Fast, Ligand- and Solvent-Free Synthesis of 1,4-Substituted Buta-1,3-diynes by Cu-Catalyzed Homocoupling of Terminal Alkynes in a Ball Mill

A method for the Glaser coupling reaction of alkynes by using a vibration ball mill has been developed. The procedure avoids the use of ligands and solvents during the reaction. Aryl‐ and alkyl‐substituted terminal alkynes undergo homocoupling if coground with KF–Al2O3 and CuI as a milling auxiliary and catalyst. Furthermore, an alternative protocol has been developed incorporating 1,4‐diazabicyclo[2.2.2]octane (DABCO) as an additional base allowing the use of KF–Al2O3 with a lower KF loading. Besides Cu salts, the homocoupling of phenylacetylene is also catalyzed by Ni or Co salts, as well as by PdCl2. TMS‐protected phenylacetylene could be directly converted into the homocoupling product after in situ deprotection of the alkyne by fluoride‐initiated removal of the trimethylsilyl group. Alkynes put through the mill: A method for the highly selective, fast, and solvent‐free homocoupling of terminal alkynes in a ball mill is presented (see scheme). The reaction yields the corresponding buta‐1,3‐diynes in medium‐to‐high yields in the presence of CuI and KF–Al2O3 as catalyst and base, respectively. The Cu catalyst can also be replaced by Ni or Co salts. The selectivity is high and independent of the metal salt.