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

Cationic (eta(6)-p-cymene) ruthenium(II) and (eta(5)-C5Me5) rhodium(III) complexes containing chiral chelate P,N ligands (1R,2R)-N-1-(4,8-di-tert-butyl-2,10-dimethoxydibenzo[d,f][1,3,2]dioxaphosphepin-6-yl)-N-1,N-2-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(eta(6)-p-cymene) (kappa-P,kappa-N)-(L1-L2)Cl]SbF6 1-2, the diastereomer having an S-Ru configuration was formed with a de >= 98%. For rhodium complexes [Rh(eta(5)-C5Me5)(kappa-P,kappa-N)-(L1-L2)Cl]SbF6 3-4, 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.