HETEROGENEOUS CATALYSIS IN ORGANIC-CHEMISTRY .10. EFFECT OF THE CATALYST SUPPORT ON THE REGIOCHEMISTRY OF THE HECK ARYLATION REACTION

Data have been presented which show that dispersed metals can be consi dered as viable catalysts for synthetically useful organometallic reac tions. In addition, the material used as the catalyst support has been shown to influence the regiochemistry of the reaction. In the Heck ar ylation an acidic support, such as silica, withdraws electrons from th e palladium and this leads to predominant beta enol ether formation. P alladium supported on a basic support, such as magnesia, which is elec tron donating, produces primarily the alpha product. Electron withdraw al also increases the rate of the reaction. As with a number of other synthetically useful reactions the heterogeneously catalyzed Heck aryl ation takes place on the coordinately unsaturated corner atoms and ada toms on the metal surface. These single atom active sites are the only ones having a sufficient number of orbitals available for interaction with both the aryl species and the enol ether at the same time, a pro cedure needed for the reaction to take place. A mechanism for the reac tion on a tetrahedral corner atom is proposed. This is based on the Fr ontier Molecular Orbitals of this site and the corresponding mechanism established for the homogenously catalyzed reaction.