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 catalyzedby (cyclopentadienone)ironcarbonyl complexes was studied by means of kinetic isotope effect(KIE) measurements, intermediate isolation, and density functionaltheory calculations. The (cyclopentadienone)iron-amine intermediateswere isolated and characterized by H-1 and C-13 NMR spectroscopy as well as X-ray crystallography. The isolatediron-amine complexes are quite stable and undergo a formal beta-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 stepwisedehydrogenation. The density functional calculations corroborate astepwise mechanism involving a rate-determining hydride transfer fromamine to iron to yield a metal hydride and an iminium intermediate,followed by a proton transfer from the iminium ion to the oxygen ofthe cyclopentadienone ligand.