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

Novel chiral N,N′-bisaryl bis­(NHC) ligand precursors H₂[(S)-2]­Cl₂ on a spiro scaffold and H₂[(S)-3b–g]­Cl₂ 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 single-crystal 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) Å), indicating a Pd^II–Pd^II intramolecular interaction (<3.00 Å). 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.