Vibrational and electronic circular dichroism of a chiral terthiophene sulfonamide

The new terthiophene sulfonamide derivative, TTS, was synthesized and its IR, VCD, UV and ECD spectra were measured and interpreted by means of the DFT calculations. The TTS conformational space analyzed at the molecular mechanics, B3LYP/TZVP/PCM, and B3LYP/def2TZVP/PCM levels, was reduced to 72 conformers none of which appeared to be definitely dominant. A detailed examination of the slight deviations in mutual positions of the three thiophene rings revealed that for each conformer an additional 16 minima are apparently important but only six of them appear to be meaningful in 300 K. Apart from the permanent stereogenic center at the C6 carbon atom, the N-atom constitutes another (labile) stereogenic center connected to the presence of a lone electron pair at this very atom. This is significant because we found that the most stable conformers exhibit almost exclusively the (S)-configuration at the N-atom. The experimental IR and VCD spectra of TTS were fairly well reproduced if its monomers and dimers were taken into account. On the other hand, the experimental ECD spectra were difficult to reproduce because the spectra with slightly deviated thiophene rings produce practically mirror image ECD spectra. Calculated and recorded VCD spectra confirm configuration at the C6 of TTS, which was earlier predicted using stereochemical model for 1,2-addition to N-sulfinylimines proposed by Ellman.