Stability of the benzyl cation

This reflects the greater stability of the benzylic cation (32), though only secondary, compared with the tertiary cation (33) that would be—but in fact is not—obtained by its rearrangement (c/. p. 105). 

Combining the above values of pAR with the value for the p-methoxybenzyl cation measured by Toteva and Richard158 allows the effect of the three oxygen substituents on the stability of the benzyl cation to be compared in Scheme 22. The values of pAR may also be compared with effects of similar oxygen substitutions at the a-position of the benzyl cation from Table 3, which are also shown in Scheme 22. As expected, the relative magnitudes of the O-, HO, and MeO substituent effects exhibit similar patterns in the a- and... 

It has been shown that an electron-donating group on the aryl substituent promotes C-S bond cleavage because of better stabilization of the benzylic cation. In the case of R3 = R4 = Me, the aldehyde is obtained after hydrolysis of the imine (Scheme 26) <1998T8941>. 

In general, polymerizations with these salts are much more efficient than those with the unsubstituted salt. Considering that the structure of the propagating species and the rate of the polymerization (kp) are expected to be the same in all three cases, the enhanced activity may be attributed to the stabilization of the benzyl cation by the substituents. Similar effects were observed with the benzylic sulphonium salts. However, more detailed studies [30] of polymerization and hydrolytic properties of various p-substituted... 

Addition to conjugated alkenylbenzenes orientation. Stability of the benzyl cation... 

By a third mechanism, the reaction occurs with loss of stereochemistry at the metal-bound carbon. This stereochemistry is observed if reaction with the electrophile leads to a relatively stable carbocation. For example, the reaction of HgX in Equation 12.18 forms racemic a-methylbenzyl chloride from the optically active a-methylbenzyl complex, presumably due to the stability of the benzylic cation. 

FIGURE 13.71 The resonance stabilization of the benzylic cation makes its formation relatively easy. Accordingly, SnI reactions of benzylic compounds containing good leaving groups are quite fast. 

Since the allyl cation is stabilized by resonance with the double bond, similar to the stabilization of the benzylic cation by an aromatic ring, the reactivity of the corresponding allyl halide l-bromobut-2-ene TM 2.12a is enhanced, resembling that of benzyl bromide. The anionic C3 synthon we already met in the former example will appear in many of the disconnections that follow. 

For mesitylene and durene, the kinetics have been followed by specular reflectance spectroscopy [159]. The results indicated that mesitylene produces a fairly stable radical cation that dimerizes. That of durene, however, is less stable and loses a proton to form a benzyl radical, which subsequently leads to a diphenylmethane. The stability of the radical cation increases with increasing charge delocalization, blocking of... 

From the relative solvolysis rates of 213 and 215, Shimizu and coworkers calculated a stabilization by 7 kcalmoD1 of the benzyl cation by the /1-Si—Si bond compared with a /1-C—Si bond. Due to the antagonistic effects of the a- and the /)-silyl groups, the net effect of the disilanyl group is relatively small86. Thus, 213 is only 12 times as reactive as a-methylbenzyl bromide 219, X = Br (Table 7 in 30% acetone the relative rate is decreased to 0.66). This might be compared with the rate acceleration of 1.05 x 105 by... 

Why then are trityl cations still more stable than benzylic cations An aryl residue that is rotated out of the nodal plane of the 2py AO of the benzylic cation center by an angle % provides resonance stabilization that is decreased by cos2 -fold. Three aryl residues in a trityl cation can thus provide up to 3 x cos2(30°) = 2.25 times more resonance stabilization than one... 

The relative stabilities of substituted benzyl cations [21C ] are correlated by equation (27) with a high resonance demand parameter Tq = 1.29 (Mishima et al., 1987, 1995). The linear correlation for the whole range of substituents down to the 3,5-(Cp3)2 group (Fig. 27), contrasts with the concave Y-T plot (Fig. 7) of the solvolytic reactivities of [21]. Note that the Tq value for the gas-phase stabilities of [21C ] is identical with the r value assigned for the SnI solvolysis of [21] tosylates hence, the r value of 1.29 must be an intrinsic index inherent in [21C ], rather than a correlational artifact of a non-linear relationship for the complex solvolysis mechanism. 

Fig. 27 The Y-T plot of gas-phase stabilities of substituted benzyl cations [21C ] r = 1.29. For interpretation of symbols, see Fig. 1. Reproduced with permission from Mishima et al. (1995). Copyright 1995 Chemical Society of Japan.

For mesitylene and durene, the kinetics have been followed by specular reflectance spectroscopy [17]. The results indicated that mesitylene produces a fairly stable radical cation that dimerizes. That of durene, however, is less stable and loses a proton to form a benzyl radical, which subsequently leads to a diphenylmethane. The stability of the radical cation increases with increasing charge delocalization, blocking of reactive sites, and stabilization by specific functional groups (phenyl, alkoxy, and amino) [18]. The complex reaction mechanisms of radical cations and methods of their investigation have been reviewed in detail [19a]. Fast-scan cyclovoltammetry gave kinetic evidence for the reversible dimerization of the radical cations of thianthrene and the tetramethoxy derivative of it. Rate constants and enthalpy values are reported for this dimerization [19b]. 

The stability of a benzyl cation—relative to the compounds from which it is made—is also accounted for by resonance involving the benzene ring. Both the carbonium ion and the compound from which it is made are hybrids of Kekule structures. In addition, the carbonium ion can be represented by three other structures, I, II, and III, in which the positive charge is located on the ortho and para carbbn atoms. Whether we consider this as resonance stabilization or simply... 

We have depicted the pinacol rearrangement as a two-step process with an actual carbonium ion as intermediate. There is good evidence that this is so, at least when a tertiary or benzylic cation can be formed. Evidently the stability of the incipient cation in the transition state permits (SNl-lihe) loss of water without anchimeric assistance from the migrating group. This is, we note, in contrast to what happens in migration to electron-deficient nitrogen or oxygen. 

The reaction proceeds according to the principles of the r-amino effect (Scheme 11). A wide range of substituted benzyl derivatives followed this pathway. Debenzylation rather than decyanoethylation occurred—a fact attributable to the stabilization of the iminium cation that occurs in the former process, whereas destabilization would result in the latter. Furthermore, the reaction gave low yields with cation-destabilizing benzyl groups such as nitro- and difluoro-substituted derivatives. In the absence of the para-methyl group, normal para-formylation occurred and the perfluoro-phenylmethyl derivative (27, Ar = QH5) formylated normally in the ortho position. The quinazolinium intermediate 29 was mooted (see type 3 reactions for more information on this reaction). 

The study of substituent effects on the stability of substituted benzyl cations is important as a source of thermodynamic information as well as a means to provide a conceptual link between the stabilities of a number of important carbocations. Mishima s study, in particular, shows that the stabilities in terms of standard Gibbs energy for 4-methoxy- and 4-nitro-substituted benzyl cations differ by some 24.9 kcal mol The former has a stability comparable to that of bicyclo[3.3.3]undecyl (manxyl) cation, while the latter is comparable to the c-pentyl cation. We present these two extreme cases in Table 4. The relative stabilities of other substituted benzyl cations are given in Refs. 50 and 51. 

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