A Two-Ring Flip as the Threshold Enantiomerization Route for a Triarylvinyl Propeller. 1D and 2D NMR and Static and Dynamic Stereochemistry of Trimesitylethylene

1,2,2-Trimesitylethylene (16) was prepared by dehydration of 1,2,2-trimesitylethanol. Its 1D ¹H and ¹³C NMR spectra at room temperature show separate signals for pairs of groups (o-Me, o-C, m-C, and m-H) on the same ring, in agreement with a propeller conformation in solution. Assignment of the signals and derivation of the threshold rotational mechanism were aided by the use of 2D NMR techniques. Two rotational barriers were measured by dynamic NMR. The lower barrier (16.8 ± 0.1 kcal mol^-1) is for the threshold mechanism which is an [α,β]-two-ring flip. The higher barrier was calculated for the interchange of diastereotopic groups on the β‘-ring (20.5 ±0.1 kcal mol^-1). The difference in the threshold mechanisms of 16 and of trimesitylethenol (a three-ring flip) was ascribed to the presence of the small hydrogen on 16 which allows the during cis to it to undergo a nonflip process in a transition state with an energy lower than that of the three-ring flip for 16 or the two-ring flip for trimesitylethenol. The distinct rotational threshold mechanisms for Mes₂C=C(X)Mes (X = H, and OH, (Mes = mesityl)) are clearly reflected in their different 2D exchange NMR spectra.