Development of N-Heterocyclic Carbene (NHC)-palladium Complex Catalyst and Immobilized NHC-palladium Complex Catalyst

Conventional methods for preparing catalysts for the immobilization of N-heterocyclic carbenes (NHC)-metal complex use the substituent on the nitrogen as the immobilization site with the support, and the bulkiness of the substituent on the nitrogen, which is a characteristic of NHC ligands, cannot be fully utilized. In this study, we focused on the backbone carbon of NHC and devised a new catalyst design that uses backbone carbon as the bind-ing point with the support. First, various silyl groups were introduced into the backbone carbon of IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), and Si-IPr-Pd complex catalysts were synthesized. The electronic properties of the introduced silyl groups were investigated, and it was confirmed that the introduction of electron-donating silyl groups enhanced the catalytic activity in C-N coupling reaction. Next, we applied this knowledge to the immobilization of Si-IPr-Pd complex catalyst with a dimethylchlorosilyl group on polystyrene resin to develop a new immobilized NHC-Pd complex catalyst with an electron-donating oxosilyl group as the binding point to the support. Finally, the catalytic performance was evaluated in the C-N coupling reaction, and the catalyst exhibited high activity, and the amount of residual Pd in the filtrate after the reaction was below the detection limit (<1 ppm).