Asymmetric hydrogenation in the core of dendrimers

The transition metal complexes containing chiral phosphorus ligands are the most widely and successfully used catalysts in asymmetric hydrogenation of unsaturated compounds. However, a major problem associated with these homogeneous catalytic systems is the separation and recycling of the often expensive and easily oxidized chiral catalysts. In addition, many hydrogenation reactions still lack efficient chiral catalysts, and the stereoselectivities in many hydrogenation reactions are substrate-dependent. Therefore, the development of highly effective and recyclable chiral phosphorus catalysts is highly desirable. Over the past few decades, a number of chiral catalysts have been successfully anchored onto different supports, such as cross-linked polymeric resins and inorganic materials. However, most of the classical supported chiral catalysts suffered from inferior catalytic properties to their homogeneous counterparts due to poor accessibility, random anchoring, and disturbed geometry of the active sites in the solid matrix. To overcome this drawback, dendrimers, which have well-defined and globular macromolecular architectures serve as a promising type of soluble catalyst support. The catalytic sites are generally located at the core or on the periphery of the dendrimer, and the resulting dendritic catalysts are designable. Incorporation of a chiral catalyst into a sterically demanding dendrimer will create a specific microenvironment around the catalytic site and thus influence the catalytic performance of the metal center, like an enzyme does. In this Account, we survey the development of core-functionalized chiral dendritic phosphorus ligands for asymmetric hydrogenation mainly by our research group. Several series of chiral dendritic phosphorus ligands, including diphosphines, monodentate phosphoramidites, and P,N-ligands, have been synthesized by attaching the corresponding chiral phosphorus units into the core or the focal point of Fréchet-type dendrons. Their transition metal (Ru, Rh, or Ir) complexes have been applied in the asymmetric hydrogenation of prochiral olefins and ketones, as well as some challenging imine-type substrates. All reactions were carried out in a homogeneous manner, and the structure-property relationships in some cases were established. The sterically demanding dendritic wedges were found to play important roles in catalytic properties, and better catalytic activities or enantioselectivities or both than those obtained from the corresponding monomeric catalysts were achieved in most cases. In addition, the dendritic catalysts could be readily recycled by means of solvent precipitation, water- or temperature-induced two-phase separation. Our study has thus demonstrated that dendrimer catalysis could combine the advantages of both classical heterogeneous and homogeneous catalysis.