Atropisomerism in Triarylboranes: Lewis Base Assisted Rotation at C-B Stereogenic Axis in Asymmetrical Boron Lewis Acids.

The synthesis, properties and structural requirements for atropisomerism at the C-B bond in boron Lewis acids such as triarylboranes have been understudied so far. We report the first series of atropisomeric triarylboranes constituted of a naphthyl rotor and a dihydro-9-bora-anthracenyl stator subunits, connected by a C-B stereogenic axis. Through systematic crystallographic, kinetic, photophysical and quantum chemical studies, the mechanisms, rates and barriers of diastereomerization and enantiomerization were determined. The orthogonal arrangement between the naphthyl and the dihydro-9-bora-anthracenyl scaffold moiety hamper the rotation of these two moieties around their Csp-B bonds, enabling chiral resolution leading to enantiopure triarylboranes of high configurational stability. Furthermore, we fully elucidated a Lewis-base assisted pathway controlling the rotation speed at the C-B stereogenic axis, enabling the atropisomerical behavior of these triarylboranes be controlled by a Lewis base.