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.本研究通过动力学同位素效应（kinetic isotope effect, KIE）测试、中间体分离以及密度泛函理论（density functional theory, DFT）计算，对(环戊二烯酮)羰基铁配合物催化的胺脱氢反应机理开展了系统研究。研究中成功分离得到(环戊二烯酮)铁-胺中间体，并通过1H核磁共振波谱、13C核磁共振波谱以及X射线晶体学对其进行了全面表征。所分离获得的铁-胺配合物稳定性优异，可经形式上的β-氢消除反应生成亚胺与氢化铁配合物。针对4-甲氧基-N-(4-甲基苄基)苯胺的铁催化脱氢反应所观测到的动力学同位素效应，与分步脱氢机理完全契合。密度泛函计算结果证实了该分步反应机理：决速步为氢从胺向铁的转移过程，生成金属氢化物与亚胺鎓中间体，随后质子从亚胺鎓离子转移至环戊二烯酮配体的氧原子位点。