Synthesis and catalytic activity of chiral linker-bridged bis-N-heterocyclic carbene dipalladium complexes

A series of new chiral-bridged N-heterocyclic carbene ligands with different substituents and their corresponding palladium complexes have been synthesised. The effect of the substituents on the catalytic activity of these Pd complexes was investigated in the Suzuki reaction of p-bromotoluene with phenylboronic acid, and the results showed that the complexes with aryl substituents performed better than those with alkyl substituents. The complex with the most sterically hindered substituent (diisopropyl on a phenyl group) performed best, and it was also an efficient catalyst for the reaction of various arylboronic acids with aryl halides having different electronic and steric properties. In addition, it was employed as a catalyst in the asymmetric Suzuki reaction, but only moderate yields of 1,1-binaphthalenes with less than 20% enantiomeric excess were obtained.