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Effectiveness and Mechanism of the Ene(amido) Group in Activating Iron for the Catalytic Asymmetric Transfer Hydrogenation of Ketones

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journal contribution
posted on 31.12.2020, 20:03 by Qingquan Xue, Rongliang Wu, Di Wang, Meifang Zhu, Weiwei Zuo
π-interacting ligands of the diphosphino amido-ene­(amido) type are effective in activating iron to resemble the properties of precious metals in the catalytic asymmetric transfer hydrogenation of ketones. To further verify the effectiveness of the ene­(amido) group, we synthesized four amine­(imine) diphosphine iron precatalyst complexes with substituents at α and β positions relative to imino groups (13) or with enlarged chelate ring sizes (5,5,6-membered rings) (4). In comparison with the parent trans-(R,R)-[Fe­(CO)­(Cl)­(PPh2CH2CHNCHPhCHPhNHCH2CH2PPh2)]­BF4 (I), the introduction of a methyl group in 1 and 2 reduced the catalytic activity but led to undiminished enantioselectivity as reaction proceeded. In comparison to the iron complexes 13 with a 5,5,5-coordination geometry, the complex 4 derived from the new (R,R)-P-NH-NH2 tridentate ligand showed high reactivity comparable to that of I but was unfortunately not enantioselective. The catalytic reactivity of 1, 2, and 4 illustrates the effectiveness of the ene­(amido) group. An electronic structure study on the important catalytic intermediate amido-ene­(amido) complex 1b proved that iron was activated by an additional π-back-donation-interaction ligand to participate in the traditional metal–ligand bifunctional pathway in the asymmetric transfer hydrogenation reactions.