Iron-catalyzed, hydrogen-mediated reductive cyclization of 1,6-enynes and diynes: Evidence for bis(imino)pyridine ligand participation

(Chemical Equation Presented) The bis(imino)pyridine iron dinitrogen complex (^iPrPDI)Fe(N₂)₂ catalyzes the hydrogen-mediated reductive cyclization of enynes and diynes with turnover frequencies comparable to those of established precious metal catalysts. Amino, oxygenated, and carbon-based substrates are readily cyclized to the corresponding hetero- and carbocycles with 5 mol % iron and 4 atm H₂ at 23°C. Stoichiometric reactions between selected substrates and the iron compound under a N₂ atmosphere established transfer dehydrogenation from an isopropyl aryl substituent to either the enyne or diyne substrate. In situ monitoring of the catalytic reaction by ¹H NMR spectroscopy coupled with deuterium labeling experiments established rapid cyclization followed by turnoverlimiting hydrogenation.