A New Class of Versatile Chiral-Bridged Atropisomeric Diphosphine Ligands: Remarkably Efficient Ligand Syntheses and Their Applications in Highly Enantioselective Hydrogenation Reactions

A series of chiral diphosphine ligands denoted as PQ-Phos was prepared by atropdiastereoselective Ullmann coupling and ring-closure reactions. The Ullmann coupling reaction of the biaryl
diphosphine dioxides is featured by highly efficient central-to-axial chirality transfer with diastereomeric
excess >99%. This substrate-directed diastereomeric biaryl coupling reaction is unprecedented for the
preparation of chiral diphosphine dioxides, and our method precludes the tedious resolution procedures
usually required for preparing enantiomerically pure diphosphine ligands. The effect of chiral recognition
was also revealed in a relevant asymmetric ring-closure reaction. The chiral tether bridging the two aryl
units creates a conformationally rigid scaffold essential for enantiofacial differentiation; fine-tuning of the
ligand scaffold (e.g., dihedral angles) can be achieved by varying the chain length of the chiral tether. The
enantiomerically pure Ru- and Ir-PQ-Phos complexes have been prepared and applied to the catalytic
enantioselective hydrogenations of α- and β-ketoesters (CO bond reduction), 2-(6‘-methoxy-2‘-naphthyl)propenoic acid, alkyl-substituted β-dehydroamino acids (CC bond reduction), and N-heteroaromatic
compounds (CN bond reduction). An excellent level of enantioselection (up to 99.9% ee) has been attained
for the catalytic reactions. In addition, the significant ligand dihedral angle effects on the Ir-catalyzed
asymmetric hydrogenation of N-heteroaromatic compounds were also revealed.