Novel Phosphine Ligands With Cobaltocenium Backbone: Synthesis And Application In Suzuki Coupling Reaction In Ionic Liquids

The palladium-catalyzed Suzuki cross-coupling reaction is one of the most efficient methods for the construction of C-C bond and has found widespread use in organic synthesis. However, the separation of the products from the reaction mixture, the recovery of the catalysts, and the need for organic solvents are the major disadvantages in the Suzuki cross-coupling reactions. To overcome these problems, palladium-centered catalysis in ionic liquids has been particular attention. Phosphine ligands with cobaltocenium backbone can be used in ionic liquids. We have studied on the synthesis and catalytic properties of novel phosphine ligands with cobaltocenium backbone. The main results are as follows:1. Six novel phosphine- substituted cobaltocenium salts 1, 1'-bis (dicyclohexylphosphino) cobaltocenium hexafluorophosphate(di-cypc+PF6-)(1a), 1,1'-bis (di-iso-propylphosphino) cobaltocenium hexafluorophosphate (di-isoppc+PF6-) (2a), 1,1'-Bis (di-tert-butylphosphino) cobaltocenium hexafluorophosphate (di-tbpc+PF6-)(3a), the monophosphine ligand Cc+PR2PF6- (Cc+ = cobaltocenium, 1b, R = Cy;2b, R = i-Pr;3b, R = t-Bu) (figure 1), and three novel phosphine-palladium complexes were synthesized, These compounds have been fully characterized by elemental analysis, spectroscopy, and X-ray crystal diffraction techniques.2. These ligands were proved to be air-stable and highly efficient. PdCl2/ ligand / [bmim]PF6 catalytic system has been developed for Suzuki cross-coupling of various aryl bromides with aryl boronic acids. The catalytic system ( PdCl2 / dppc+PF6- / [bmim]PF6 ) can be recycled at least ten times with minimal loss of activity.