General Base Catalysis, Structure-Reactivity interactions, and Merging of Mechanisms for Elimination Reactions of (2-Arylethyl)quinuclidinium Ions

Article abstract of DOI:10.1021/ja00371a024

Structure-reactivity parameters and interaction coefficients are reported for elimination reactions of N-(2-arylethyl)quinuclidinium ions and for 2-arylethyl halides and tosylates in 60percent Me2SO/wares at 40 deg C, based on direct measurements of Bronsted β values for general base catalysis by oxyanion buffers, Hammet ρ values, and β_1g for substituted quinuclidines.The Bronsted slopes increase from β - 0.67 for N-(2-(p-nitrophenyl)ethyl)quinuclidinium ions, which react by an E1cB_irr mechanism, to β ca. 0.9 for the reactions of other N-(2-arylethyl)quinuclidinium ions by a concerted E2 elimination.There is no detectable interaction between the base catalyst and the leaving group for E1cB elimination, so that the interaction coefficient p_xy = dβ_1g/dpK_BH = dβ/dpK_1g is ca.0 for the N-(2-(p-nitrophenyl)ethyl)quinuclidinium ions.In contrast, values of β_1g become less negative with increasing pKa of the base catalyst for the p-cyano and other N-(2-arylethyl)quinuclidinium ions, giving a constant value of p_xy = 0.018 for the E2 elimination reactions of these compounds.The positive p_xy coefficient for the N-(2-phenylethyl)quinuclidinium ions is confirmed by the observation of less negative values of β_1g as the effective basicity of aqueous tetramethylammonium hydroxyde is increased by the addition of Me2SO.An increase in β with poorer leaving groups in the series of 2-(p-nitrophenyl)ethyl halides also corresponds to a positive p_xy coefficient and an E2 mechanism.The interaction between the leaving group and central atoms is shown by the less negative values of β_1g with electron-withdrawing substituents on the β-phenyl group, which corresponds to a negative coefficient p_yy' = -dβ_1g/d?^- = -dρ/dpK_1g = -0.09.A small decrease in β with electron-withdrawing substituents on the β-phenyl group suggests an interaction between the base catalyst and the central atoms that is described by a negative coefficient p_xy' = dβ/d?^- = dρ/dpK_BH = -0.07.The sign of the p_yy' and p_xy' coefficient are consistent with an important component of proton transfer in the transition state.These properties of the E2 elimination reactions of N-(2-arylethyl)quinuclidinium ions can be described by a reaction coordinate that is rotated 24 deg counterclockwise from the x coordinate (for proton transfer) on a reaction coordinate-energy diagram that is defined by the observed structure-reactivity parameters.In contrast, β increases with increasing ? for elimination reactions of 2-arylethyl bromides with a positive value of p_xy' = 0.07.This suggests more diagonal character to the transition state on the reaction coordinate diagram.The change from E1cB to an E2 mechanism is more easily described as a transformation of mechanism than as a change between two coexisting mechanisms.