Triarylmethyl cations

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic UV absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylearbinols and the corresponding carbocations. 

The diarylmethyl cations listed in Table 5.1 are 6-7pATr+ units less stable than the corresponding triarylmethyl cations. This indicates that the additional aryl group has a cumulative, although not necessarily additive, effect on stability of the carbocation. Primary benzylic cations (monparylmethyl cations) are generally not sufficiently stable for determination of pATr+ values. A particularly stable benzylic ion, the 2,4,6-trimethyl-phenylmefliyl cation, has a pATr+ of — 17.4. 

A number of benzylic cations have been obtained in solution as SbFg salts, Diarylmethyl and triarylmethyl cations are still more stable. Triphenylchloro-methane ionizes in polar solvents that do not, like water, react with the ion. In SO2, the equilibrium... 

Triarylmethyl cations (14) are propeller shaped, though the central carbon and the three ring carbons connected to it are in a plane The three benzene rings cannot all be in the same plane because of steric hindrance, though increased resonance energy would be gained if they could. 

For a review of crystal-structure determinations of triarylmethyl cations and other carbocations that can be isolated in stable solids, see SundaraUngam, M. Chwang, A.K. in Olah Schleyer, Ref. 2, vol. 5, p. 2427. 

In the older literature there are reports of colorless but conducting solutions of triarylmethyl derivatives. It seems well-established that triarylmethyl cations are colored, but an oxonium ion formed by the covalent attachment of a carbonium ion to an ether oxygen atom, for example, would be colorless. 

Ritchie was the first to directly measure the absolute reactivity of cations toward solvent and added nucleophiles. The cations were highly stabilized examples, triarylmethyl cations bearing stabilizing substituents such as 30 and 31, xanthylium ions (e.g., 32) and tropylium ions (e.g., 33). The feature (and requirement) of these cations was that they had a lifetime in water such that kinetics could be followed by conventional or stopped-flow spectroscopy whereby one solution containing the pre-formed cation was added to a second solution. The time required to mix these solutions was the important factor and limited measurements to cations with lifetimes longer than several milliseconds. The lifetimes in water for 30-33 are provided below. Lifetime is defined as the reciprocal of the first-order rate constant for the decay of the cation in solvent. 

Even for a series with varying aromatic substituents, the correlations with deviate significantly from linearity. Typical behavior is illustrated with data for monosubstituted triarylmethyl cations in Figure 1.2. Significant deviations are observed in the points for the para n donors. Moreover, these deviations are in the direction that indicates that these substituents have kinetic stabilizing effects greater than indicated by ct+. In fact, there are good correlations with a parameter based on NMR chemical shifts of benzylic-type cations obtained under stable ion conditions. 

Triarylmethyl cations are among the most stable carbocations known. They are intensely colored and are formed readily when the corresponding triaryl-methanols are dissolved in strong acids ... 

Several types of carbocation have proved to be stable when entrapped in a zeolite cage. For instance, (64) and related dimeric cations can be prepared from indene,105 and (65) and several other cations can be prepared from 4-vinylanisole, some of the latter being stable for several weeks.106 Absolute lifetimes of cumyl cations (66) have been measured in non-acidic zeolites, and their reactivities with co-absorbed alcohols have been studied.107 Triarylmethyl cations can be prepared by ship-in-a-bottle syntheses in large-pore zeolites.108... 

Compared to simple alkyl cations, triarylmethyl cations are well characterized in terms of structure and stability, and react with a number of readily oxidized species including aliphatic amines (El tsov et al., 1978) and ethers (Kabir-ud-Din and Plesch, 1978). Their reduction by alcohols in acidic medium have an historic importance in the area (Bartlett and McCollum, 1956), and the use of triarylmethanes as hydride carriers is an area of continuing interest (Ivanov et al., 1982). 

Primary kinetic hydrogen isotope effects in the reductions of triarylmethyl cations by formate in aqueous trifluoroacetic acid have been determined (Stewart and Toone, 1978). For 16 cations with pK + ranging from —2.25 (4-methoxy-4 -methyl) to — 7.63 (4,4 -dichloro), plots of log kH and log kD against pA"R+ show poor linear fits with the best straight lines (least squares) having slopes of 0.473 (r = 0.973) for the H-data and 0."491(r = 0.983) for the D-data. Both plots show downward curvature, with the effect being more emphatic for the H-data. Least squares fit to quadratics better... 

Like most photochemical reactions, benzylic fragmentation has been known from the beginning of the twentieth century. The photoinduced cleavage of triphenylmethylcarbinol has been noted by Gomberg in Ann Arbor in 1913 [1] and the generation of triarylmethyl cationic dyes from their leuco form by Lifschitz in Zurich in 1919 [2]. A solution of the colorless derivative obtained from p-rosaniline and cyanide became colored in a few seconds when exposed to an iron arc lamp (Sch. 1, old and new notation). 

Thermal and photochemical decomposition of triarylmethyl azides proceeds by loss of molecular nitrogen. In contrast, the electron impact-induced fragmentation occurs by loss of N3, possibly because of the great stability of triarylmethyl cations. However, decomposition of the molecular ion of azidophenylacetic acid involved loss of N2 and CO2 as in the photochemical reaction (138). In the mass-spectrometric... 

Solvolysis of 1-arylethenyl sulphonates forming vinyl cations 303 Highly electron-deficient carbocation systems 304 Carbocation formation equilibria 315 Triarylmethyl cations 315 Benzhydryl cations 319 1,1-Diarylethyl carbocations 322... 

The pXr values of diarylmethyl cations [22C ] were basically those determined according to the same method as for the triarylmethyl cations in aqueous sulphuric acid solutions by Deno and coworkers (Deno et ai, 1955 Deno and Schriesheim, 1955 Deno and Evans, 1957). For reasons of solubility, a wide set of pXr+ values in aqueous acetic acid solutions were determined by Mindl and coworkers (Mindl and Vecera, 1971, 1972 Mindl, 1972). Unfortunately, however, the pXr+ values, determined by means of the protonation-dehydration method for diarylmethanols, could only be obtained for more stable derivatives and not for those less stable than the unsubstituted... 

The rate constants for the solvent-recombination process of the carbocations [3C (X,Y,Z)] were determined by the use of the azide clock method (Richard etal., 1984 Richard and Jencks, 1984a,b,c McClelland et al., 1991) and the rate constant of the forward reaction was derived using (38b) as /Ch = /CwXr+ (McClelland et al., 1989,1991). While ordinary Hammett-type relationships were found to be inapplicable to the substituent effects on solvent recombination, there is a rate-equilibrium correlation for all available data on triarylmethyl cations, shown as the linear log/c , vs. p/Cr<- plot, in Fig. 34 with a slope of 0.64. Such a relationship was earlier suggested by Arnett and Hofelich (1983) and Ritchie (1986). The correlation of ky, with the cr scale was... 

Ritchie, 1986 McClelland et al., 1989, 1991) for a wide set of triarylmethyl cations, and that there is a reasonably linear correlation encompassing the entire set of triarylmethyl carbocations over 16 p/Cr+ units with a small amount of scatter (Fig. 34). The implication of this behaviour is that despite a change in the cation stability, there is a little change in the apparent position of the transition state, at least as revealed in this rate-equilibrium relationship. 

The earlier review drew attention to photo-oxidation reactions which ensue on irradiation of complexes formed between hydrocarbons and oxygen, sulphides and oxygen, and amines and oxygen. Triarylmethanes are photo-oxidised (38) to give triarylmethyl cations (Kuder et ai, 1979). The ease... 

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