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

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journal contribution
posted on 16.04.2020, 21:29 by Dao Zhang, Jueqin Yu
Novel chiral N,N′-bisaryl bis­(NHC) ligand precursors H2[(S)-2]­Cl2 on a spiro scaffold and H2[(S)-3bg]­Cl2 with a binaphthyl linkage were rationally designed and their cyclometalated cis-chelated NHC palladium complexes (S)-5, (S)-6, and (S)-7bg 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 PdII–PdII intramolecular interaction (<3.00 Å). These N,N′-bisaryl-bis­(NHC)-Pd complexes together with N,N′-bisalkyl analogues {[(S)-1ad]­PdX2} (X = I, (S)-4a; X = Br, (S)-4bd) 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 7ag 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.