Coupling of Aryl Chlorides by Nickel and Reducing Metals

Article abstract of DOI:10.1021/jo00364a002

An efficient synthesis of biaryls (diarenes) from aryl chlorides has been developed and investigated.The coupling reagent is a catalytic mixture of anhydrous nickel salt and triphenylphosphine in the presence of a reducing metal (Zn, Mg, or Mn).The reaction occurs rapidly under mild conditions, tolerates a variety of functional groups, employs air-stable starting materials, and can be run conveniently in ordinary laboratory glassware.Excess reducing metal drives the coupling process and allows even aryl chlorides to be coupled to high yields, a distinguishing advantage over previous coupling processes which employed stoichiometric zero-valent nickel reagents.The reaction is pseudo-zero-order in aryl chloride and is promoted by 2,2'-bipyridine and by halide salts (F < Cl < Br < I).A number of reaction variables have been examined and their effects on the kinetics and yields are discussed.Reduction and aryl transfer from triphenylphosphine, the major side reactions, can be minimalized in most cases.A coupling mechanism consistent with the observed kinetics and reaction parameters is proposed in which the key step is reduction of an arylnickel species by zinc to an arylnickel(I) complex.