Unprecedented Detection of the Enantiomerisation pi-Barriers due to Restricted Aryl Torsion: the Case of 1,8-Di-arylbiphenylenes

Dynamic NMR spectroscopy allowed, for the first time, the determination of the pi-barriers responsible for the enantiomerisation 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 the experimental data.