Protoadamantyl cation

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. 

Takeuchi and co-workers736 have reported that ionization of 3,4-dimethyl-4-homoadamantanol in Magic Acid results in ionization and rearrangement to yield the 3-ethyl-5-methyl-l-adamantyl cation 178 observed by 13C NMR spectroscopy at —30°C, which, after quenching with methanol, gives ether 179 [Eq. (5.281)]. A series of known rearrangement steps and intermediates including protoadamantyl cations can account for the observation. 

Finally, a solvolytically initiated 7T-route ring closure to a protoadamantyl cation has been found to give minor amounts of protoadamantyl products (Eq.(25))84>. 

Olah and coworkers NMR study of the 8,9-dehydro-2-adamantyl cation, obtained from the corresponding alcohol in superacidic medium, showed the equivalence of C2, C8 and C9 carbons ((5 C 157.0) in agreement with the conclusions of solvolytic studies Even at -120 °C the structure of the cation could not be frozen out to a static cation, showing the extremely fast equilibration of the threefold degenerate cyclopropylcarbinyl cation 74. An identical NMR spectrum was obtained from the ionization of the 2,5-dehydro-4-protoadamantanol, which prompted the suggestion of the intermediacy of the 2,5-dehydro-4-protoadamantyl cation 75. The ion rearranges to an ally lie cation 76 at -78 (equation 45). 

Two different dimethyl-8,9-dehydroadamantyl cations have also been investigated by Olah and coworkers. The 1,2-dimethyl-8,9-dehydroadamantyl cation [141] prepared by Olah et al. (1978b) was found to be a static cyclo-propylcarbinyl cation from —95°C to —10°C. No contribution from the 2,5-dehydro-4-protoadamantyl cation [142], and no degenerate rearrangement was observed for [141]. The other isomer, the 2,8-dimethyl-8,9-dehydro-... 

Obviously the epimeric 4-protoadamantyl precursors, (860) and (866), do not react by way of a common 4-protoadamantyl cation (859). Assisted ionization with rearrangement, leading directly to the 2-adamantyl cation (858), would explain the enhanced reactivity of (860) (exo endo = 104). Even if the intermediate is bridged, the bridging should be weak and it must be unsymmetrical. The carbon skeleton of (858) is much more stable than that of (859) any tendency of the 2-adamantyl cation to bridge must be offset, at least in part, by an increase in ring strain. The obvious way to improve this situation is to stabilize (859), or to destabilize (858). 

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)] ... 

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