13C NMR Spectroscopic Investigation of Tertiary Spiro[cyclopropane-3'-norbornan]-2'-yl Cations and their Rearrangements
2'-Methylspiro[cyclopropane-3'-norbornan]-2'-yl (9), 2'-phenylspiro[cyclopropane-3'-norbornan]-2'-yl (10), and 2-cyclopropylspiro[cyclopropane-3'-norbornan]-2'-yl (11) cations were prepared from the corresponding alcohols with Magic Acid (1:1 SbF5 + FSO3H) at low temperatures and characterized by 13C NMR spectroscopy. Cation 11 is extremely stable up to -20°C. Cation 10 rearranges to the 2-phenyl-4-methylbicyclo[3.2.1]oct-3-en-2-yl cation (13) at about -70°C. The cation 9, and its rearranged isomer 2,4-dimethylbicyclo[3.2.1]oct-3-en-2-yl cation (12), exist in an 1:1 ratio even at -90°C. The chemical shift assignments were aided by ab initio IGLO calculations. The study shows the overwhelming delocalizing ability of the 3-spirocyclopropyl group over C1-C6 σ-bond participation. The latter nonclassical σ-participation, however, was shown to persist to some extent even in the tertiary 2-norbornyl cations.
G. A. Olah et al., "13C NMR Spectroscopic Investigation of Tertiary Spiro[cyclopropane-3'-norbornan]-2'-yl Cations and their Rearrangements," Journal of Organic Chemistry, vol. 57, no. 4, pp. 1114-1118, American Chemical Society (ACS), Feb 1992.
The definitive version is available at http://dx.doi.org/10.1021/jo00030a016
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