The Extraordinarily Long Lifetimes and Other Properties of Highly Destabilized Ring-Substituted 1-Phenyl-2,2,2-trifluoroethyl Carbocations

A large bromide common ion rate denression is observed for the reactions of 1-(4-methoxyphenyl)-and 1-(4-(methylthio)phenyl)-2,2,2-trifluoroethyl bromides in largely aqueous solutions when the ionic strength is maintained with weakly nucleophilic NaClO₄ but not when it is maintained with NaN₃. The rates of the reactions of ring-substituted (σ⁺ ≤ -0.32) 1-phenyl-2,2,2-trifluoroethyl tosylates, mesylates, and bromides in 50:50 (v/v) water/trifluoroethanol, at increasing [I^-] or [N₃ ^-], are independent of nucleophile concentration, but at 0.5 M [Nu] the reactions give yields of the nucleophile adduct which range from 30% to quantitative (»99%), depending on the ring substituent. These results show that the highly unstable carbocations XArCH(CF₃)⁺ are readily generated as intermediates of the solvolysis reactions of XArCH(CF₃)Y. The rate constant ratios for partitioning of XArCH(CF₃)⁺ between capture by azide and a solvent of 50:50 (v/v) water/trifluoroethanol (k_az/k_s), calculated from product ratios, decrease from 6 × 10⁶ M^-1 for capture of 4-N(CH₃)₂ArCH(CF₃)⁺ to 0.8 M^-1 for capture of 4-CH₃ArCH(CF₃)⁺ and give k_s values ranging from 800 s^-1 to 6 × 10⁹ s^-1, assuming a diffusion-controlled rate constant of 5 × 10⁹ M^-1 s^-1 for k_az. There is a modest correlation of log k_s values to σ⁺ with a slope ρ⁺ _s = 4.8. The k_s values for the reactions of XArCH(CF₃)⁺ are nearly identical with k_s values for comparably ring-substituted XArCH(CH₃)⁺ despite the fact that the former carbocations are at least 8 kcal/mol more unstable relative to a neutral reactant. It is proposed that XArCH(CF₃)⁺ possess an unusual kinetic stability, which is due to unusually large resonance, electrostatic, or homoconjugative barriers to XArCH(CF₃)⁺ capture by solvent.