Highly Enantioselective Hydrogenation of Simple Ketones Catalyzed by a Rh ± PennPhos Complex**

The development of new chiral ligands plays a crucial role in expanding the utility of transition metal catalyzed asymmetric reactions.[1] A major research goal in asymmetric catalysis is to impart high enantioselectivity and activity to important reactions by the invention of new chiral ligands and the optimization of reaction conditions for use of these ligands. Many effective chiral bisphosphanes contain a diarylphosphane as the key steric group that defines the electronic properties.[1] Recently, we designed conformationally rigid endo-2,5-dialkyl-7-phosphabicyclo[2.2.1]heptanes as new chiral scaffolds, and demonstrated that these monophosphane species can be more effective for some asymmetric reactions[2] than the conformationally flexible 2,5-disubstituted phospholanes characteristic of the DuPhos and BPE ligands[3] (Figure 1). Herein we report the synthesis of a novel class of conformationally rigid chiral bisphosphanes, P,P'-1,2-phenylenebis(endo-2,5-dialkyl-7-phosphabicyclo[2.2.1]heptanes (PennPhos, 5 and 6).[4] Although PennPhos shares some features with DuPhos, such as electron-donating properties and a modular structure, it is bulkier, more rigid, and does not form C2-symmetric compounds with many transition metals. In addition, PennPhos can be made in large quantities from inexpensive starting materials and is an air-stable solid.