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The aryl halide-arylacetylene-arylboronic acid three-component coupling in the presence of a Pd catalyst containing no organic ligands is described. The reaction routes that take place in the reaction system are shown to include the consecutive carbopalladation of two arylacetylene molecules, leading to unsaturated polyaromatic compounds. The possibility of controlling the alternative reaction routes is demonstrated.

This paper presents the results of a kinetic study on the step of activation of aryl iodides and aryl bromides under conditions of ligand-free catalysis in the direct arylation of indole at a C–H bond performed using an analysis of the integral kinetic curves of consumption of the competing aryl halides. In the course of an experiment under the conditions of competition between a pair of aryl halides having different reactivity, an acceleration in the consumption of a less active substrate was observed, which coincided in time with the almost complete consumption of a more active one. The experimental data unambiguously indicated a rapid character of the stage of oxidative addition with the participation of not only reactive aryl iodides but also nonactivated aryl bromides.

The kinetic approach for distinguishing between homogeneous and heterogeneous catalysis mechanisms by the analysis of differential selectivity was applied for direct C–H arylation of an indole with aryl halides under “ligandless” conditions. The differential selectivity of competing arylation of an indole with two aryl iodides or bromides and the differential regioselectivity of a parallel formation of C2- and C3-regioisomers of arylated indoles were studied by the simple method of phase trajectories needed for raw kinetic data only. The results obtained indicated that the selectivities strongly depended on the type (soluble or insoluble) and concentration of the Pd precursor. Considering the pathways of Pd transformations under the reaction conditions, this indicates a substantial contribution of catalysis occurring on the surface of metallic Pd (including nanoparticles).

The results of a comparative study of phosphine-containing and phosphine-free catalytic systems of the Heck reaction using aromatic carboxylic anhydrides as arylating agents are presented. It was demonstrated that the patterns of diff erential selectivity of the reaction under competition of two aromatic anhydrides or two alkenes are independent of the presence of a tertiary phosphine additive in the system. It was established that palladium complexes with no phosphine ligands in their coordination sphere are catalytically active at the step of activation of aromatic carboxylic anhydride and alkene. The patterns of diff erential selectivity for regioisomers of arylated products provide the evidence of the participation of phosphine-containing anionic palladium complexes in the regioselectivity-determining step of the catalytic cycle. The data obtained are in agreement with the phosphine involvement in the catalyst transformations proceeding outside the main catalytic cycle.

A kinetic study of reaction of alkene arylation by anhydrides of aromatic acids (modified Mizoroki–Heck reaction) was carried out using differential selectivity as the main measured parameter. The results obtained under conditions of competition between a pair of alkenes or a pair of aromatic anhydrides point to the occurrence of the reaction via a homogeneous mechanism of catalysis analogously to a conventional variant of reaction with aryl halides as arylating reagents. The hypothesis that the active catalyst is homogeneous agrees with the results of kinetic studies of processes of catalyst formation and deactivation.

Differential selectivity of the direct arylation of indole with aryl halides under competing and noncompeting conditions with a varying set of reaction parameters was determined using phase trajectories. The results described herein allow for conclusions to be drawn regarding the character of active complexes (cationic, neutral, or anionic) as well as realization of the indole electrophilic substitution in the catalytic cycle using the ligand-free catalytic system.

Differential selectivity in the competitive Heck reaction with aromatic carboxylic anhydrides has been found to depend on the nature of both the cation and anion of a salt used as an additive for the catalytic system. This fact indicates that the anions present in the system are involved in active anionic species.

Investigating the mechanism of a complex catalytic reaction is usually quite complicated because many processes, such as active catalyst formation and deactivation, are linked to the main catalytic cycle. Using artificial multi-route reactions with similar competing substrates is a unique way to elucidate the fine details of a reaction mechanism. Applying such multi-route reaction-based methods to the cross-coupling of alkynes with aryl halides (Cu-free Sonogashira reaction) and alkenes with aromatic carboxylic anhydrides (modified Heck reaction) catalysed by ligand-free Pd allowed us to elucidate the fast, reversible, and rate-determining steps involved in the relevant catalytic cycles.