84Among the numerous coupling reactions of aryl halides with various compounds (alkenes, alkynes, amines, organometallics), the Heck and Suzuki reac tions ((I) and (II), respectively) are of greatest importance and are the focus of researchers' atten tion [1, 2]:The nature of catalysis in the Heck reaction has been keenly discussed in the literature since the late 1990s. The authors of some reviews [3][4][5][6] have arrived at the conclusion that homogeneous catalysis in this reaction is more likely, irrespective of the nature of the catalyst precursor generating active species. All authors believe that the same catalytic species are involved in the reaction, namely, dissolved molecular complexes of Pd(0) and Pd(II), colloidal palladium particles in solution and/or on the supports surface, and larger particles of palladium metal. The heteroge neous palladium species are only sources of catalytic, dissolved Pd(0) and Pd(II) complexes, the species that are directly involved in the catalytic cycle. It is believed that all active and inactive species generated by the catalyst precursor undergo interconversion during the catalytic reaction and the most important role in these conversions is played by the aryl halide (Scheme 1). Hal R Ph Ph R Hal R R Ph [Pd] NaOAc + R = H, COMe; Hal = Br, I + PhB(OH) 2 [Pd] NaOAc R = H, COMe, Me; Hal = Br, I (I) (II) , .Abstract-Using a simple method based on an analysis of the phase trajectories of competing reactions of several substrates, it has been established that the selectivity of catalytically active species in the Suzuki reac tion of aryl bromides depends on the nature of the catalyst precursor. This indicates that there is a consider able contribution from heterogeneous catalysis. At the same time, in the reaction involving aryl iodides, when the catalyst concentration in the solution is much higher, the selectivity of the catalyst is precursor indepen dent, suggesting that homogeneous catalysis is dominant. In the Heck reaction of both aryl bromides and aryl iodides, pure homogeneous catalysis takes place.
