Axial Chirality at the Boron-Carbon Bond: Synthesis, Stereodynamic Analysis, and Atropisomeric Resolution of 6-Aryl-5,6-dihydrodibenzo[c,e][1,2]azaborinines.

6-Aryl-5,6-dihydrodibenzo[c,e][1,2]azaborinines display restricted rotation at the boron-carbon aryl bond, yielding conformational isomers or atropisomers. The stereodynamic processes were monitored by variable-temperature NMR (D-NMR), dynamic enantioselective HPLC (D-HPLC), or kinetic racemization measurements. The absolute configuration of stable atropisomers (compound 1d, ΔG⧧rac = 26.0 kcal·mol-1) was determined by TD-DFT simulation of the electronic circular dichroism (ECD) spectra. The racemization energy of 1d is more than 12 kcal·mol-1 smaller than its isostere 9-(2-methylnaphthalen-1-yl)phenanthrene.