Mechanistic Studies on Iron-Catalyzed Dehydrogenation of Amines Involving Cyclopentadienone Iron ComplexesEvidence for Stepwise Hydride and Proton Transfer

The mechanism of dehydrogenation of amines catalyzed by (cyclopentadienone)iron carbonyl complexes was studied by means of kinetic isotope effect (KIE) measurements, intermediate isolation, and density functional theory calculations. The (cyclopentadienone)iron–amine intermediates were isolated and characterized by 1H and 13C NMR spectroscopy as well as X-ray crystallography. The isolated iron–amine complexes are quite stable and undergo a formal β-hydride elimination to produce imine and iron hydride complexes. The KIEs observed for the iron-catalyzed dehydrogenation of 4-methoxy-N-(4-methylbenzyl)aniline are in accordance with stepwise dehydrogenation. The density functional calculations corroborate a stepwise mechanism involving a rate-determining hydride transfer from amine to iron to yield a metal hydride and an iminium intermediate, followed by a proton transfer from the iminium ion to the oxygen of the cyclopentadienone ligand.