1.3.5.7- Tetramethyl-2-adamantyl cation

S.2.8. The 1,3,5,7-Tetramethyl- and 1,2,3,5,7-Pentamethyl-2-adamantyl Cations. The nature of the 2-adamantyl cation 571 has been difficult to study under stable ion conditions since it undergoes facile rearrangements to the more stable 1-adamantyl cation 49.182 This difficulty was circumvented by Lenoir, Schleyer, Saunders, and co-workers999 by blocking all four bridgehead positions by methyl groups in a study of 1,3,5,7-tetramethyl- and l,2,3,5,7-pentamethyl-2-adamantyl cations 572 and 573. 

An entirely different spectrum was obtained for 572 with the C(2)13C resonance at 813C 92.3, more than 200 ppm removed from the position expected for a static classical cation. Since a static structure like 572 was incompatible with the observed spectrum, a chemical shift estimate was made for the protoadamantyl cation 575. However, the discrepancy between these estimated chemical shifts and those observed was too large to explain the behavior of the l,3,5,7-tetramethyl-2-adamantyl cation within the properties of an equilibrating set of ions 575 even with the partial contribution of 572. This left the set of o-bridged ions 574 equilibrating according to the mechanism in Eq. (3.141) as the only possible structure for this ion. 

The n.m.r. spectra of the polymethyl-2-adamantyl cations have been studied. That of the tetramethyl cation shows the proton on the tervalent carbon at 8 p.p.m. higher field than expected and this is interpreted as being due to the presence of a set of four rapidly interconverting partially bridged ions (951) cf. ref. 92). 

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