Mechanistic Insights into the Pd(BINAP)-Catalyzed Amination of Aryl Bromides: Kinetic Studies under Synthetically Relevant Conditions

Abstract

Kinetic studies using reaction calorimetry were carried out under synthetically relevant conditions to study the mechanism of the amination of bromobenzene with primary and secondary amines using Pd₂(dba)₃/BINAP mixtures as well as preformed (dba)Pd(BINAP), (p-tolyl)(Br)Pd(BINAP), and Pd(BINAP)₂ complexes. The presence of a significant induction period in the reaction was attributed to the slow activation of the catalytic precursor, resulting in an increase in the concentration of active species within the catalytic cycle. The induction period can mask the true kinetics of the reaction, which exhibits positive order dependences on aryl bromide and amine and zero-order dependence on base. It is also determined that the bis-ligand complex Pd(BINAP)₂ does not play a role directly on the catalytic cycle. In addition to the conventionally accepted pathway involving oxidative addition of the aryl halide to (BINAP)Pd as the first step, a pathway initiated by addition of the amine to the catalyst is proposed and supported by kinetic modeling of sequential reaction experiments. A subtle dependence of the reaction mechanism on the relative concentrations of substrates is revealed in these studies. The dependence of the catalyst resting state on reaction conditions is also discussed. This work suggests that conclusions from kinetic studies may be meaningful only for the conditions under which they are carried out, calling into question the use of conventional kinetic methods in this system.