The Effect of β-Fluorine Substituents on the Rate and Equilibrium Constants for the Reactions of α-Substituted 4-Methoxybenzyl Carbocations and on the Reactivity of a Simple Quinone Methide

The reactions of 4-MeOArCH(R)Cl (R = CH₂F, CHF₂, CF₃) and 4-MeOArC(CF₃)₂Y (Y = OTs and Br) with nucleophilic reagents were studied in 50:50 (v/v) trifluoroethanol/water. There is chloride common ion rate depression of the solvolysis of 4-MeOArCH(R)Cl. The large leaving group effect, k_OTs/k_Br = 9 × 10⁴, for the solvolysis of 4-MeOArC(CF₃)₂Y suggests that the reaction of the tosylate is accelerated by the relief of ground-state strain. The solvolysis of 4-MeOArC(CF₃)₂OTs gives a 33% yield of 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone which forms by the aromatic substitution reaction of water at 4-MeOArC(CF₃)²⁺. This quinone methide reacts slowly with solvent (t_1/2 ≈ 11 h) and rapidly with azide ion. All of these compounds react with azide ion by an SN1 mechanism. Values of k_az/k_s and k_a/k_t (M^-1) for the partitioning of the carbocation intermediates between capture by azide or chloride ion and solvent are reported. Absolute rate constants k, (s^-1) and ky (M^-1 s^-1, Y^- = Cl^-, Br^-) for the capture of 4-MeOArCR^*(R₂)⁺ were calculated from the values of k_a/k_t (M^-1) or k_az/k_y and an estimated value of k_az = 5 × 10⁹ M^-1 s^-1 for the diffusion-limited reaction of N₃. Values of K for the ionization of 4-MeOArCR^-(R₂)Y to give 4-MeOArCR₁(R₂)⁺ were calculated as the ratio of the microscopic rate constants k_solv/k_y. A single β-fluorine at 4-MeOArCH(CH₃)⁺ causes a 2-fold increase in ks, due to a small inductive substituent effect. Additional β-fluorine substituents cause k_s to decrease due to a steric effect; the value of k_s for the capture of 4-MeOArC(CF₃)²⁺ is 40-fold smaller than k_t for 4-MeOArCH₂⁺. The effect of β-fluorine substituents on log K are consistent with steric congestion at the more highly substituted substrates that destabilizes 4-MeOArCR^-(R₂)Y relative to 4-MeOArCR^-(R₂)⁺. These results support the proposal that the low reactivity of 4-MeOArCH(CF₃)⁺ toward solvent is due, in part, to a resonance effect.