Fine Tuning of Chiral Bis(N-heterocyclic carbene) Palladium Catalysts for Asymmetric Suzuki-Miyaura Cross-Coupling Reactions: Exploring the Ligand Modification

摘要

Novel chiral N,N'-bisaryl bis(NHC) ligand precursors H-2[(S)(2)-]Cl-2 on a spiro scaffold and H-2[(S)-3b-g]Cl-2 with a binaphthyl linkage were rationally designed and their cyclometalated cis-chelated NHC palladium complexes (S)-5, (S)-6, and (S)-7b-g have been synthesized and fully characterized. Complexes 6 and 7b were further confirmed by X-ray singlecrystal analysis. Both complexes adopted a slightly distorted square planar geometry around the Pd(II) center. The structure of 6 consists of a rare dimeric arrangement incorporating two palladium(II) centers bonded through a short metal-metal bond (2.853(2) angstrom), indicating a Pd-II-Pd-II intramolecular interaction (<3.00 angstrom). These N,N'-bisaryl-bis(NHC)-Pd complexes together with N,N'-bisalkyl analogues {[(S)-1a-d]PdX2} (X = I, (S)-4a; X = Br, (S)-4b-d) have been used in the asymmetric aryl-aryl cross-coupling reactions of arylboronic acids and aryl halides. The enantioselectivity of the biaryl products was greatly improved within 24 h (up to 74% ee) when complexes 7a-g were used as catalysts. The results show that for these types of bis(NHC) palladium catalysts the structural characters of the chiral scaffolds play a decisive role in the enantioselectivities of cross-coupling reactions.