posted on 2021-09-27, 21:14authored byBing Zu, Yonghong Guo, Chuan He
The construction of main group heteroatom-stereogenic
compounds
is of great importance due to their intriguing chemical, physical,
biological, and stereoelectronic properties. Despite that organoboron
compounds are widely used in organic chemistry, the creation of a
tetrahedral boron-stereogenic center in one enantiomeric form remains
highly challenging. Given the labile nature of ligands attached to
the tetracoordinate boron atom, only a handful of enantioenriched
boron-stereogenic compounds have been reported via resolution or a
chiral substrate-induced diastereoselective approach. To date catalytic
asymmetric synthesis of boron-stereogenic compounds has remained unknown.
Here, we demonstrate the first catalytic enantioselective construction
of boron-stereogenic compounds via an asymmetric copper-catalyzed
azide–alkyne cycloaddition (CuAAC) reaction. This enantioselective
CuAAC reaction not only gives access to a wide range of novel highly
functionalized boron-stereogenic heterocycles in high yields with
good to excellent enantioselectivities but also produces optically
active terminal alkyne and triazole moieties with various potential
application prospects. Further transformation of the chiral tetracoordinate
boron compounds delivers several complex heterocyclic entities bearing
boron-stereogenic centers without the loss of enantiopurity. Moreover,
the X-ray structure, the barrier to racemization, and the HOMO/LUMO
gap of selected tetracoordinate boron compounds are investigated.
Notably, these novel N,N π-conjugated boron-stereogenic compounds
exhibit bright fluorescence. The optical properties, including circular
dichroism, quantum yield, and circular polarized luminescence spectroscopies,
are examined. These features expand the chemical space of the chiroptical
boron-based dye platform, which could have great potential applications
in chiral optoelectronic materials.