Rearrangement protoadamantyl-adamantyl

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. 

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). 

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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