2-Arylethyl cations

The results of analysis based on the gas-phase Y-T equation (27) using gas-phase substituent constants given in Table 15 are summarized for proton-transfer equilibria of ethylenes and acetylenes in Table 16. As an example, the Y-T plot for a-CFa-a-arylethyl cations [5C ] in Fig. 25 may be compared with the plot for the corresponding solvolysis in Fig. 3. The AAG(cc)h for the a-neopentyl-a-r-butyl series [4C" ] has been found to correlate linearly with the gas-phase Y-T equation (27) with ro = 0.81 (Fujio et al., 1997a). a-f-Butyl-a-methylbenzyl cations [18C ] also showed a reduced ro value of 0.89 (Mishima et al., 1992b), a value which is close to r = 0.91... 

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. 

The behaviour of a-arylethyl cations has been analysed by Richard and coworkers (Richard etal., 1984 Richard and Jencks, 1984a,b,c) using the azide clock technique. Combination of the rate constants for the reaction of the carbocations with water and the acid-catalysed cleavage of 1-phenylethanols... 

Full details of a study of leaving group-promoted solvolytic elimination reactions of 1-(1-methyl-l-arylethyl)pyridinium cations in 25 vol.% acetonitrile (aqueous) have been reported. Reactions of (34) and (35) are found to proceed via a common carbocation intermediate of ion-molecule pair type to give the suhstitution product (36) and elimination product (37) (Scheme 4). The total rate of reaction of (35) exceeds that for (34) by 1100-fold, corresponding to a Bronsted parameter of )S g = —0.93, and the fraction of (37) obtained is governed by = 0.12 for the dehydronation (kg) of the ion-molecule pair by the leaving group the product ratio is hardly affected hy the presence of substituted pyridines. For (34) and (35), = 1.85 0.10 (60 °C) and... 

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