Cyclometalated and Alkoxyphenyl-Substituted Palladium Imidazolin-2-ylidene Complexes. Synthetic, Structural, and Catalytic Studies

Some new N,N ‘-bis(aryl)imidazolinium salts ( 3a − h ), in which the N-aryl groups have one unsubstituted ortho position and alkyl (Me, Pr i ) or alkoxy (OMe, OPr i ) substitution at the other ortho and/or para positions, have been prepared. They were used for the synthesis of N-heterocyclic carbene complexes of palladium in which the N-heterocyclic carbene is attached to cyclometalated or electron-rich aromatic rings ( 4, 5, 11d, e ). The reaction of Pd(tmed)(CH 3 ) 2 with the N-heterocyclic carbene generated from 3b and base gave the dimethyl complex 6, which quantitatively eliminated ethane to form the Pd(0) complex 7 . The latter was converted to a new type of “pincer” complex ( 8 ) by facile cyclometalation of both aromatic groups of the N-heterocyclic carbene ligand. The activities of the new complexes 4 and 9d, e in the Heck reaction of aryl halides were compared. At higher temperatures the complexes 9d, e show low activity in the coupling of aryl chlorides.