Half-sandwich ruthenium(II) and rhodium(III) complexes bearing chiral amino-phosphoramidite ligands: Synthesis, characterization and application in asymmetric transfer hydrogenation of acetophenone

Cationic (η6-p-cymene) ruthenium(II) and (η5-C5Me5) rhodium(III) complexes containing chiral chelate P,N ligands (1R,2R)-N1-(4,8-di-tert-butyl-2,10-dimethoxydibenzo[d,f][1,3,2]dioxaphosphepin-6-yl)-N1,N2-dialkylcyclohexane-1,2-diamine (alkyl = Me L1, Et L2) have been synthesized and characterized by multinuclear one- and two-dimensional NMR spectroscopy, conductivity measurements, mass spectroscopy and elemental analysis. In the case of the ruthenium complexes [Ru(η6-p-cymene)(κ-P,κ-N)-(L1–L2)Cl]SbF61–2, the diastereomer having an SRu configuration was formed with a de ⩾ 98%. For rhodium complexes [Rh(η5-C5Me5)(κ-P,κ-N)-(L1–L2)Cl]SbF63–4, the stereoselectivity of the chelation process and the configurational stability of the complexes depend on the reaction conditions and N-substituent of amino nitrogen.Some of these complexes have been tested as precatalysts in the asymmetric transfer hydrogenation of acetophenone by propan-2-ol.

Graphical abstractCationic (η6-p-cymene) ruthenium(II) and (η5-C5Me5) rhodium(III) complexes containing chiral chelate amino-phosphoramidite ligands have been synthesized and characterized. Diastereoselective synthesis of (SRu)-[Ru(η6-p-cymene)(κ-P,κ-N)-(L1–L2)Cl]SbF6 complexes 1–2 have been obtained. Some of these cationic ruthenium and rhodium complexes are active catalysts for AHT in the presence of KOH or NEt3.Figure optionsDownload full-size imageDownload as PowerPoint slide