Synthesis of Gold Catalyst Featuring Z-Type Ligand and Its Catalyic Reaction

Green described a novel method for classifying covalent bonds in transition metals in 1995. Based on this definition, there are three types of ligands, the L-, X- and Z-types, which are categorized depending on the bonding functions for the ligating atom of a ligand. The X-ligand provides one electron to the metal forming the covalent bond (M-X) exemplified by a hydride, alkyl, and halogen. The L-ligand (e.g., P, N, CO, and olefin etc.) provides two-electrons for occupying an empty orbital on the metal center. The so-called Lewis acidic atom (B, Al, and Si etc.) is classified as a Z-type ligand. Although there is quite a wide variety of metal complexes bearing the X- and L-type ligands, the metal complexes featuring the Z-type ligand (M -> Z interactions) are still limited. Therefore, the catalytic reaction utilizing the M -> Z interaction has been rarely reported. Recent effort in our laboratory focused on the synthesis of gold complex Au (DPB)X (DPB = diphosphine-borane) featuring Z-type ligand, and its catalytic reaction. As a result, the gold catalysts showed the higher activity than the general catalysts (without Z-ligand) for the several cyclization reactions presumably due to the decrease of the electron density at the gold center by the electron-withdrawing effect of Z-ligand. This article describes the structure analysis of the Au -> Z complex and the catalytic reactions based on the alkyne activation.