Norticyclic-Norbornenyl Cation. Isotopic Perturbation and Isotopic Scrambling

Article abstract of DOI:10.1021/ja00246a035

The stable carbocation which is formed from either 3-nortricyclyl or norbornenyl precursors was studied with the deuterium isotopic perturbation method. 13C NMR signals from the two carbons, expected to be videly split by isotopic perturbation if an equilibrium exist between rapidly interconverting norbornenyl structures, showed only a small, temperature-independentsplitting.It was concluded that a maximum of ca. 1percent of the equilibrating norbornenyl ions might be present.A single structure (nortricyclyl with extensive charge delocalization) adequately represents the cation.This is verified by di-13C-labeled cation precursor, which also provides the complete set of J₁₂ 13C-13C coupling constants.NMR spectra of the deuteriated ion and of 1,7-di-13C-labeled ion revealed that the cation is undergoing at least three different scrambling processes below 0 deg C.The spectra show that the fastest process involves reversible skeletal rearrangement to the bicyclo<3.1.1>heptenyl cation.A still slower process including a hydride shift also occurs.The observed rearrangements ultimately result in complete scrambling of all carbons and hydrogens in the cation.