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

摘要

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 CD_3CN solution between two isomers of the trans structure.The reaction of the palladium complex with quinuclidine in CH_3CN,at 25 deg 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 (k_Q~(HPLC) = 2.3 x 10~(-3) M~(-1) s~(-1)) is nearly 3 times slower than a rate constant monitored spectrophotometrically (k_Q = 6.5 x 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 lambda = 295 nm of trans-bis[2-(2-chloroethyl)pyridine-d_2]palladium chloride with quinuclidine (Q) has shown the presence of a significant primary kinetic isotope effect,k_Q(H)/k_Q(D) = 1.8,for the elimination reaction within the Pd complex,1.The second-order rate constant for the beta-elimination reaction from 2-(2-chloroethyl)pyridine induced by quinuclidine in CH_3CN at 25 deg C is k_Q~(FREE) = 6.2 x 10~(-6) M~(-1) s~(-1).It can be observed as a significant activation (about 3 orders of magnitude) of the beta-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.