First observable CD spectra from n-σ* excitation: TD-DFT calculation and determination of absolute configuration of 2,6-dithiaspiro[3.3]heptane 2,6-dioxide

Abstract

Recently we reported that 2,6-dithiaspiro[3.3]heptane 2,6-dioxide 1a exists as enantiomers at ambient temperature and is able to be resolved. This is because the lone pair(s) on the sulfur atom(s) are rich in s-character, which results in high barrier for flipping the conformation of the four-membered ring, i.e., racemization of the spiro framework of 1a. During the HPLC analysis, we noticed considerably strong absorption of UV at 210 nm. We expected that the absorbance comes from n-σ* excitation. The acute bond angle of C-S-C leads to high p-character of the C-S bond(s), which lowers the σ* orbital energy. We performed the TD-DFT calculations of a model, constrained dimethylsulfoxide (DMSO) and 1a. The strong UV absorption of 1a is reproduced with the TD-DFT calculation. However, there are two sulfoxide groups arranged with chiral position in 1a, we could expect that the circular dichroism (CD) spectra are observable by coupling of two sulfoxide groups. The TD-DFT calculation showed that the Cotton effects should appear in the observable UV range. We separated the enantiomers of 1a by HPLC equipped with a chiral stationary phase column. The enantiomers are subjected to the CD analysis. We determined the absolute configuration of 1a, comparing the calculation and the obtained spectra. This is the first report of the CD spectra of n-σ* excitation.