explored. Styrene is rather reluctant to react with the trityl ion and does not react at all with the tropylium ion. The latter observation has been reported by Ledwith and Sherrington vidro describe an experiment in diichp-methoxystyre-... 

Sambhi returned to the HgCl2 —PhaCCl system in 1970 . For this investigation of the polymerisation of styrene, both the decay of trityl ion (413 nm) and that of styrene (291.5 nm) were followed spectroscopically in ethylene chloride at 30°C. No special precautions were reported to exclude adventitious moisture from the reaction media. A plot of trityl ion concentration vs. the product [PhsCCl] x [HgCl2 ] was linear indicating that ion pairs were the predominant species. The rest of the kinetic treatment was entirely similar to that given in the previous paper by Sambhi and Treloar... 

The extreme slowness of these polymerisations makes one wonder if the trityl ion (thought to be present exclusively as the pair PhaC HgCl in the above two japers, but see below for a reassessment of this conclusion) was the real initiator and not the weak Lewis acid used to form it. No blank experiments were reported with mercuric chloride alone as catalyst, to verify the absence of polymerisation at the concentrations used for the study with trityl chloride. One could well envisage that in the presence of traces of moisture of hydrogen chloride (note that HgCl2 was used without purification) this Lewis acid, present in concentrations of about 10 M, could have catalysed the polymerisation of styrene with half lives of tens of hours. 

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