Epimerization of trans‐Cycloalkenes with the X−C=C−SeR*‐Unit – The Steric Demand of X=H, F, Cl, Br, I, Me, Et and CF₃

Abstract

Trans‐cycloalkenes with the X–C=C–SeR*‐unit and ring sizes from 9 to 20 have been synthesized. Bond the selenium atom is the chiral (S)‐o‐(1‐Methoxypropyl)phenyl‐residue R*, and X = H, F, Cl, Br, I, Me, Et and CF3. The planar‐chiral trans‐cycloalkenes in combination with the chiral residue R* exist as two diastereomers. These can be distinguished in principle by NMR spectroscopy. We have studied the epimerization of the trans‐cycloalkenes, i.e., the 180° rotation of the X–C=C‐unit through the cavity of the ring. The measurements were done with variable temperature 13C NMR spectroscopy in the range from –110 to 140°C. The obtained values of the Gibbs energy of activation ΔG‡C depend strongly on the ring size. Furthermore, the ΔG‡C values show dramatic steric effects due to the groups X. The steric requirement of X increases in the series H << F << Cl < Me < Br < I < Et < CF3. Here, F is significantly larger than H, and CF3 is larger than Et. The corresponding iPr‐compounds could not be synthesized. The transition state structures of the ring inversion for ring sizes 8–20 were calculated at the DFT level of theory.