Chiral linker-bridged bis-N-heterocyclic carbenes: design, synthesis, palladium complexes, and catalytic properties

Abstract

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.