Bridgehead Adamantyl, Diamantyl, and Related Cations and Dications

George A. Olah, G. K. Surya Prakash, Joseph G. Shih, V. V. Krishnamurthy, Gheorge D. Mateescu, Gao Liang, Gyorgy Sipos, Volker Buss, Tamara M. Gund, Paul V. Ragué Schleyer

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Abstract

The bridgehead 1-adamantyl cation 3a and a series of methyl-substituted adamantyl cations 3b-d have been prepared in superacid medium and characterized by 1H and 13C NMR spectroscopy. The relative deshielding of bridgehead γ carbons with respect to β-methylenes has been rationalized in terms of a C-C hyperconjugative effect. The cation generated from the ionization of exo-5, 6-trimethylene-2-norbornyl derivatives has been shown to be the equilibrating 2, 3-trimethylene-2-norbornyl cation 12. The temperature-dependent 13C NMR spectroscopic studies on 12 indicate the barrier for the degenerate exo-2, 3-hydrogen shift, ΔG* = 7.2 kcal/mol. The cation 12 in a solution of SO2ClF is stable even at room temperature; however, in a solution of SO2, it smoothly and irreversibly rearranges to bridgehead 1-adamantyl cation 3a, On the other hand, 2-methyl-5, 6-exo-trimethylene-2-norbornyl cation 17 was found to be quite stable. The 1-adamantyl cation was also studied as its isolated SbF6-salt by solid-state CPMAS 13C NMR spectroscopy. The bridgehead 1-and 4-diamantyl cations 4 and 27 were also prepared and studied. Heteronuclear-correlated 2D NMR spectra were also obtained for 3a and 4 and used in many assignments of the NMR shifts. Cation 27 slowly and irreversibly rearranges to 4. The cation 4 was also generated by the ionization of 3-diamantyl derivatives. The above rearrangements to the thermodynamically more stable ion 4 has been rationalized in terms of facile intermolecular hydrogen shifts rather than intramolecular shifts. The bridgehead diamantyl cations 4 and 27 also derive their stability from C-C hyperconjugative effects similar to their adamantyl analogues. In contrast to the instability of secondary 3-diamantyl cation the corresponding 3-methyl-and 3-phenyl-substituted analogues were found to be rather stable. 3, 3'-(l, 1-Biadamantyl) dication 6 and 4, 9-diamantyl dication 5 were also generated and studied. The C-C hyperconjugative stabilizing interaction which is found “operative” in all of the bridgehead cations including dication 6 is absent in the 4, 9-diamantyl dication 5. Charge-charge repulsion in 5 seems to diminish such C-C hyperconjugative interaction. All attempts to prepare the 1, 3-adamantyl dication 8 or the 1, 6-diamantyl dication 7 were unsuccessful.

Original languageEnglish (US)
Pages (from-to)2764-2772
Number of pages9
JournalJournal of the American Chemical Society
Volume107
Issue number9
DOIs
StatePublished - Jan 1 1985
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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