Metal Ion Catalysis in the β-Elimination Reactions of N-[2-(4-Pyridyl)ethyl]quinuclidinium and N-[2-(2-Pyridyl)ethyl]quinuclidinium in Aqueous Solution

摘要

Catalysis of the β-elimination reaction of N-[2-(4-pyridyl)ethyl]quinuclidinium (1) and N-[2-(2-pyridyl)ethyl]quinuclidinium (2) by Zn~(2+) and Cd~(2+) in OH-/H_2O (pH = 5.20-6.35, 50℃, and μ = 1 M KCl) has been studied. In the presence of Zn~(2+), the elimination reactions of both isomers occur from the Zn~(2+)-complexed substrates (C). The equilibrium constants for the dissociation of the Zn~(2+)-complexes are as follows: K_d = 0.012 +- 0.003 M (isomer 1) and K_d = 0.065 +- 0.020 M (isomer 2). The value of k_(H_2O)~C for isomer 1 is 4.81 x 10~(-6) s~(-1). For isomer 2 both the rate constants for the "water" and OH~--induced reaction of the Zn~(2+)-complexed substrate could be measured, despite the low concentration of OH~- in the investigated reaction mixture [k_(H_2O)~C = 1.97 x 10~(-6) s~(-1) and k_(OH~-)~C = 21.9 M~(-1) s~(-1), respectively]. The measured metal activating factor (MetAF), i.e., the reactivity ratio between the complexed and the uncomplexed substrate, is 8.1 x 10~4 for the OH~--induced elimination of 2. This high MetAF can be compared with the corresponding proton activating factor (Alunni, S.; Conti, A.; Palmizio Errico, R. J. Chem. Soc., Perkin Trans. 2 2000, 453), PAF = 1.5 x 10~6 and is in agreement with an E1cb irreversible mechanism (A_(xh)D_E~* + D_N) (Guthrie, R. D.; Jencks, W. P. Acc. Chem. Res. 1989, 22, 343). A value of k_(H_2O)~C ≥ 23 x 10~(-7) s~(-1) is estimated for the Cd~(2+)-complexed isomer 2, while catalysis by Cd~(2+) has not been observed for isomer 1.