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, kH/kD, 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 kintra/kinter is not less than 3.6 × 102 mol dm–3.