1451092-92-4Relevant academic research and scientific papers
Chiral linker-bridged bis-N-heterocyclic carbenes: Design, synthesis, palladium complexes, and catalytic properties
Zhang, Dao,He, Yu,Tang, Junkai
, p. 11699 - 11709 (2016/07/28)
A series of chiral bis(benzimidazolium) salts 10-19 with (1R,2R)-cyclohexene, (1R,2R)-diphenylethylene and (aR)-binaphthylene linkers have been designed and synthesized in 30-94% yield. Ten chiral bis(NHC) palladium complexes 20-28 have been synthesized and characterized by NMR, HRMS, elemental analysis and further confirmed by X-ray single crystal analysis. These bis(NHC)-Pd complexes showed obviously different catalytic properties in the asymmetric Suzuki-Miyaura coupling reactions. The (1R,2R)-cyclohexene-bridged bis(NHC)-Pd complex, (R,R)-23, achieved the highest yield of 90%, while complex (aR)-28, with a binaphthylene linker, showed the best enantioselectivity of 60 ee%. The structural analysis of these complexes suggested that such difference of catalytic performance has a close relationship with their coordination surroundings around metal centres.
Chiral 1,2-cyclohexane-bridged bis-NHC palladium catalysts for asymmetric Suzuki-Miyaura coupling: Synthesis, characterization, and steric effects on enantiocontrol
Li, Yinle,Tang, Junkai,Gu, Jun,Wang, Quanrui,Sun, Peipei,Zhang, Dao
, p. 876 - 884 (2014/03/21)
The series of chiral 1,2-cyclohexane-bridged bis-N-heterocyclic carbene ligand precursors H2[(1R,2R)-(1a-i)]Br with different substituent groups and their neutral and cationic diaqua palladium complexes, namely {Pd[(1R,2R)-(1a-i)]Br2} (2a-i), {Pd[(1R,2R)-(1a)]X2} (X = Cl (3), OAc (4-OAc), OC(O)CF3 (4-OCH(O)CF3)), and {Pd[(1R,2R)-(LOMe)](OH2)2}X2 (X = OTf (5-OTf), SbF6 (5-SbF6)) have been prepared in moderate to good yields. These chiral palladium complexes were fully characterized by elemental analysis, high-resolution mass spectra, 1H and 13C NMR, and optical rotation determinations. The crystal structures of the chiral complexes 2a and 5-OTf were further confirmed to adopt a distorted-square-planar coordination geometry around the palladium center. The obtained chiral NHC-Pd compounds were able to catalyze the asymmetric Suzuki-Miyaura couplings of aryl halides with arylboronic acids in good yields (up to 96%) and moderate enantioselectivities (up to 64% ee). The coligand and steric effects were studied carefully. The coligands, including Br-, Cl-, AcO-, CF3COO-, and water molecules, have little influence on the catalytic results. However, a strong steric effect of the two aromatic substituents R on the enantiocontrol has been proved in the catalytic asymmetric Suzuki-Miyaura coupling reaction. The highest enantioselectivity of 64% ee could be achieved under the standard reaction conditions.
