Ullmann condensation using copper or copper oxide as the reactant. Arylation of active hydrogen compounds (imides, amides, amines, phenol, benzoic acid, and phenylacetylene)

Abstract

Ullmann condensation using Cu or copper oxide as the reactant and using no base has been studied. Reactions of active N—H, O—H, and C—H compounds, RZH (imides, amides, amines, phenol, benzoic acid, and phenylacetylene), with mixtures of aryl halides and more than a stoichiometric amount of Cu or copper oxide (Cu2O or CuO) without addition of base lead to facile arylation of the active hydrogen compounds. In the arylation reaction Cu and copper oxides serve not only as the catalyst but also as the acceptor of halogen from aryl halide. The reactivity of phenyl halide, PhX, increases in the order PhCl < PhBr < Phl. Cu generally gives higher yields than Cu2O and CuO and, of the two copper oxides, Cu2O has the higher activity. The stronger the acidity of the active hydrogen compound (RZH), the more easily the compound is phenylated by the mixture of PhX and Cu; a roughly linear correlation between Ti, the temperature where the phenylation starts, and the pKa value of the active hydrogen compound has been observed. A mechanism involving initial formation of an RZ—Cu species by the reaction between RZH and Cu or copper oxide and ensuing attack of organic halide at the RZ—Cu species is proposed.