Arylbiphenylenes Atropisomers: Structure, Conformation, Stereodynamics and Absolute Configuration

Anti and syn conformers, due to restricted sp2-sp2 bond rotation, were detected in hindered 1,8-diarylbiphenylenes, the aryl moieties being phenyl groups bearing ortho alkyl substituents such as Me, Et, i-Pr, t-Bu. By means of low temperature NOE experiments the corresponding structures were assigned and were found to be in agreement with the results of single crystal X-ray diffraction. The interconversion barriers of these conformers were determined by line shape simulation of the variable temperature NMR spectra and the experimental values were reproduced satisfactorily by DFT calculations. In the case of the bulkiest aryl substituent investigated (i.e. 2-methylnaphthalene) the syn and anti atropisomers were stable enough as to be separated at ambient temperature. The two enantiomers (M,M and P,P) of the isomer anti were also isolated by enantioselective HPLC and the theoretical interpretation of the corresponding CD spectrum allowed the absolute configuration to be assigned.