The Polymethyl 2-Adamantyl Cations

The Polymethyl 2-Adamantyl Cations The nature of the 2-adamantyl cation (153) has been difficult to study since it undergoes facile rearrangement to the more stable 1-adamantyl cation (154). This difficulty was circumvented by Lenoir et al. by blocking all four bridgehead positions with methyl groups inastudy that involved the 1,3,5,7-tetramethyl- (155) and 1,2,3,5,7-pentamethyl-2-adamantyl (156) cations. 

The H NMR spectrum of 155 in superacid had the right number of peaks to fit the symmetry of a static 2-adamantyl cation but the chemical shift value of the CH proton at the C(2) presumed carbocation center was only 5 H 5.1. This is 8 ppm to higher field than expected for typical static secondary carbenium ions such as the isopropyl cation.  

Since the symmetry of the spectrum was incompatible with either a static bridged 2-adamantyl cation (157) or a static protoadamantyl cation (158), two mechanisms were postulated involving sets of the cations from 157 or from 158 undergoing rapid degenerate rearrangements at -47°C [Eqs. (5.23) and (5.24)]  

Since it was difficult to make an exclusive choice between bridged (involving 157) and unbridged (involving 158) mechanistic pathways for the degenerate rearrangement and thus the structure of 155 on the basis of NMR data alone, further evidence were derived from a solvolytic study, and the mechanism involving 157 became the preferred explanation for the behavior of 155 in superacid. 

An entirely different spectrum was obtained for the presumed 155 ion the C(2) resonance was located at 5 C 92.3. This value was 200 ppm more shielded than expected for a static classical cation. Since a static structure like 155 was clearly incompatible with the observed spectrum, a chemical shift estimate was made for the protoadamantyl cation 158. However, the discrepancy between the estimated and observed values was too large to explain the behavior of the l,3,5,7-tetramethyl-2-adamantyl cation with properties of an equilibrating set of ions such as 158, even with the partial contribution of 155. This left the set of equilibrating nonclassical bridged ions (157) as the only possible structure for the cation [Eq. (5.23)]. 

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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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