Ruthenium Complexes of Phosphino-Substituted Ferrocenyloxazolines in the Asymmetric Hydrogenation and Transfer Hydrogenation of Ketones: A Comparison

Three novel routes have been developed for the synthesis
of ferrocenyl-based phosphino-oxazolines in which the phosphino unit
is attached to a ferrocenylmethyl or a ferrocenylethyl side chain.
In two of the routes the phosphino-substituted ethyl side chain was
built up diastereoselectively. Ruthenium complexes of the type [RuCl2PPh3(L)] of 12 bidentate phosphine-oxazoline ligands
were synthesized, characterized, and tested in the transfer hydrogenation
of acetophenone. For the best performing complexes a total of 12 additional
ketones were screened in transfer hydrogenations and hydrogenations
under transfer hydrogenation conditions. Two catalyst precursors in
particular delivered products with an enantiomeric excess of up to
98% in transfer hydrogenations and 99% ee in hydrogenations. The transfer
hydrogenation results obtained with all novel ligands were compared
to those of two well-established FOXAP ligands. Furthermore, a qualitative
comparison with the hydrogenation data was carried out. In both cases
surprising similarities in product enantiomeric excess and product
absolute configuration were found. Attempts were made to rationalize
some of the observed features by considering a transition-state model.
The molecular structures of one synthesis intermediate, two catalyst
precursors, and two corresponding acetonitrile complexes were studied
by X-ray diffraction.