Reactions of Substituted 1-Phenylethyl Carbocations with Alcohols and Other Nucleophilic Reagents¹

Selectivities of a series of substituted 1-phenylethyl carbocations toward alcohols and other nucleophiles have been determined by product analysis. The l-(4-(dimethylamino)phenyl)ethyl carbocation exhibits a high selectivity in its reactions with alcohols, with k_EtOH/k_TFE = 140 and β_nuc = 0.5. The selectivity for activation-limited reactions with alcohols decreases progressively with increasing reactivity of the carbocation, in contrast to the behavior expected from the N⁺ scale of reactivity. A sharper drop in selectivity for carbocations that react faster than ∼ 10⁹ s⁻¹ is attributed to an approach to limiting rate constants for the more reactive alcohol. The limiting selectivity of k_EtOH/k_TFE = 2 for carbocations with k_s ∼ 10¹¹ s⁻¹ may represent reaction from a pool of solvent molecules in which there is a modest charge-dipole interaction between the alcohol and carbocation. The relatively low reactivity of water corresponds to that expected for an alcohol of pK_a ∼ 13. This is ascribed to an imbalance between charge development and solvation of the transition state compared with H₃O⁺. Substituted acetate anions react with the l-(4-methoxyphenyl)ethyl carbocation with β_nuc ⁼ 0.13. The selectivity decreases with increasing cation reactivity as the carboxylate ions approach limiting rate constants of ∼5 × 10⁸ M⁻¹ s⁻¹. This relatively low limit is attributed to a requirement for desolvation of basic oxygen anions before reaction. A dependence of solvent selectivity on the leaving group shows that the l-(4-methylphenyl)ethyl carbocation reacts with solvent, in part, through an ion pair. Azide ion reacts from a pool that can be described by an equilibrium constant of K_as = 0.3 M⁻¹. Styrene formation from this carbocation is catalyzed by a leaving carboxylate ion and by added buffers, with β = 0.14. The equilibrium constant for the formation of a reactive base-cation pair is ∼0.04 M⁻¹. Rate constants for collapse of the ion pair, to form ester, and for proton removal, to form 4-methylstyrene, were estimated to be approximately 1.6 × 10¹⁰ s⁻¹ and 6 × 10⁷ s⁻¹ respectively. The rate constants for deprotonation and for hydration of the styrene give the acid dissociation constant of the carbocation to form 4-methylstyrene, pK_A = −11.2.