Preparation, chromatographic enantiomer separation, ultraviolet–visible and circular dichroism spectra, and barriers to rotation of chiral twisted 1-acyl-1-thioacyl- and 1,1-bis(thioacyl)-2,2-diaminoethylenes

Three 1-acyl-1-thioacyl-2,2-diaminoethylenes and three dithio analogues with the amino groups included in a six-membered ring and carrying unlike groups (Pri and PhCH2) have been prepared by thiation of the oxygen analogues. These compounds are strongly twisted about the double bond, and four of them with unlike (thio)acyl groups are chiral and have been preparatively resolved by chromatography on swollen, microcrystalline triacetylcellulose. C.d. spectra have been recorded, and free energy barriers to rotation through the planar state (123–127 kJ mol–1) have been determined by monitoring the thermal recemization. Owing to the twist about the formal double bond, the compounds are best described as internal amidinium 1-(thio)acylvinyl-2-thiolates, and the electronic transitions are interpreted with the aid of CNDO/S calculations to occur partly in the (thio)acylvinylthiolate group, partly as charge transfer between this and the amidinium group. The latter transitions are made possible by overlap between the thiocarbonyl lone pair orbitals and the π* orbital in the amidinium group.