Conformational Studies by Dynamic NMR. 79.1 Dimesityl Sulfine Revisited: Detection of the Helical Antipodes and Determination of Their Enantiomerization Pathways

By means of low-temperature NMR spectra, it is demonstrated that dimesityl sulfine (Mes2CSO) adopts in solution the same chiral propeller conformation (C1 symmetry) determined by X-ray diffraction in the crystalline state. With the help of MM calculations, it has been also shown that a correlated rotation (cog wheel effect) of the two mesityl rings reverses the molecular helicity according to an enantiomerization process entailing a one-ring flip pathway with ΔG⧧ = 5.9 kcal mol-1 and a two-ring flip pathway with ΔG⧧ = 13.8 kcal mol-1. On the contrary the Z- and E-isomers of mesityl phenyl sulfine (MesPhCSO) adopt essentially achiral conformations (Cs symmetry), having the Ph−CSO rotation barriers equal to 5.2 and 5.8 kcal mol-1, respectively, and the mesityl−CSO rotation barriers equal to 21.3 and 15.1 kcal mol-1, respectively.