106988-35-6Relevant articles and documents
Thermodynamics, kinetics, and mechanism of exchange of cyclopalladated ligands
Ryabov, Alexander D.
, p. 1252 - 1260 (2008/10/08)
The exchange between cyclopalladated complexes and free ligands has been studied in acetic acid. An equilibrium study of the system based on N,N-dimethylbenzylamine derivatives [PdX(YZC6H2CH2NMe2)]2 + 2C6D5CD2NMe2 ? [PdX-(C6D4CD2NMe2)]2 + 2YZC6H2DCH2NMe2 (K1) has revealed that Pd(II) binds preferably with the electron-poorest ligand at equilibrium; K1 is 114, 0.59, 0.125, 0.008, and 0.0034 for 4-Y (5-Z) = MeO (MeO), H (Me), H (H), H (Cl), and H (NO2), respectively, at 55°C in D3CCOOD/CDCl3, X = MeCO2-. A procedure for regioselective ortho palladation of bifunctional derivatives such as 1-(3,4-dimethoxyphenyl)-2-methyl-3-(4-nitrophenyl)-2-azapropane (7) is put forward. In aprotic chloroform, Pd(II) acetate metalates the electron-rich ring of 7 to yield 8a, but the electron-poor ring is ortho palladated in acetic acid to yield 9a. A dissociative exchange mechanism is proposed on the basis of a kinetic study of reactions between [PdX-(YZC6H2CH2NMe2)]2 and 2-phenylpyridine or azobenzene to afford the corresponding cyclopalladated complexes. Preequilibrium measurements have indicated that in the former case the reactive species are monomers formed via acetate-bridge cleavage by 2-phenylpyridine but in the latter case the complexes react as dimers. Despite this, all of the reactions are first order in complex and zero order in entering ligand. The rate constants of the 2-phenylpyridine case at 75°C are (104k) 12.6, 3.9, 2.35, 2.6, 0.44, and 0.0225 s-1 for 4-Y (5-Z) = MeO (MeO), H (Me), H (H), H (MeO), H (Cl), and H (NO2), respectively. On the basis of substituent and solvent kinetics isotope effects, values of activation parameters, and data obtained previously, it has been suggested that cleavage of the Pd-N bond of the leaving ligand occurs first, followed by acidolysis of the Pd-C bond. Both steps can contribute to the overall rate. The two are followed by the rapid activation of the C-H bond of the incoming ligand. Reasons for the pseudonucleophilic behavior of Pd(II) toward C-H bonds of benzylamines in acetic acid have been evaluated on the basis of the proposed mechanism.
