Atropisomerism of 2,2-Diaryl-1,1-binaphthalenes Containing Three Stereogenic Axes: Experimental and Computational Study

The rotational barriers of four 2,2-diaryl-1,1-binaphthalenes have been determined on the basis of peak volume integration in variable-temperature H-1 EXSY NMR spectra. They were consistent with those determined at higher temperatures by complete line shape analysis. DFT calculations at the B2-PLYP//B3LYP-D3/6-311++G(d,p) level provided relative enthalpy values in very good agreement with the experimentally measured data both for particular atropisomers and for transition states. Calculations showed that the simultaneous rotation of both aryl groups has a much higher barrier than the individual rotations. The tendency of the atropisomerization barriers to increase for the first three derivatives correlates with the sizes of the substituents in the ortho positions of the phenyl rings (methoxy < dimethylamino < methyl), whereas the barriers for the derivative bearing isopropyl groups are, surprisingly, in the same range as for that bearing methyl substituents. We assume that in the latter case tilting of the binaphthalene moiety is responsible for the lowering of rotational barriers.