Design and Synthesis of Janus-Type Chiral Dendritic Diphosphanes and Their Applications in Asymmetric Hydrogenation

A series of chiral diphosphane-functionalized Janus dendrimers (up to 16 BINAP units) have been readily synthesized by using liquid-phase organic synthesis with the third-generation Frechet-type poly(aryl ether) dendron as the soluble support. The resulting dendritic ligands were purified by simple solvent precipitation without the need for chromatographic separation at the end of reaction. Complete functionalization of the dendrimers with BINAP moieties was confirmed by 1H NMR, MALDI-TOF mass spectroscopy, and elemental analyses. Their ruthenium complexes were applied to the asymmetric hydrogenation of 2-arylacrylic acids. It was found that only slightly lower enantioselectivities were achieved than the corresponding small-molecular Ru catalyst. Interestingly, a clear increase in activity of the dendritic catalysts was observed on going to the higher generations. In addition, the third-generation catalyst could be recycled without significant loss of catalytic activity or enantioselectivity before the fifth catalytic run.