K. Maeda et al., DOES THE THRESHOLD ENANTIOMERIZATION ROUTE OF CROWDED TETRAARYLETHENES INVOLVE DOUBLE-BOND ROTATION, Journal of the American Chemical Society, 117(38), 1995, pp. 9686-9689
The chiral tetraarylvinyl propeller 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 diastereomer
ic 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 chromato
graphy. The enantiomerization process which was studied in n-pentadeca
ne 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 e
nantiomerization conditions. Consequently, we conclude that the thresh
old enantiomerization process of 4 does not involve a double bond rota
tion. A lower limit of 48.7 kcal mol(-1) is estimated for the barrier
of the latter rotation.