Affiliation:
1. Institut für Chemie und Biochemie, Anorganische Chemie Freie Universität Berlin Fabeckstr. 34–36 14195 Berlin Germany
2. Institut für Chemie, Analytische Chemie Universität Potsdam Karl-Liebknecht-Straße 24–25 14476 Potsdam, OT Golm Germany
3. Institut für Chemie und Biochemie, Physikalische und Theoretische Chemie Freie Universität Berlin Takustr. 3 14195 Berlin Germany
Abstract
AbstractTrans‐cycloalkenes with the X−C=C−SeR*‐unit and ring sizes from 9 to 20 have been synthesized. R* bonding the selenium atom is the chiral (S)‐o‐(1‐Methoxypropyl)phenyl‐moiety, 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. Here, 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 within 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.
Funder
Deutsche Forschungsgemeinschaft
Freie Universität Berlin
Subject
Organic Chemistry,Physical and Theoretical Chemistry