Half-Sandwich Complexes of Osmium(II) with l-α-Amino Carboxylate Ligands as Asymmetric Transfer Hydrogenation Catalysts. On the Origin of the Enantioselectivity

The preparation and characterization of optically active half-sandwich osmium­(II) complexes with l-α-amino carboxylate ligands (Aa) of formula [(η6-p-MeC6H4iPr)­Os­(Aa)­Cl] are reported. Chloride abstraction renders the corresponding cationic trimers [{(η6-p-MeC6H4iPr)­Os­(Aa)}3]3+, which were isolated as tetrafluoroborate salts. Trimerization most probably occurs through the chiral-at-metal mononuclear species [(η6-p-MeC6H4iPr)­Os­(Aa)­(MeOH)]+ and takes place with chiral self-recognition: i.e., only the equal configurations at metal ROs,ROs,ROs and SOs,SOs,SOs diastereomers are detected. The molecular structures of the mononuclear complexes [(η6-p-MeC6H4iPr)­Os­(Aa)­Cl] (Aa = MePhe, Pip, MePro) as well as those of the trimers [{(η6-p-MeC6H4iPr)­Os­(Aa)}3]­[BF4]3 (Aa = Pro, Pip) are also reported. The metal and, for amino carboxylates with secondary or tertiary amino moieties, also the nitrogen atom are stereogenic centers. Characterization of the new compounds includes determination of their absolute configuration. Assignment of the configurations in solution has been carried out by NOE and circular dichroism experiments. While both configurations are detected for the osmium, the configuration at nitrogen is induced by that of the carbon of the starting l-α-amino acid. Both neutral mononuclear compounds and cationic trimers catalyze the asymmetric transfer hydrogenation from 2-propanol to ketones with up to 82% ee. From stereochemical considerations, the enantiodifferentiation achieved can be accounted for by assuming that Noyori’s bifunctional mechanism is operating.