Elimination Reactions of N-(2-(p-Nitrophenyl)ethyl)alkylammonium Ions by an E1cB Mechanism

Article abstract of DOI:10.1021/ja00340a021

Elimination reactions of N-(2-(p-nitrophenyl)ethyl)quinuclidinium and 2-(p-nitrophenyl)ethyl)trimethylammonium ions catalyzed by hydroxide ion and buffer bases undergo a change in rate-determining step from an (E1cB)₁ to an (E1cB)_R mechanism with increasing buffer concentration in aqueous solution.Exchange of labeled 2H or 3H with solvent was shown for the quinuclidine and trimethylamine derivatives in the presence of buffers.Large inverse solvent deuterium isotope effects on the initial rate confirm the E1cB mechanism with leaving group expulsion partly or entirely rate determining under the same conditions.The dependence of log k on the pK of the leaving quinuclidine gives β₁^lg = -0.17 for k₁ (proton abstraction), β_p = 0.47 for k_-1/k₂ (partitioning of the intermediate zwitterion), and β_lg = -0.64 for k₁k₂/k_-1 (rate-determining expulsion of the leaving group).The ratio k₂/k_-1 increases from 10 to 59 with increasing ethanol concentration up to 40percent as the result of an increase in the rate of amine expulsion from the zwitterionic intermediate in the presence of organic cosolvents; k₂/k_-1 also increases with increasing temperature.Rate constants for elimination of 2-(p-nitrophenyl)ethyl halides increase with increasing leaving ability of the halide, consistent with a concerted E2 mechanism for all but the fluoride derivative.