Catalysis of the Reversible Elimination Reactions of Substituted N-(β-Phenylethyl)quinuclidinium Ions in Aqueous Solution

Article abstract of DOI:10.1021/ja00526a047

The rate constants for elimination reactions of substituted N-(β-p-nitrophenylethyl)quinuclidinium ions induced by hydroxide ion in water at 25 deg C show only a small sensitivity to the pK_a of the leaving quinuclidine, with β_lg = -0.18.The primary isotope effect is k_H/k_D = 8.5 and the secondary solvent isotope effect is k_OD/k_OH = 1.55 for the quinuclidine derivative.The rates of these elimination reactions are comparable to or faster than that of 2-p-nitrophenylethylbromide.The reaction is readily reversible in quinuclidine buffers and the rate constants of the addition reaction to p-nitrostyrene show a large dependence on the pK_a of substituted quinuclidines with β_nuc = 0.69; the equilibrium constants in the elimination direction follow β_eq = -0.89.The addition reaction shows general acid catalysis by protonated quinuclidines and the elimination reaction shows general base catalysis, with a Broensted coefficient of β = 0.68 for elimination from the diazabicyclobutane derivative.Elimination reactions from the corresponding phenyl compounds are ca. 10³ slower and show a more negative value of β_lg = -0.35 in water at 40 deg C; in EtONa/EtOH the value of β_lg is -0.28.The change in β_lg for the phenyl compounds corresponds to a negative structure-reactivity coefficient p_yy' = <*>β_lg/-<*>? = <*>p/-<*>pK_lg, consistent with the expected E2 mechanism for the phenyl compounds.However, it is uncertain whether the p-nitrophenyl compounds react by an E2 or an irreversible E1cB mechanism.