The role of phosphines in the activations of coupled substrates in the Suzuki-Miyaura reaction

The regularities of differential selectivity with respect to competing substrates in phosphine-containing palladium catalytic systems of the Suzuki-Miyaura reaction were analyzed. The detected sensitivity of differential selectivity of two competing aryl bromides to the nature and concentration of a tertiary phosphine suggests palladium phosphine-containing complexes to be involved in the activation of aryl halides. At the same time, the selectivity over competing arylboronic acids remains unchanged when the catalytic system is supplemented with tertiary phosphines, which suggests the steps of activation of arylboronic acids to involve palladium species free of phosphine ligands. The detected regularities coupled with literature data on how phosphines influence the enantioselectivity of the asymmetric Suzuki-Miyaura reaction agree with the fact that the reaction proceeds via a non-linear cooperative catalysis mechanism.