Mechanochemical Aerobic Activation of Metallic Copper for the Synthesis of 1,4‐Allenynes

Mechanical activation of zero-valent metals is emerging as a new paradigm in sustainable synthesis, opening up a broad swath of chemical space under more practical conditions than solution-phase methods. Contrary to its neighboring elements in the periodic table, copper has been significantly overlooked in this domain, especially for CC bond formation. Herein, a mechanochemical method for synthesizing >25 previously unreported conjugated allenynes-a valuable scaffold in organic synthesis is presented-through copper-catalyzed homocoupling of propargylic esters. This approach utilizes ball milling to activate inexpensive zero-valent copper under solvent-minimized conditions, sourced from both powders and recycled waste materials, including wires and bolts, into catalytically active Cu^I species under aerobic conditions. The synthetic utility of the synthesized compounds is showcased through an unprecedented rhodium-catalyzed cyclocarbonylation and a gold-catalyzed cyclization reaction.