Conformational Consequences of the Dynamic Processes in the Stereolabile Atropisomers of Acyl-Substituted Meta-Terphenyl Derivatives

By means of low temperature NMR spectroscopy the conformers (stereolabile atropisomers) due to the restricted rotation about the Ar-Ar and Ar-C(O)R bonds were detected in a number of acylphenyl derivatives, substituted in the positions 2 and 6 by the 3-isopropyl phenyl moiety (compounds 1-3, R = H, Me, t-Bu, respectively). The conformational assignment was accomplished on the basis of the symmetry of the low temperature 13C NMR spectra with the added support of ab-initio calculations. The interconversion barriers were also determined by complete line shape simulation of the NMR spectra and the experimental values were satisfactorily reproduced by ab-initio calculations. In the case of the asymmetric derivative 4, two enantiomers, generated by the restricted t-BuC(O)-Ar rotation, were found sufficiently stable as to allow their separation by means of the enantioselective HPLC technique at ambient temperature and to obtain the corresponding CD spectra.