Sulfone Gelander Helices: Revealing Unexpected Parameters Controlling the Enantiomerization Process

The two sulfonyl-bridged Gelander helices 1a and 2a are obtained by oxidation of the corresponding sulfide bridged precursors 1b and 2b. Both Gelander structures are fully characterized by NMR, high-resolution mass spectrometry, and optical spectroscopies. X-ray diffraction with a single crystal of 2a provides its solid-state structure. Both Gelander helices 1a and 2a are separated into enantiomers, and their racemizations are monitored by circular dichroism. For 1a, consisting of two equally sized macrocycles, a substantial increase in the enantiomerization barrier is observed upon going from the sulfide to the sulfone, and only a subtle rise is detected for the constitutional isomer 2a with two macrocycles of different size during the same transformation. This results not only in 1a with the highest configurational stability in the series of hitherto investigated Gelander structures but also challenges the so far hypothesized correlations between bridging structures and the Gibbs free energy of enantiomerization. The simulation of the enantiomerization process in the macrocyclic subunits suggests the proximity of the endotopic hydrogens as parameter responsible for the heights of the enantiomerization barrier.

Automated summary

The study involves the synthesis and characterization of two sulfonyl-bridged Gelander helices obtained from oxidation of sulfide bridged precursors. NMR, mass spectrometry, optical spectroscopy, and X-ray diffraction were used for full characterization. The results show a significant increase in enantiomerization barrier for 1a upon transformation from sulfide to sulfone, challenging previous assumptions about the correlation between bridging structures and enantiomerization Gibbs free energy.