Structural Conformers of (1,3-Dithiol-2-ylidene)ethanethioamides: The Balance between Thioamide Rotation and Preservation of Classical Sulfur–Sulfur Hypervalent Bonds

The
reaction of N-(2-phthalimidoethyl)-N-alkylisopropylamines and S2Cl2 gave
4-N-(2-phthalimidoethyl)-N-alkylamino-5-chloro-1,2-dithiol-3-thiones
that quantitatively cycloadded to dimethyl or diethyl acetylenedicarboxylate
to give stable thioacid chlorides, which in turn reacted with one
equivalent of aniline or a thiole to give thioanilides or a dithioester.
Several compounds of this series showed atropisomers that were studied
by a combination of dynamic NMR, simulation of the signals, conformational
analysis by DFT methods, and single crystal X-ray diffraction, showing
a good correlation between the theoretical calculations, the experimental
values of energies, and the preferred conformations in the solid state.
The steric hindering of the crowded substitution at the central amine
group was found to be the reason for the presence of permanent atropisomers
in this series of compounds and the cause of a unique disposition
of the thioxo group at close-to-right angles with respect to the plane
defined by the 1,3-dithiole ring in the dithiafulvene derivatives,
thus breaking the sulfur–sulfur hypervalent bond that is always
found in this kind of compounds.