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 (Mes(2)C=SO) adopts in solution the same chiral propeller conformat ion (C-1 symmetry) determined by X-ray diffraction in the crystalline state . With the help of MM calculations, it has been also shown that a correlate d rotation (cog wheel effect) of the two mesityl rings reverses the molecul ar helicity according to an enantiomerization process entailing a one-ring flip pathway with hG double dagger = 5.9 kcal mol(-1) and a two-ring flip p athway with DeltaG double dagger = 13.8 kcal mol(-1). On the contrary the Z - and E-isomers of mesityl phenyl sulfine (MesPhC=SO) adopt essentially ach iral conformations (C-s symmetry), having the Ph- CSO rotation barriers equ al to 5.2 and 5.8 kcal mol-l, respectively, and the mesityl-CSO rotation ba rriers equal to 21.3 and 15.1 kcal mol(-1), respectively.