Asymmetric hydrogenation of N-alkyl and N-aryl ketimines using chiral cationic Ru(diamine) complexes as catalysts: the counteranion and solvent effects, and substrate scope

• Published in: Tetrahedron Vol. 68; no. 26; pp. 5248 - 5257
• Main Authors: Chen, Fei, Ding, Ziyuan, He, Yanmei, Qin, Jie, Wang, Tianli, Fan, Qing-Hua
• Format: Journal Article
• Language: English
• Published: OXFORD Elsevier Ltd 01.07.2012; Elsevier
• Subjects: amines; Anions; Asymmetric catalysis; Bearing; Catalysis; Catalysts; Cationic; Chemistry; Chemistry, Organic; Diamine ligand; enantiomers; Hydrogenation; Imines; Ketimines; methylene chloride; phosphoric acid; Physical Sciences; Ruthenium; Science & Technology; solvents; Diamine ligand; Ruthenium; Ketimines; Asymmetric catalysis; Hydrogenation; ALDOL ADDITIONS; PHOSPHINE-OXAZOLINE LIGANDS; C-SYMMETRIC COPPER(II) COMPLEXES; ACYCLIC IMINES; KETONES; HIGHLY ENANTIOSELECTIVE HYDROGENATION; IRIDIUM COMPLEXES; LEWIS-ACIDS; EFFICIENT; REDUCTIVE AMINATION
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/j.tet.2012.03.019

Asymmetric hydrogenation of N-alkyl and N-aryl ketimines catalyzed by chiral cationic η6-arene-(N-monosulfonylated diamine) Ru(II) complexes has been investigated. Strong counteranion and solvent effects on the enantioselectivity were observed. The ruthenium catalyst bearing non-coordinating BArF− anion was found to be particularly effective for the hydrogenation of acyclic and exocyclic N-alkyl ketimines in the presence of (Boc)2O in dichloromethane or even under solvent-free conditions, providing chiral amines with up to >99% ee and full conversions. Alternatively, the ruthenium catalyst bearing achiral phosphate anion together with corresponding phosphoric acid as the additive was also efficient for the hydrogenation of N-alkyl ketimines in the absence of (Boc)2O with excellent enantioselectivities and full conversions. For N-aryl ketimines lower enantiomeric excesses were observed by using the ruthenium catalyst bearing BArF− anion. This catalytic protocol thus provides a facile and practical access to optically active amines and has been successfully employed in the gram-scale synthesis of enantiomerically pure (+)-sertraline. [Display omitted]