Radical Reactions of 1,4-Alkadiynes: Metal Coordination as an Effective Tool for Controlling the Regio- and Stereoselectivity of the C–C Bond Formation

A novel method for selective generation of radicals in 1,4-alkadiynes is developed by employing a π-bonded Co₂(CO)₆ core as a triple-bond immobilizing, cation-stabilizing, and radical-guiding auxiliary group. The isolation of α-alkynyl-cobalt-complexed propargyl cations and their reduction with zinc occurred in a regio- and stereoselective manner, giving rise to tetraethynylethanes with predominant formation of d,l-diastereomers (82–93%). The methodology provides an easy access to tetraethynylethanes and, upon oxidation, to practically important tetraethynylethenes. We also report on the novel phenomenon of “chiralization by metal complexation”, with achiral tetraynes being converted to chiral bis-clusters due to highly regio- and stereoselective complexation of the acetylenic groups.