Conformational studies by dynamic NMR. 79. 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 (Mes2C=SO) 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 delta G++ = 5.9 kcal mol-1 and a two-ring flip pathway with delta G++ = 13.8 kcal mol-1. On the contrary the Z- and E-isomers of mesityl phenyl sulfine (MesPhC=SO) 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.