Reactions of Ring-Substituted 1-phenyl-2,2,2-trifluoroethyl Carbocations with Nucleophilic Reagents: A Bridge between Carbocations Which Follow the Reactivity-Selectivity “Principle” and the N₊ Scale

The effect of changing carbocation reactivity on nucleophile selectivity has been determined for the reactions of ring-substituted 1 -phenyl-2,2,2-trifluoroethyl carbocations, XArCH(CF₃)⁺, with amines, alcohols, and carboxylate ions. Rate constants, k_s, for the capture of XArCH(CF₃)+ by 50/50 (v/v) trifluoroethanol/water range from 1 × 10¹⁰ s^-1 for 4- MeArCH(CF₃)⁺ to ≤200 s^-1 for 4-Me₂NArCH(CF₃)⁺. β_nuc = 0.29 was determined for the reaction of alkylamines with 4-Me₂NArCH(CF₃)+. β_nuc for reaction of RCO₂^- decreases from 0.35 for 4-Me₂NArCH(CF₃)⁺ to 0.05 for 4-MeOArCH(CF₃)+. This decrease is due, at least in part, to a Hammond effect on the location of the reaction transition state along the reaction coordinate. β_nuc for reaction of alcohols decreases from 0.48 for 4-Me₂NArCH(CF₃)⁺ to 0.09 for 4-MeArCH(CF₃)⁺. The plot of log (k_EtOH/k_TFE) for capture of XArCH(CF₃)⁺ by ethanol and trifluoroethanol against log k_s has a shallow negative slope for the more stable XArCH(CF₃)⁺, which steepens with destabilization of the carbocation. This change in slope is due, in part, to a change in the magnitude of the Hammond effect, which corresponds to a third-derivative structure-reactivity effect, p*_yyy′ = ∂p_yy′/-∂σ > 0. There is considerable overlap between the reactivities of the most unstable triarylmethyl carbocations and the most stable XArCH(CF₃)⁺, and there are also marked similarities in the reactivity-selectivity behavior of these species in the region of this overlap. Models are considered to explain the spectrum of reactivity-selectivity behavior that is observed on moving from very unreactive to very reactive carbocations.