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

The bis(imino)pyridine iron dinitrogen complex (((i)Pr)PDI)Fe(N(2))(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 H(2) at 23 degrees C. Stoichiometric reactions between selected substrates and the iron compound under a N(2) atmosphere established transfer dehydrogenation from an isopropyl aryl substituent to either the enyne or diyne substrate. In situ monitoring of the catalytic reaction by (1)H NMR spectroscopy coupled with deuterium labeling experiments established rapid cyclization followed by turnover-limiting hydrogenation.