Elucidating the reaction mechanism of a palladium-palladium dual catalytic process through kinetic studies of proposed elementary steps

Abstract

In the synergistic dual catalytic process, the kinetics of the catalytic cycles must be balanced for the successful outcome of the reaction. Therefore, the analysis of the kinetics of the independent catalytic cycles is essential for such reactions, as it enables their relational optimization as well as their design. Here we describe an analysis of the mechanism of a catalytic synergistic bimetallic reaction through the experimental study of a palladium-catalysed cross-coupling of aryl halides with terminal alkynes, an example of a monometallic dual catalytic process. The proposed mechanism of the investigated reaction was disassembled into two palladium catalytic cycles and further into elementary reactions, and each step was studied independently. The described mechanistic analysis allowed us to identify the rate-determining step of the catalytic process by comparing the rates of the elementary reactions under similar reaction conditions, balanced kinetics of the palladium catalytic cycles, and also in which step which reagent enters the catalytic cycle and how.

New Mechanistic Insights into the Copper-Free Heck–Cassar–Sonogashira Cross-Coupling Reaction

“…In addition, the different coordination–carbopalladation processes appear to be a function of several factors, such as the nature of the leaving group, the p K a of the alkyne, the solvent, and the strength and nucleophilicity of the base. In 2018, Košmrlj and co-workers proposed a general mechanism for the Heck–Cassar protocol based on a second cycle of palladium, as shown in Figure . The mechanism was supported by DFT calculations, kinetic studies, and 31 P NMR analysis of intermediate complexes.…”

“…The presence of the nitro substituent on the aromatic ring of aryl iodide 1 NO2 is able to slow down the RE step and increase the chances of detecting intermediate complexes of the catalytic cycle. The Pd­(II) complexes B NO 2 I , C , M , and L (Figure ) were individually synthetized according to previously reported procedures . For this purpose, only TMG and PYR were screened as the base, while TEA was not tested, because of its inefficiency in generating Pd(0) species at room temperature.…”