Activation of Aryl Chlorides in the Suzuki-Miyaura Reaction by "Ligand-Free" Pd Species through a Homogeneous Catalytic Mechanism: Distinguishing between Homogeneous and Heterogeneous Catalytic Mechanisms

A distinction between the homogeneous and heterogeneous catalytic mechanisms of a myriad of Pd catalytic systems for the Suzuki-Miyaura reaction has been made based on soluble metal complexes or heterogenized Pd compounds. To date, hypotheses that concern whether the reaction proceeds in the solution phase or on the surface of solid particles, including nanosized ones, have been based on experimental results obtained under different reaction conditions. Moreover, convincing evidence was obtained for reactions proceeding simultaneously through homogeneous and heterogeneous catalytic mechanisms. However, most of the experimental evidence was obtained for relatively reactive aryl bromides and aryl iodides, while insufficient data exists for nonreactive aryl chlorides to make any well-grounded conclusions regarding the homogeneous or heterogeneous nature of the active species. Here, we report the use of "ligand-free" conditions, which excludes the need to add strong organic ligands, such as phosphines, amines, and carbenes, for the activation of aryl chlorides in the Suzuki-Miyaura reaction. The results suggest that the reaction proceeds in the solution phase through a homogeneous mechanism even when heterogeneous precursors are used. The main reason for this conclusion is the independence of the reaction differential selectivity under the competition of two aryl chlorides or two arylboronic acids, which is based on numerous parameters that only influence the nature of the possible heterogeneous Pd forms when the homogeneous mechanism is realized.