Sulfone "Geländer" Helices: Revealing Unexpected Parameters Controlling the Enantiomerization Process.

The two sulfonyl-bridged Geländer helices and are obtained by oxidation of the corresponding sulfide bridged precursors and . Both Geländer structures are fully characterized by NMR, high-resolution mass spectrometry, and optical spectroscopies. X-ray diffraction with a single crystal of provides its solid-state structure. Both Geländer helices and are separated into enantiomers, and their racemizations are monitored by circular dichroism. For , 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 with two macrocycles of different size during the same transformation. This results not only in with the highest configurational stability in the series of hitherto investigated Geländer 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.