Novel Chiral PNNP Ligands with a Pyrrolidine Backbone – Application in the Fe‐Catalyzed Asymmetric Transfer Hydrogenation of Ketones

The PNNP ligand ( R , R )‐{PPh 2 (2‐C 6 H 4 )CH=N(pyrrolidine‐NBn)‐} 2 2 was prepared by condensation between 2‐diphenylphosphino benzaldehyde and the pyrrolidine‐substituted diamine 1 . Reduction with NaBH 4 in MeOH afforded ( R , R )‐{PPh 2 (2‐C 6 H 4 )CH–NH(pyrrolidine‐NBn)‐} 2 3 . The corresponding iron(II) complexes [FeCl 2 ( 2 )] ( 4 ), [Fe(CH 3 CN) 2 ( 2 )](BF 4 ) 2 ( 5 ) and [Fe(CH 3 CN) 2 ( 2 )](PF 6 ) 2 ( 6 ) were synthesized and fully characterized by NMR, ESI‐HRMS, and EA. DFT calculations for complexes [FeCl 2 ( 2 )] ( 4 ) and [Fe(CH 3 CN) 2 ( 2 )](BF 4 ) 2 ( 5 ) were carried out in order to investigate the stability of related structures. The PNNP ligands 2 and 3 in combination with Fe 3 (CO) 12 as iron source were tested as catalysts in the asymmetric transfer hydrogenation of a variety of ketones with conversions higher than 95 % and enantioselectivities up to 97 %.