Catalysis of the beta-elimination reaction of N-[2-(4-pyridyl)ethyl]quinuclidinium (1) and N-[2-(2-pyridyl)ethyl] quinuclidinium (2) by Zn2+ and Cd2+ in OH-/H2O (pH ) 5.20-6.35, 50 °C, and ionic strength 1 M KCl) has been studied. In the presence of Zn2+, the elimination reactions of both isomers occur from the Zn2+-complexed substrates (C). The equilibrium constants for the dissociation of the Zn2+ complexes are as follows: Kd = 0.012 M (isomer 1) and Kd = 0.065 M (isomer 2). The value of kH2O for isomer 1 is 4.81x10-6 s-1. For isomer 2 both the rate constants for the “water” and OH- induced reaction of the Zn2+ complexed substrate could be measured, despite the low concentration of OH- in the investigated reaction mixture [kH2O = 1.97x10-6 s-1 and kOH = 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 81000 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 ) 1500000 and is in agreement with an E1cb irreversible mechanism (Guthrie, R. D.; Jencks, W. P. Acc. Chem. Res. 1989, 22, 343). A value of kH2O higher than 23x10-7 s-1 is estimated for the Cd2+ complexed isomer 2, while catalysis by Cd2+ has not been observed for isomer 1.
Metal ion catalysis in the beta-elimination reactions of N-[2-(4-pyridyl)ethyl]quinuclidinium and N-[2-(2-pyridyl)ethyl]quinuclidinium in aqueous solution.
DE MARIA, Paolo;FONTANA, Antonella;
2004-01-01
Abstract
Catalysis of the beta-elimination reaction of N-[2-(4-pyridyl)ethyl]quinuclidinium (1) and N-[2-(2-pyridyl)ethyl] quinuclidinium (2) by Zn2+ and Cd2+ in OH-/H2O (pH ) 5.20-6.35, 50 °C, and ionic strength 1 M KCl) has been studied. In the presence of Zn2+, the elimination reactions of both isomers occur from the Zn2+-complexed substrates (C). The equilibrium constants for the dissociation of the Zn2+ complexes are as follows: Kd = 0.012 M (isomer 1) and Kd = 0.065 M (isomer 2). The value of kH2O for isomer 1 is 4.81x10-6 s-1. For isomer 2 both the rate constants for the “water” and OH- induced reaction of the Zn2+ complexed substrate could be measured, despite the low concentration of OH- in the investigated reaction mixture [kH2O = 1.97x10-6 s-1 and kOH = 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 81000 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 ) 1500000 and is in agreement with an E1cb irreversible mechanism (Guthrie, R. D.; Jencks, W. P. Acc. Chem. Res. 1989, 22, 343). A value of kH2O higher than 23x10-7 s-1 is estimated for the Cd2+ complexed isomer 2, while catalysis by Cd2+ has not been observed for isomer 1.File | Dimensione | Formato | |
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