Gas-phase substituent effects in highly electron-deficient systems. II. stabilities of 1-aryl-2,2,2-trifluoroethyl cations based on chloride-transfer equilibria

Article abstract of DOI:10.1246/bcsj.70.1163

The relative stabilities of 1-aryl-2,2,2-trifluoroethyl cations were determined based on the chloride ion-transfer equilibria in the gas phase. An application of the Yukawa-Tsuno equation to this substituent effect on the equilibrium constants gave a remarkably larger r⁺ of 1.53 and a ρ of-10.6, supporting our previous conclusion that the highly electron-deficient benzylic carbocation systems are characterized by extremely high resonance demands. This r⁺ value, furthermore, conformed a linear relationship between the r⁺ value and the relative stability of the unsubstituted member of the respective benzylic carbocations, clearly demonstrating a continuous spectrum of varying resonance demands characteristic of the stabilities of carbocations. The π-delocalization of the positive charge into the aryl π-system increases with the destabilization of a carbocation by the α-substituent(s) linked to the central carbon. In addition, the r ⁺ value of 1.53 for 1-aryl-2,2,2-trifluoroethyl cations was found to be in complete agreement with that for the solvolysis of 1-aryl-2,2,2- trifluoroethyl tosylates in 80% aq acetone. This reveals that the r⁺ value observed for this solvolysis must be the intrinsic resonance demand of a highly electron-deficient cationic transition state in the S_N 1 ionizing process. The identity of the r⁺ value was consistent with our previous observation for other benzylic carbocation systems, indicating that the degree of the π-delocalization of the positive charge is identical between the cationic transition state and an intermediate cation for all benzylic systems, which cover a wide range of reactivity and stability of the carbocation. This leads us to the conclusion that the geometry of the transition state in the ionizing process of the S_N1 solvolysis, which is a highly endothermic reaction, closely resembles the high-energy product, an intermediate cation.