15 - Copper Halide-Catalysed Oxidative Coupling of Acetylenes

Abstract

This chapter discusses the methods, scope, and limitations of copper halide-catalyzed oxidative coupling of acetylenes. The original procedure using pre-formed copper acetylide has evolved to a number of variants in which the acetylene is oxidized with oxygen or air in the presence of catalytic amounts of copper(I) halide. Instead of introducing oxygen, copper(II) salts can be used in stochiometrical amounts. Copper has a unique role. Salts of cobalt and iron are not capable of catalyzing the coupling. A wide variety of acetylenic compounds have been oxidatively “dimerized.” The oxidative coupling is carried out in an organic solvent or in aqueous medium depending on the acidity of the ethynyl proton in RC≡CH, the nature of R, and other factors. As the oxidative couplings proceed smoothly over a wide pH range, acid- as well as base-sensitive acetylenes can be dimerized with satisfactory results. Acetylenic amines can be coupled as their HCl salts. Under basic conditions the order of the reaction rates of oxidative couplings is found to be parallel with the expected order of acidities of the acetylenes. A number of heterosubstituted acetylenes do not give the coupling products because of the presence of strongly complexing groups in ethynylphosphines. Application of dimethylformamide (DMF), acetone, or other volatile organic solvents provides complementary possibilities for oxidative couplings. Water is a suitable solvent for couplings of the lower acetylenic alcohols such as HC≡CCH2OH.