Conformational Studies by Dynamic NMR. 60.1 Structure and Stereomutation of the Enantiomers of Dialkoxy Disulfides

Abstract

The structure of dialkoxy disulfides ROSSOR has been established by single-crystal X-ray diffraction of 1, ArCH2OSSOCH2Ar (Ar = 4-nitrophenyl). The gauche conformation (OSSO dihedral angle = 85.4°) observed in the solid state entails the existence of M and P enantiomers that were actually detected in solution, in the case of MeOSSOMe, t-BuOSSOBu-t, and t-BuOSSOMe (3, 4, 5, respectively), by means of NMR spectroscopy in a chiral environment. The activation parameters (ΔG⧧, ΔH⧧, and ΔS⧧) for the stereomutation of this type of enantiomers, brought about by the S−S bond rotation at high temperature (50−110 °C), were measured by computer line shape simulation of the dynamic NMR spectra of appropriate derivatives bearing alkyl substituents with potentially diastereotopic geminal groups. The free energies of activation for such a process (18−19 kcal mol-1, depending on the compound) were found to be much higher than for the corresponding disulfides (RSSR), owing to the larger S−S double-bond character, as indica...