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

Trans-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 degrees rotation of the X-C=C-unit through the cavity of the ring. The measurements were done with variable temperature C-13 NMR spectroscopy within the range from -110 to 140 degrees C. The obtained values of the Gibbs energy of activation Delta G(C)(not equal) depend strongly on the ring size. Furthermore, the Delta G(C)(not equal) values show dramatic steric effects due to the groups X. The steric requirement of X increases in the series H << F << Cl更多