Abstract
Several new nickel complexes bearing bulky N-heterocyclic carbene (NHC) ligands were developed as highly active catalysts in order to achieve efficient cross-coupling reactions of aryl halides. These complexes were identified on the basis of NMR, IR, MS spectroscopy and determined by X-ray crystallography. It should be noted that the steric requirements of the most bulky carbene, IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), controlled the reactivity of the nickel compounds to form catalytically active unsaturated species. That is, the NHC ligand stabilizes unusual paramagnetic or diamagnetic monovalent nickel compounds, which have performed high catalytic activity via a specific catalytic cycle between Ni(I)-Ni(I) and Ni(II)-Ni(II) species in the cross-coupling reactions. Kumada-Tamao-Corriu cross-coupling and Buchwald-Hartwig amination of aryl halides, and α-arylation of linear ketones were achieved to form corresponding coupling products in sufficient yields. In particular, amination with diarylamines proceeded efficiently under mild conditions to form triarylamine derivatives which are very important in optical or electronic organic devices, in contrast to the corresponding palladium chemistry.
