Development of a Tunable Chiral Pyridine Ligand Unit for Enantioselective Iridium-Catalyzed C-H Borylation

Although pyridine derivatives are versatile supporting ligands in catalysis, the development of their chiral versions has been relatively limited. Herein, we report the design, synthesis, and proof-of-concept application of a structurally tunable chiral pyridine framework featuring an annulated compact ring system. Using an N,B-bidentate ligand skeleton containing the chiral pyridine moiety, we have developed an enantioselective iridium-catalyzed desymmetrizing C-H borylation reaction of diaryl(2-pyridyl)methane compounds with up to 96% ee and 93% yield. The resulting borylation products could be readily transformed into various chiral tri(hetero)arylmethane compounds. Density functional theory investigations revealed that the two chair conformations of the flexible ketal motif both favored the enantiomer that was consistent with experimental results. This work has thus introduced an effective and tunable chiral pyridine ligand framework that may be used in many catalytic asymmetric transformations.