Switching Amination Pathways: How Ru(II) Oxidation State Favors Ru–O/N over Ru–C Bonded Intermediates

Employing the Ru(II)-complex ([Ru–H]) as an
efficient catalyst for the reductive amination of 5-hydroxymethylfurfural
(HMF) with H2 and NH3 to 2,5-bis(aminomethyl)furan
(BAMF) in tert-amyl alcohol solution, the reaction
mechanisms have been investigated at the M06/def2-TZVP, 6–311++G
(d,p) theoretical level. The aromatic precatalyst [Ru–H] serves as the precursor to the catalytically active species, undergoing
in situ conversion to the dearomatized complex [Ru-0], a process facilitated by the aldehyde group of HMF. In [Ru-0], the Lewis acidic Ru(II)-center facilitates the cleavage of multiple
bonds (N–H, C–OH, H–H, and C–H), and the
Lewis basic N-site in the ligand acts as a proton transfer-station,
assisted by the Cl– anion as a stable mediator.
The positive charge of the Ru(II)-center favors reaction pathways
involving Ru–O or Ru–N bonded intermediates over those
featuring Ru–C bonds. In contrast, the Ru(0)-center, with its
metallic nature, preferentially accesses and stabilizes the Ru–C
bonded pathway. The optimal reaction pathway proceeds through the
reductive amination of the aldehyde group rather than the amination
dehydration of the hydroxymethyl group. The rate-determining step
is associated with the cleavage of the C–OH bond for the amination
dehydration of aldehyde to imine. The findings of this study offer
valuable theoretical insights for the rational design of novel catalysts
targeting the reductive amination of aldehydes and hydroxymethyl-containing
compounds.

本研究以二价钌配合物([Ru–H])为高效催化剂，在叔戊醇溶液中实现5-羟甲基糠醛(5-hydroxymethylfurfural, HMF)与氢气(H₂)、氨气(NH₃)的还原胺化反应制备2,5-二(氨甲基)呋喃(2,5-bis(aminomethyl)furan, BAMF)，并在M06/def2-TZVP、6–311++G(d,p)理论水平下对该反应的机理展开了研究。该芳香型预催化剂[Ru–H]作为催化活性物种的前驱体，可在HMF醛基的辅助下原位转化为脱芳香化配合物[Ru⁰]。在[Ru⁰]中，路易斯酸性的Ru(II)中心可促进N–H、C–OH、H–H及C–H等多根化学键的断裂，而配体中的路易斯碱性氮位点则作为质子转移位点，在氯离子(Cl⁻)这一稳定媒介的协助下完成质子传递过程。Ru(II)中心的正电荷更倾向于形成Ru–O或Ru–N键合中间体的反应路径，而非Ru–C键合路径。与之相反，具有金属特性的Ru(0)中心则优先结合并稳定Ru–C键合的反应路径。最优反应路径经由醛基的还原胺化过程进行，而非羟甲基的胺化脱水过程。决速步与醛胺化脱水生成亚胺过程中的C–OH键断裂相关。本研究的结果为合理设计针对醛类及含羟甲基化合物还原胺化的新型催化剂提供了宝贵的理论参考。