Developing asymmetric iron and ruthenium-based cyclone complexes; complex factors influence the asymmetric induction in the transfer hydrogenation of ketones.

The preparation of a range of asymmetric iron and ruthenium-cyclone complexes, and their application to the asymmetric reduction of a ketone, are described. The enantioselectivity of ketone reduction is influenced by a single chiral centre in the catalyst, as well as by the planar chirality in the catalyst. This represents the first example of asymmetric ketone reduction using an iron cyclone catalyst.

The importance of the N-H bond in Ru/TsDPEN complexes for asymmetric transfer hydrogenation of ketones and imines.

Ru(II) complexes of TsDPEN containing two alkyl groups on the non-tosylated nitrogen atom are poor catalysts for asymmetric transfer hydrogenation of ketones and imines; this observation provides direct evidence for the importance of the N-H interaction in the transition state for ketone reduction.

Ru(II) complexes of N-alkylated TsDPEN ligands in asymmetric transfer hydrogenation of ketones and imines.

N-Alkylated TsDPEN derivatives bearing a small alkyl group act as highly efficient ligands in Ru(II) complexes for the asymmetric transfer hydrogenation of imines and ketones. A larger alkyl group serves to significantly reduce the activity of the catalyst; however, high enantiomeric excesses are still obtained. An X-ray crystal structure of the N-benzyl derivative reveals a conformation that permits hydrogen transfer through a six-membered transition state. A transition state structure for the imine reduction process is proposed.

Asymmetric transfer hydrogenation of C=O and C=N bonds by tethered Rh(III) catalysts.

Rh(III) catalysts containing a tetramethylcyclopentadienyl group linked by a 'tether' to a tosylated diamine ligand have previously been reported by our group for the asymmetric transfer hydrogenation (ATH) of ketones. The extension of these catalysts to the asymmetric reduction of imines, as well as to more highly functionalized substrates is reported. In some cases, the catalysts give better ee values than other methods for these transformations at lower catalyst loadings. The introduction of a methoxy group into the tethering aryl ring does not negate the performance of the catalyst, thus opening up a route to supported derivatives.