Acid-catalyzed cleavage of 4-halonortricyclanes. Evidence that the 2-norbornyl cation is an unsymmetrical species

Abstract

Nortrieyclanes 4a, 4c, 4d, and 4e were prepared and ring opened in D2SO4/CD3COOD. We found that the ratios of endo/exo deuterium at C(6) were 1.09 ± 0.02, 1.30 ± 0.03, 1.40 ± 0.04. and 1.45 ± 0.05 for cleavage of 4a, 4c, 4d, and 4e, respectively. The amount of 6,2-hydride shift decreased from 14.3 ± 1.0% in the case of nortricyclane (4a) to 6.0 ± 1.1%, 6.4 ± 0.5%, and 8.1 ± 0.7% for 4c, 4d, and 4e, respectively. Although four distinct mechanisms involving either nonclassical or rapidly-equilibrating cations arc possible, cleavage most probably occurs to generate 6,2-unsymmetrically-bridged norbornyl cations.