Cu-and Ni-catalyzed couplings

The Ullmann-type reaction that is homocoupling of aryl or vinyl halides is conveniently mediated by copper at high temperature. The copper powder serves as a zerovalent metal. The classical Ullmann reaction reported in 1901 has long been employed by chemists to generate a carbon-carbon bond between two aromatic nuclei. 

Nickel in its zerovalent state can also be used for the coupling of aryl halides to biphenyls and vinyl halides to 1,3-dienes. Bis(l,5-cyclooctadiene)nickel, Ni(COD)2, is the best source of zerovalent nickel for such couplings. 

Although copper-catalyzed homocoupling of alkynes and Ni(0)-catalyzed homocoupling of aryl halides were well known, Cadiot and Chodkiewicz gave first-time synthetically more useful copper-catalyzed cross-coupling of alkynes. 

The copper(I)-catalyzed cross-coupling of a terminal alkyne and an alkynyl halide to yield diyne is known as the Cadiot-Chodkiewicz coupling.  

The Castro-Stephens coupling also involves the reaction of copper(I) acetylide with aryl halides to form diarylacetylenes. 

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