Unprecedented detection of enantiomerization pi-barriers due to restricted aryl torsion: case of 1,8-di-arylbiphenylenes.

Dynamic NMR spectroscopy allowed, for the first time, the determination of the pi-barriers responsible for the enantiomerization processes in derivatives bearing two aryl substituents bonded to a planar framework: this could be achieved in the case of 1,8-di-m-tolylbiphenylene (1) and 1,8-di-m-xylylbiphenylene (2). In derivative 1, the three possible conformers predicted by DFT computations (anti-in, syn, and anti-out) were detected, and in addition, the steric barrier responsible for their interconversion could be measured. The barriers predicted by DFT calculations were found in satisfactory agreement with experimental data.