Synthesis, Characterization, and Reactivity of Arylpalladium Cyanoalkyl Complexes:  Selection of Catalysts for the α-Arylation of Nitriles

A new coupling process, the palladium-catalyzed α-arylation of nitriles, was developed by exploring the structure and reactivity of arylpalladium cyanoalkyl complexes. Complexes of 1,2-bis(diphenylphosphino)benzene (DPPBz), 1,1‘-bis(di-i-propylphosphino)ferrocene (DⁱPrPF), racemic-2,2‘-bis(diphenylphosphino)-1,1‘-binaphthyl (BINAP), and diphenylethylphosphine (PPh₂Et) were prepared. Coordination to palladium through the α-carbon was observed for DPPBz-ligated complexes and for complexes of primary and benzylic nitrile anions. However, the anion of isobutyronitrile was coordinated to palladium through the cyano-nitrogen when the complex was ligated by DⁱPrPF. The isobutyronitrile anion displaced a phosphine ligand to form a C,N-bridged dimer when generated from PPh₂Et-ligated palladium. These results suggest that the nitrile anion preferentially coordinates to palladium through the carbon atom in the absence of steric effects. Thermolysis of the arylpalladium cyanoalkyl complexes led to reductive elimination that formed α-aryl nitriles. The high yields and short reaction times observed for BINAP-ligated complexes suggested that BINAP-ligated palladium catalysts might be appropriate for the arylation of nitriles. Initial results on a palladium-catalyzed process for the direct coupling of aryl bromides and primary, benzylic, and secondary nitrile anions to form α-aryl nitriles in good yields are reported.