Four Enantiomerization Routes of 1,2,2-Trimesitylvinyl Acetate. Enantioselective Liquid Chromatography of (E)- and (Z)-2-m-Methoxymesityl-1,2-dimesitylvinyl Acetates Elimelech Rochlin, † Zvi Rappoport,* ,† Fritz Kastner, ‡ Nikola Pustet, ‡ and Albrecht Mannschreck ‡ Department of Organic Chemistry, The Hebrew University, Jerusalem 91904, Israel, and Institut fu ¨ r Organische Chemie der Universita ¨ t, Universita ¨ tstrasse 31, D-93040, Regensburg, FRG Received June 29, 1999 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 ΔG q is obtained: R-2-ring flip >R′-2-ring flip > ′-2-ring flip >R′-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. Triarylvinyl systems exist in the solid state and in solution in a chiral propeller conformation, and the static and dynamic stereochemistry of these triarylvinyl propel- lers were studied extensively during the past decade. 1 The stereoisomerization pathways in triarylvinyl propel- lers were described 1,2 in terms of “flip” processes. 3 In a flip process the flipping ring passes during the rotation via a plane perpendicular to the double bond plane, while concurrently nonflipping rings pass via the CdC plane. The mechanism is designated by the number of the flipping rings. Thus, for a triarylvinyl propeller where each ring has C 2 symmetry, one 0-ring flip, three isomeric 1-ring flips, three isomeric 2-ring flips, and one 3-ring flip, all leading to enantiomerization, are possible. Figure 1 displays the four latter pathways with the correspond- ing transition states for a tris(o,o′-dimethylphenyl)vinyl-X system. However, the experimental data on the barriers (∆G q ) for the different alternative enantiomerization routes in a single system are limited to two systems 1 and 2. Two competing enantiomerization routes were reported for 1,2,2-trimesitylethylene 1 4 where the order of the barriers was ∆G q R′ > ∆G q R , where the subscripts indicate the flipping rings in the transition state of the enantiomerization. In a previous study the dynamic stereochemistry of 1,2,2-trimesitylvinyl isopropyl ethers 2a-d 5 was inves- tigated. The activation barriers for the four enantiomer- ization routes depicted in Figure 1 were measured by a DNMR technique by labeling one at a time each of the mesityl rings by a m-OMe group, which converts the R′- 3-ring flip enantiomerization route to a diastereomeriza- † The Hebrew University. ‡ Institut fu ¨ r Organische Chemie der Universita ¨ t, Regensburg. (1) Rappoport, Z.; Biali, S. E. Acc. Chem. Res. 1997, 30, 307. (2) Biali, S. E.; Rappoport, Z. J. Am. Chem. Soc. 1984, 106, 477. (3) (a) Kurland, R. J.; Schuster, I. I.; Colter, A. K. J. Am. Chem. Soc. 1965, 87, 2276. (b) Mislow, K. Acc. Chem. Res. 1976, 9, 26. (c) Mislow, K.; Gust, D.; Finocchiaro, P.; Boettcher, R. J. Topics in Current Chemistry, No 47, Stereochemistry 1; Springer-Verlag: Berlin, 1974; p 1. (4) Biali, S. E.; Rappoport, Z. J. Org. Chem. 1986, 51, 2245. (5) Rochlin, E.; Rappoport, Z. J. Org. Chem. 1994, 59, 3857. Figure 1. Four enantiomerization routes and idealized transition state structures of a triarylvinyl propeller. The structures in the middle are the corresponding transition states with the rings either in the CdC plane or perpendicular to it. 8840 J. Org. Chem. 1999, 64, 8840-8845 10.1021/jo9910435 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/30/1999