Gas-Phase Substituent Effects in Highly Electron-Deficient Systems. I. Intrinsic Stabilities of 1-Aryl-1-(trifluoromethyl)ethyl Cations

The relative stabilities of 1-aryl-1-(trifluoromethyl)ethyl cations were determined by measuring the proton-transfer equilibria of 1-aryl-1-(trifluoromethyl)ethylenes or the chloride-transfer equilibria of 1-aryl-1-(trifluoromethyl)ethyl chlorides in the gas phase. The stability of 1-phenyl-1-(trifluoromethyl)ethyl cation was found to be 16 kcal mol−1 lower than that of the α-cumyl (1-phenyl-1-methylethyl) cation. The substituent effect on the stability of this cation can be correlated in terms of the Yukawa–Tsuno equation, giving an r+ of 1.41 and a ρ of −10.0 (in log K/Ko−1 unit). While the ρ value is nearly identical to that of the α-cumyl cation series, the r+ value is remarkably higher than the value of unity for the α-cumyl cation, indicating that such a highly electron-deficient carbocation system should be characterized by an extremely large r+ value compared with that of the stable α-cumyl cation. In addition, this r+ value agrees with that for the SN1 solvolysis of 1-aryl-1-(trifluoromethyl)ethyl tosylates. Such agreement of the r+ values between the gas phase and solvolysis reactions has been generally observed for the benzylic carbocation systems. It is concluded that the enhanced r+ value for the solvolysis of 1-aryl-1-(trifluoromethyl)ethyl tosylates must reflect the intrinsic resonance demand characteristic of the parent 1-phenyl-1-(trifluoromethyl)ethyl cation, itself, and that the extremely large ρ+ values given by a simple correlation with σ+ (r+ = 1) are an artifact due to an improper analysis of underestimating the resonance demand for such highly deactivated substrates.