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This account presents previous and recent mechanistic studies of the Heck reactions reported by the authors whose attention has been focused on a problem of the coupling of catalyst transformations within and without the catalytic cycle. Based on the understanding of the interaction mechanism between these processes a development of new methods of Heck chemistry became conceivable. In particular, such a rational approach allowed an elaboration of the ligand-free systems, which are effective not only in the reaction with non-activated aryl bromides but also with nonactivated aryl chlorides. 1 Introduction 2 Main Catalytic Cycle 2.1 Pd(0)/Pd(II) Catalytic Cycle 2.2 Rate-Limiting Step of the Main Catalytic Cycle in the Heck Reaction with Aryl Iodides 3 Reactions without the Main Catalytic Cycle of the Heck Reaction 3.1 Reduction of Palladium 3.2 Oxidation of Palladium 3.3 Agglomeration of Palladium 3.4 Dissolution of Palladium 3.5 Dissolution of Palladium as a Rate-Limiting Step of the Heck Reaction with Aryl Bromides 4 Coupling Effects of the Catalyst Reactions within and without the Main Catalytic Cycle 4.1 Reaction with Aryl Iodides 4.2 Reaction with Aryl Bromides and Aryl Chlorides 4.3 Reaction with the Heterogeneous Catalyst Precursors 5 Conclusions and Outlook

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An increase of reaction rate with decreasing catalyst concentration by performing the Heck reaction with aryl bromides in the presence of several catalytic systems has first been observed. Taking into account palladium colloidal particles formation established experimentally in the Heck reaction with aryl bromides, a new approach to the description of unusual kinetics is suggested. That approach is based on a concept of ''magic'' number clusters.

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