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, OPri) 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)(CH3)(2) with the N-heterocyclic carbene generated from 3b and base gave the dimethyl complex 6, which quantitatively eliminated ethane to form the Pd(O) 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.