Synthesis and structural characterization of new chiral mixed phosphine phosphoramidite complexes of ruthenium

New phosphoramidite complexes of ruthenium chiral at the metal were synthesized, structurally characterized and their electrochemical and catalytic properties were studied. Reaction of the known chiral phosphoramidites (RO)2PNR2′ (R = naphthyl, R′ = CH3, 1a; R = naphthyl, R′ = benzyl, 1b; R = octahydronaphthyl, R′ = benzyl, 1c) with CpRu(PPh3)2Cl afforded the title compounds CpRu(PPh3)(1a–c)(Cl) (2a–c) in 46–74% isolated yields. Fractional crystallization of 2b and 2c afforded the corresponding diastereopure complexes which are chiral both at the metal and at the ligand. The molecular structures of 2b and 2c were determined, revealing a pseudo octahedral coordination geometry about the ruthenium center. Electrochemical studies by cyclic voltammetry showed reversible electrochemical behavior of the metal complexes 2a–c. The new metal complexes are catalytically active in the Mukaiyama aldol reaction (24 h, room temperature, 31–53% yield), but almost no enantiomeric excesses for the products were obtained.

Graphical abstractThe synthesis of novel phosphoramidite complexes of ruthenium is described, which are chiral at the metal. Two of the complexes were characterized structurally, and their electronic and steric properties are discussed. The novel complexes were also applied as catalysts in the Mukaiyama aldol reaction.Figure optionsDownload full-size imageDownload as PowerPoint slide