Design And Synthesis Of Chiral Benzimidazole Carbene Ligands And Their Application In The Asymmetric Reaction

N-heterocyclic carbenes have several advantages:stronger a-donors property, easier modification of structure, and better stability to air, thermal and moist than the corresponding phosphine ligands. These advantages make NHCs be widely used in asymmetric catalysis. The structure of chiral ligands is one of the key factors in the successful application of asymmetric catalysis. Herein, we designed and synthesized a series of new chiral NHC precursors with benzimidazolium scaffold, including four C₂ symmetric benzimidazolium salts and fourteen C₂ asymmetric benzimidazolium salts. Additionally, seven NHC-Rh complexes were also prepared. There were twenty one new compounds among the ligands synthesized above, and the structures of these compounds were characterized by 1H NMR,13C NMR and HR-MS spectra. The absolute structure of NHC-Rh complexe 5e was further determined by X-ray diffraction.There are mainly two aspects for the application of the new ligands in asymmetric catalysis:1. Palladium-catalyzed intramolecular a-arylation:After deprotonation of the benzimidazolium salts with base and formation of the palladium complexes in situ, four C₂-symmetric ligands were tested as catalysts in asymmetric intramolecular a-arylation. The salt 1q which incorporated S-1-cyclohexylethyl group showed the best enantioselectivity. The corresponding oxindoles were obtained with high yields and moderate enantioselectivities.2. Asymmetric arylation of aromatic aldehydes with arylboronic acids:C₂-asymmetric benzimidazole carbene precursor salts were used in rhodium-catalyzed arylation of aromatic aldehydes. The salt 1h in which the substituents of nitrogen were 2-naphthyl and S-3,3-dimethyl-2-butyl showed the best enantioselectivity. A number of chiral diarylmethanols were obtained with high yields ?up to 99% yield? and moderate enantioselectivities ?up to 61% ee?. In addition, 1h was also applicated in Pd-catalyzed arylation of aromatic aldehydes. The products were obtained with moderate yields but poor enantioselectivities ?<10% ee?. Subsequently, several carbene-Rh complexes were prepared as catalysts for the same reaction, giving the corresponding diarylmethanols with high yields ?up to 99% yield? and moderate enantioselectivitities ?up to 46% ee?.