Four Enantiomerization Routes of 1,2,2-Trimesitylvinyl Acetate. Enantioselective Liquid Chromatography of (E)- and (Z)-2-m-Methoxymesityl-1,2-dimesitylvinyl Acetates.

The vinyl propellers (E)- and (Z)-2-m-methoxymesityl-1,2-dimesitylvinyl acetates (3c and 3d) were prepared and their geometries assigned. The stereoisomerization barriers of the trimesityl vinyl acetate system were determined by DNMR and by enantioselective LC resolution and polarimetric monitoring of the behavior of the two diastereomeric racemates of 3c and 3d. Combined with data for trimesitylvinyl-OAc 3a and its 1-m-methoxymesityl analogue 3b, the following order of barriers DeltaG() is obtained: alphabeta-2-ring flip > alphabeta'-2-ring flip > betabeta'-2-ring flip > alphabetabeta'-3-ring flip (the threshold enantiomerization barrier). This order which differs from the previously found orders for trimesitylvinyl-X, X = H, OPr-i was rationalized and discussed.