The synthesis of N-heterocyclic carbene-substituted palladacyclic complexes and their catalytic properties

Palladium-catalyzed coupling reactions provide an efficient and powerful method for the formation of C-C and C-heteroatom bonds. Thus, the design and application of new palladium catalysts and the search for new catalytic systems for these coupling reactions have attached many researchers interest.Great progress has been obtained for the coupling reactions by using catalysts with bulky electron-rich phosphines. However, the phosphines ligands are expensive, toxic and unrecoverable; usually it is difficult to remove these ligands and their degradation by-products, phosphines oxides. Thus a lot of effort has been made to explore alternatives for phosphines, such as N-heterocyclic carbenes (NHC). NHC are better s-donors than phosphines and one of the most active catalysts for the formation C-C and C-heteroatom bond.Recently, the combination of a palladacycle framework and N-heterocyclic carbene (NHC) has been reported. These catalysts combine the stability induced by the presence of a palladacycle framework with the high activity associated with the NHCs. The advantages of such palladacyclic catalysts include their ease of synthesis, facile modification and convenience of handling.In this study has been characterized and prepared a series NHC-substituted palladacyclic complex and also examined their catalytic activity in the Suzuki-Miyaura. Three major routes were applied for the synthesis of palladacyclic complexes. In the first step, the NHC precursor has been prepared. In the second step, acetate-bridged palladacyclic complex has been synthesized and the last step, as a result of the reaction between dimeric palladacyclic complex and NHC ligand has been synthesized NHC-substituted palladacyclic complexes. These new compounds were characterized by elemental analysis, IR, 1H- and 13C-NMR spectroscopy.