Cis-chelated palladium(II) complexes of biphenyl-linked bis(imidazolin-2-ylidene): synthesis and catalytic activity in the Suzuki-Miyaura reaction
Guangming Nan,a,b Bin Rao,a and Meiming Luoa*
aKey Laboratory of Green Chemistry and Technology of Ministry of Education,
College of Chemistry, Sichuan University, Chengdu 610064, P. R. China
bDepartment of Chemistry, Ili Teachers College, Yining 835000, Xinjiang, P. R. China
A new type of biaryl-linked bis(imidazolin-2-ylidene) ligand precursor and corresponding cis- chelated bis-N-heterocyclic carbene.Pd(II) complexes have been synthesized starting from 6,6.- dimethyl-2,2.-dinitro-1,1.-biphenyl. The catalytic activity of these NHC.Pd(II) complexes in the Suzuki.Miyaura reactions of aryl chlorides has also been investigated affording biaryls in good to excellent yields.
Keywords: Carbene ligand, N-heterocyclic carbene, palladium complexes, Suzuki cross- coupling, biaryl compounds
With unique ligating properties and stability toward heat, air, oxygen and moisture compared to metal–phosphine complexes, catalysts that incorporate N-Heterocyclic carbenes (NHCs) as ligands have been applied to a broad range of organic transformations.1 The reactivity and stability of NHC.metal complexes largely rely on the electronic and steric characteristics of the NHC ligands, which can be controlled by modifying the structures of NHC ligands. Among various NHC ligands, chelate bis-NHCs with a biaryl backbone not only yield more stable metal complexes, but also give interesting features that can provide fine-tuning of topological properties such as steric hindrance, bite angles and chirality.1j So far, two types of biaryl backbones in biaryl-linked bis-NHCs appeared in literature. The first one is 1,1.-binaphthyl. RajanBabu et al. reported the first chelated bis(imidazolin-2-ylidene) complexes of Pd(II) and Ni(II) with a binaphthyl backbone A (Figure 1) derived from 2,2.-bis(bromomethyl)-1,1.- binaphthyl.2 Crabtree et al. synthesized a chiral proligand B (Figure 1) with a binaphthyl