Electrocatalytic Semi-Hydrogenation of Terminal Alkynes using Ligand-Based Transfer of Protons and Electrons
Alkyne semi-hydrogenation is a broadly important transformation in chemical synthesis. Here, we introduce an electrochemical method for the selective semi-hydrogenation of terminal alkynes using a dihydrazonopyrrole Ni complex capable of storing an H2 equivalent (2H+ + 2e–) on the ligand backbone. This method is chemoselective for the semi-hydrogenation of terminal alkynes over internal alkynes or alkenes. Mechanistic studies reveal that the transformation is concerted and Z-selective. Calculations support a ligand-based hydrogen-atom transfer pathway instead of a hydride mechanism which is commonly invoked for transition metal hydrogenation catalysts. The synthesis of proposed intermediates demonstrates that the catalytic mechanism proceeds through a reduced formally Ni(I) species. The high yields for terminal alkene products without overreduction or oligomerization are among the best reported for any homogeneous catalyst. Furthermore, the metal-ligand cooperative hydrogen transfer enabled with this system directs the efficient flow of H-atom equivalents toward alkyne reduction rather than hydrogen evolution, providing a blueprint for applying similar strategies towards a wide range of electroreductive transformations.