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

Article abstract of DOI:10.1021/ja00317a032

Selectivities of a series of substituted 1-phenylethyl carbocations toward alcohols and other nucleophiles have been determined by product analysis.The 1-(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 ca. 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 ca. 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 ca. 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 1-(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 ca. 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 1-(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, wih β = 0.14.The equilibrium constant for the formation of a reactive base-cation pair is ca. 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.