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.