Does the Threshold Enantiomerization Route of Crowded Tetraarylethenes Involve Double Bond Rotation?

The chiral tetraarylvinyl propeller 1,2-bis(4-tert-butyl-2,6-dimethylphenyl)-1,2-dimesitylethene (4) was synthesized by irradiation of (4-tert-butyl-2,6-dimethylphenyl)mesitylketene 5. Ethene 4 exists in two diastereomeric forms (E and Z), each existing as a racemate (i.e., (+)/(—) 4Z and (+)/(-) 4E). The enantiomers were resolved by chiral HPLC and the EIZ diastereomers were separated by (achiral) supercritical fluid chromatography. The enantiomerization process which was studied in n-pentadecane has a barrier of ΔG* = 44.8 ± 0.7 kcal mol^-1. A 4Z ⇌ 4E interconversion was not observed under the enantiomerization conditions. Consequently, we conclude that the threshold enantiomerization process of 4 does not involve a double bond rotation. A lower limit of 48.7 kcal mol^-1 is estimated for the barrier of the latter rotation.