Cleavage of Carbon−Carbon Bonds in Aromatic Nitriles Using Nickel(0)

Abstract

The nickel(0) fragment [(dippe)Ni] has been found to react with a variety of aromatic nitriles. Initial π-coordination to the CC and C⋮N bonds of 2-cyanoquinoline is found to lead ultimately to C−CN oxidative addition. 3-Cyanoquinoline reacts similarly, although no η²-CN complex is observed. 2-, 3-, And 4-cyanopyridines react initially to give η²-nitrile complexes that then lead to quantitative formation of C−CN oxidative addition products. Benzonitrile reacts similarly but undergoes reversible insertion into the Ph−CN bond to give an equilibrium mixture of Ni(II) and Ni(0) adducts. A series of para-substituted benzonitriles has been studied in terms of both the position of the equilibrium between (dippe)Ni(η²-arylnitrile) ⇌ (dippe)Ni(CN)(aryl) and the rate of approach to equilibrium, and the Hammett plots indicate a buildup of negative charge at the ipso carbon both in the transition state and the Ni(II) product. Terephthalonitrile gives both η²-nitrile and oxidative addition adducts, as well as dimetalated products. No C−C or C−N cleavage of the aromatic ring is seen with quinoline or acridine; only η²-arene complexes are formed. The structures of many of these compounds are supported by X-ray data.