The trans-bis[2-(2-chloroethyl)pyridine]palladium chloride (1) has been prepared and structurally characterized by X-ray spectroscopy and computational study. The X-ray structure of 1 is consistent with the trans isomer (with respect to Pd). The NMR spectrum and the computational study are in agreement with an equilibrium in CD3CN solution between two isomers of the trans structure. The reaction of the palladium complex with quinuclidine in CH3CN, at 25 °C, leads to competing elimination and displacement reactions with formation of vinylpyridine and chloroethylpyridine in a ratio of 1.5:1. However, the rate constant for formation of uncoordinated (vinyl)pyridine monitored by HPLC (kQ HPLC ) 2.3 10-3 M-1 s-1) is nearly 3 times slower than a rate constant monitored spectrophotometrically (kQ ) 6.5 10-3 M-1 s-1). This suggests that the initial product of elimination is a palladium complex of vinylpyridine and that displacement from this complex is partially rate determining in the formation of the uncoordinated product. A study by UV spectroscopy at ì ) 295 nm of trans-bis[2-(2-chloroethyl)pyridine-d2]palladium chloride with quinuclidine (Q) has shown the presence of a significant primary kinetic isotope effect, kQ(H)/kQ(D) ) 1.8, for the elimination reaction within the Pd complex, 1. The second-order rate constant for the â-elimination reaction from 2-(2-chloroethyl)pyridine induced by quinuclidine in CH3CN at 25 °C is kQ FREE ) 6.2 10-6 M-1 s-1. It can be observed as a significant activation (about 3 orders of magnitude) of the â-elimination reaction within the complex 1 with respect to the free 2-(2-chloroethyl)pyridine. The possible mechanism in agreement with these results is discussed.
Structure and Reactivity of trans- Bis[2-(2-Chloroethyl)pyridine]palladium Chloride(1). A study on the Elimination Reaction of 1 and 2-(2-Chloroethyl)pyridine Induced by Quinuclidine in acetonitrile
ALUNNI, Sergio;BELLACHIOMA, Gianfranco;DEL GIACCO, Tiziana;OTTAVI, LAURA;ZUCCACCIA, DANIELE
2005
Abstract
The trans-bis[2-(2-chloroethyl)pyridine]palladium chloride (1) has been prepared and structurally characterized by X-ray spectroscopy and computational study. The X-ray structure of 1 is consistent with the trans isomer (with respect to Pd). The NMR spectrum and the computational study are in agreement with an equilibrium in CD3CN solution between two isomers of the trans structure. The reaction of the palladium complex with quinuclidine in CH3CN, at 25 °C, leads to competing elimination and displacement reactions with formation of vinylpyridine and chloroethylpyridine in a ratio of 1.5:1. However, the rate constant for formation of uncoordinated (vinyl)pyridine monitored by HPLC (kQ HPLC ) 2.3 10-3 M-1 s-1) is nearly 3 times slower than a rate constant monitored spectrophotometrically (kQ ) 6.5 10-3 M-1 s-1). This suggests that the initial product of elimination is a palladium complex of vinylpyridine and that displacement from this complex is partially rate determining in the formation of the uncoordinated product. A study by UV spectroscopy at ì ) 295 nm of trans-bis[2-(2-chloroethyl)pyridine-d2]palladium chloride with quinuclidine (Q) has shown the presence of a significant primary kinetic isotope effect, kQ(H)/kQ(D) ) 1.8, for the elimination reaction within the Pd complex, 1. The second-order rate constant for the â-elimination reaction from 2-(2-chloroethyl)pyridine induced by quinuclidine in CH3CN at 25 °C is kQ FREE ) 6.2 10-6 M-1 s-1. It can be observed as a significant activation (about 3 orders of magnitude) of the â-elimination reaction within the complex 1 with respect to the free 2-(2-chloroethyl)pyridine. The possible mechanism in agreement with these results is discussed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
