Kinetics and mechanism of ortho-palladation of ring-substituted NN-dimethylbenzylamines

Abstract

Addition of excess of NN-dimethylbenzylamine to a chloroform solution of [Pd₃(O₂CMe)₆] leads to instantaneous depolymerisation of the trimer giving the monomer trans-[Pd(O₂CMe)₂(PhCH₂NMe₂)₂]. Reversible dissociation of the amine from the latter affords a pseudo-three-co-ordinate 14-electron intermediate [Pd(O₂CMe)₂(PhCH₂NMe₂)] which undergoes subsequent rate-limiting ortho-palladation to form the cyclopalladated acetate-bridged dimer [{Pd(O₂CMe)(C₆H₄CH₂NMe₂)}₂]. The rate-limiting step is electrophilic in character; the slope of the corresponding Hammett plot for differently ring-substituted NN-dimethylbenzylamines is –1.6. The kinetic isotope effect, k_H/k_D, for PhCH₂NMe₂ is 2.2 ± 0.2. The activation entropy for the rate-limiting step is very negative, ca. –250 J K^–1 mol^–1 for PhCH₂NMe₂, suggesting a highly ordered tight transition state. This dissociative path is a factor of ca. 100 faster than a parallel one, involving the 16-electron monomer trans-[Pd(O₂CMe)₂(PhCH₂NMe₂)₂] but without loss of the amine. Ortho-palladation is not rate-limiting in acetic acid solvent, where slow cleavage of acetate bridges in polynuclear palladium species occurs, affording a vacant co-ordination site for subsequent rapid ortho-palladation. A comparison of intra- and inter-molecular activations of carbon-hydrogen bonds in arenes by palladium(II) in terms of ‘effective molarities’ shows that the ratio k_intra/k_inter is not less than 3.6 × 10² mol dm^–3.