Catalysis of the â-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 í ) 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 ( 0.003 M (isomer 1) and Kd ) 0.065 ( 0.020 M (isomer 2). The value of kH2O C for isomer 1 is 4.81 10-6 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 C ) 1.97 10-6 s-1 and kOHC ) 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 8.1 104 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 ) 1.5 106 and is in agreement with an E1cb irreversible mechanism (AxhDE* + DN) (Guthrie, R. D.; Jencks, W. P. Acc. Chem. Res. 1989, 22, 343). A value of kH2O C g 23 10-7 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 b-Elimination Reactions of N-[2-(2-Pyridyl)ethyl]quinuclidinium and N-[2-(2-Pyridyl)ethyl]quinuclidinium in Aqueous Solution
ALUNNI, Sergio;DEL GIACCO, Tiziana;
2004
Abstract
Catalysis of the â-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 í ) 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 ( 0.003 M (isomer 1) and Kd ) 0.065 ( 0.020 M (isomer 2). The value of kH2O C for isomer 1 is 4.81 10-6 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 C ) 1.97 10-6 s-1 and kOHC ) 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 8.1 104 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 ) 1.5 106 and is in agreement with an E1cb irreversible mechanism (AxhDE* + DN) (Guthrie, R. D.; Jencks, W. P. Acc. Chem. Res. 1989, 22, 343). A value of kH2O C g 23 10-7 s-1 is estimated for the Cd2+-complexed isomer 2, while catalysis by Cd2+ has not been observed for isomer 1.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
