Triarylmethyl carbocations

Figure 1.3. Rate-equilibrium correlation for hydration of carbocations triarylmethyl and 9-aryl-9-fluorenyl ( and , respectively, slope =—0.60) diarylmethyl (A, —0.54) aryltropylium (O, —0.68) 9-xanthylium and cyclic phenyldialkoxycarbocations ( and A, respectively, slope = —0.63).

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 wide range of caibocation stability data has been obtained by measuring the heat of ionization of a series of chlorides and cafbinols in nonnucleophilic solvents in the presence of Lewis acids. Some representative data are given in Table 5.4 These data include the diarylmediyl and triarylmethyl systems for which pX R+ data are available (Table 5.1) and give some basis for comparison of the stabilities of secondary and tertiary alkyl carbocations with those of the more stable aryl-substituted ions. 

The cleavage is an S jl reaction that occurs by protonation of the oxygen atom followed by loss of the stable triarylmethyl carbocation. 

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 presence of solvent alone, the lifetime of the intermediate of the stepwise reaction of X-l-Y in the narrow borderline between the S l and Sn2 substitution reactions of azide ion (—0.32 < a" " < —0.08, Fig. 2.2) is 1/ = 10 ° s. Azide ion is 10°-10 -fold more reactive than water toward triarylmethyl carboca-tions and related electrophiles, and this selectivity is independent of carbocation reactivity, so long as the reactions of both azide ion and solvent are limited by... 

Jencks and Richard, and others, had pioneered the use of the azide elock to quantitatively assess the lifetime of carbenium ions generated under solvolytic conditions. The method relies on the use of product yield data collected at varying [N3 ] to determine the N /solvent selectivity, expressed as the ratio of the second-order rate constant for trapping of the ion by N3 and the pseudo-lirst-order rate constant for trapping of the ion by solvent k Jk. The assumption is made that k z is diffusion limited at ca. 5 x 10 M"" s This assumption allows k to be estimated, and l/kg provides the lifetime of the ion in the solvent in the absence of added nucleophiles. McClelland and Steenken showed by direct measurement of k. , for a series of diarylmethyl and triarylmethyl carbocations that k is approximately constant at... 

In general, og k /k ) or log s for 1-arylethyl, cumyl, diarylmethyl and triarylmethyl carbocations correlate well with or The plots of... 

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 ... 

Most carbocations are too reactive to be directly observable in ordinary solvents, and until relatively recently evidence has been obtained indirectly, primarily through the study of reaction kinetics and trapping processes, experiments discussed in Sections 5.1, 5.2, and 5.4. Nevertheless, a few types of compounds have long been known to produce observable concentrations of positive ions relatively easily. The triarylmethyl derivatives were the first of this type to be investigated the halides ionize readily in non-nucleophilic solvents such as sulfur dioxide,70 and the alcohols yield solutions of the ions in concentrated sulfuric acid. Early observations by the freezing-point depression technique (see Section 3.2, p. 130) established that each mole of triphenyl carbinol yields 4 moles of ions in sulfuric acid, the reaction presumably being by way of Equation 5.14.71 Results in methane-sulfonic acid are similar.72... 

The cryoscopic method is also applicable to other triarylmethyl systems, to some diarymethyl and allylic ions, and, when ortho substituents are present, to aryl acylium ions (20) (Equation 5.15) 73 unfortunately, side reactions frustrate most attempts to generate carbocations in sulfuric acid. 

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... 

Table 3 Rate and equilibrium constants and intrinsic reaction barriers91 for the addition of nucleophiles to the quinone methide 48 and the triarylmethyl carbocation (PhC+) in water at 25°C...

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 correlations, and especially the r values, must be intimately related to the dihedral angle between the substituted phenyl ring and the plane of the carbocation. In fact, the symmetrically substituted series exhibits a linear Y-T correlation with p and r values intermediate between those for symmetrically substituted triarylmethyl and diarylmethyl cations. In the unsymmetrical (X Y) series, the p-MeO group appears to deviate from the correlation, as indicated by the SD values, and a lower r value should be assigned for these systems. The non-linear Y-T correlations with rather lower r values for protonation of diarylethylenes probably result from the conformation of the carbocation intermediates and nearly similar results are also observed in bromination. Both reactions will be considered together in a later section (p. 334). 

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... 

The results are appreciably different from those based on the analysis of the reactions of monosubstituted triphenylmethyl cations (McClelland et al., 1989). Obviously, this is due to the difficulties of correlating the substituent effects in a simple manner. The preferred results on the trisarylmethyl cation lead to the conclusion that both the forward process of the k ionization and the reverse process of the solvent-recombination step of the carbocation with various nucleophiles can be described to a good approximation by a Y-T a scale with an r value of the transition state which is essentially identical with the intrinsic r value of the thermodynamic stabilities. However, we will consider the triarylmethyl system further following similar analysis of a-arylethyl cations. 

Returning to the triarylmethyl carbocations, we saw that the relationship between log and pX R has been investigated (Arnett and Hofelich, 1983 ... 

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. 

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 units with a small amount... 

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