Synthesis, Coordination Behavior, and Structural Features of Chiral Amino-, Pyrazolyl-, and Phosphino-Substituted Ferrocenyloxazolines and Their Application in Asymmetric Hydrogenations

A highly flexible and modular synthesis of 15 chiral nonracemic ferrocenyloxazolines-all based on a ferrocenylethyl backbone-is described. Starting from N,N-dimethylaminoethyl ferrocene a range of oxazoline derivatives with various oxazoline substituents were prepared in a three-step sequence. After reaction with methyl iodide, the quaternized dimethylamino group was replaced by different amino, pyrazolyl, and phosphinyl groups. Subsequent reduction of the phosphinyl groups gave phosphinooxazoline derivatives. The molecular structures of three palladium [PdCl2(L)] and three ruthenium [RuCl(p-cymene)(L)PF6 complexes of representative ferrocenyloxazoline ligands were studied by X-ray diffraction. In addition, five ruthenium complexes [RuCl2(PPh3)(L)] were prepared. These complexes were tested, using a high-throughput screening system, in the asymmetric hydrogenation of three ketones. Enantioselectivities of up to 99% ee were obtained.