Bridgehead Adamantyl, Diamantyl, and Related Cations and Dications

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.