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 E/Z diastereomers were separated by (achiral) supercritical fluid chromatography. The enantiomerization process which was studied in n-pentadecane has a barrier of Delta G double dagger = 44.8 +/- 0.7 kcal mol(-1). A 4Z reversible arrow 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.