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.