Conformational Studies by Dynamic NMR. 63.¹Stereodynamics of Dialkylamino Ethoxy Disulfides:  S−S and S−N Rotation Processes

Abstract

The title compounds (EtOS−SNR2) display diastereotopic OCH2 hydrogens below room-temperature owing to the existence of two stereolabile enantiomers brought about by the restricted S−S rotation in the adopted gauche conformer. A second dynamic process, which makes the N-bonded alkyl groups diastereotopic, was also observed at a much lower temperature (below −80 °C). On the basis of experimental observations and of ab initio calculations, the latter has been interpreted as due to the restricted rotation about the S−N bond. Line shape analysis of the variable temperature 1H and 13C NMR spectra allowed the free energies of activation to be determined for both motions in the case of R = Me, Et, Pr i .

Solid State Optical Activity of Dichalcogenides: Isolation by Chiral Crystallization and Determination of Absolute Configuration

“…Acyclic disulfides are well-known to show skewed structures in their crystalline states − or in the gas phase, , except for disulfides with congested substituents, and many theoretical studies have indicated that such skewed geometries are global minima. ,− Dynamic NMR studies of disulfides also support these results. ,− Similar findings have been reported for diselenides 10,12,14-17 and ditellurides. ,,, The skewed geometries of dichalcogenides have been explained on the basis of a gauche effect proposed by Wolfe. ,,, Therefore, dichalcogenides can be considered to exist in chiral P- and M- helical forms . However, the barriers of rotation of chalcogen−chalcogen bonds for racemization are not high enough, as estimated by theoretical 9-11 and dynamic NMR studies, ,, to isolate the optical isomers. Thus, racemization readily occurs in solution.…”

“…Furthermore, stereochemistry of chiral crystallization could be controlled by using an optically active crystal as a seed crystal.Acyclic disulfides are well-known to show skewed structures 1 in their crystalline states [2][3][4][5] or in the gas phase, 6,7 except for disulfides with congested substituents, 8 and many theoretical studies have indicated that such skewed geometries are global minima. 7,[9][10][11] Dynamic NMR studies of disulfides also support these results. 5,[11][12][13] Similar findings have been reported for diselenides 10,12,[14][15][16][17] and ditellurides.…”

“…7,[9][10][11] Dynamic NMR studies of disulfides also support these results. 5,[11][12][13] Similar findings have been reported for diselenides 10,12,[14][15][16][17] and ditellurides. 10,16,18,19 The skewed geometries of dichalcogenides have been explained on the basis of a gauche effect proposed by Wolfe.…”

“…10,16,18,19 The skewed geometries of dichalcogenides have been explained on the basis of a gauche effect proposed by Wolfe. 5,11,20,21…”

“…1997, 62, 4924. (6) (a) Yokozeki, A.; Bauer, S. H. Therefore, dichalcogenides can be considered to exist in chiral P-and M-helical forms. 22 However, the barriers of rotation of chalcogen-chalcogen bonds for racemization are not high enough, as estimated by theoretical [9][10][11]5,11,13 to isolate the optical isomers. Thus, racemization readily occurs in solution.…”