Steric Effect at C-7 of Norbornene

Abstract

<p>The object of this work was to show unambiguously which face, syn or anti, of the norbornene system is less sterically crowded. This was accomplished by use of thermodynamics and kinetics. These studies have produced the first quantitative measure of the free energy difference between isomers at C-7 caused only by the different steric requirements of the ethano and etheno bridges. In the thermodynamic studies, isomeric mixed ketals were employed. In the course of the stereochemical assignment of these isomers, it was found that a methoxy group has a greater levelling effect on the 7-cation of norbornene than has a p-anisyl group. The knowledge of the sense and approximate magnitude of the steric effect is applied to the reactions of the 7-radical of norbornene to cast light on the nature of this species.</p>