Rearrangement adamantyl cations

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. 

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. 

The rearrangements of several twistane derivatives to adamantyl cations under the same conditions, on the other hand, appear to involve reversible, random carbonium ion formation, at least to a limited extent. Rearrangement of 2-twistanol-2-d (38) occurs with considerable intermolecular hydrogen scrambling (Eq. (15)) 40T Similar intermolecular rearrangements are observed when a 50 50 mixture of 1-adamantanol and l-adamantanol-3,5,7-d3 in S02 is treated with SbFs 40). 

Methyl-2-twistanol rearranges to the 2-methyl-2-adamantyl cation in SbFs /S02 solutions 40 The mechanism of this rearrangement can be most readily depicted in terms of 1,3-hydride shifts (protonated cyclopropanes) as illustrated in Scheme 7. Control experiments have shown that the 3-methyl-l-adamantyl cation is not involved in the rearrangement. It is stable to the rearrangement conditions despite the fact that relative solvolytic reactivities suggest that the 2-methyl-2-adamantyl cation is more stable by nearly 6 kcal/ mole 55> 56 ... 

Of course, the intramolecular nature of the rearrangement of the 2-adamantyl cation to its tertiary isomer has not been established. Intermolecular processes similar to those discussed above in connection with the 1 -adamantyl cation may be involved, and, in fact, seem likely 60 ... 

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). 

With the aim of studying a geometrically well-defined cyclopropylcarbinyl cation Baldwin and Foglesong (1968a) prepared the 8,9-dehydro-2-adamantyl 3,5-dinitrobenzoates [125 X = H, D or T]. The solvolysis of [125] in 60% aqueous acetone proceeded with considerable rate enhancement in comparison with 2-adamantyl tosylate. Scrambling of the label to the 8 and 9 positions in the solvolysis of [125 X = D] and [125 X = T] revealed a degenerate rearrangement (90) of the intermediate 8,9-dehydro-2-adamantyl cation [126]. 

Thus the interesting chemistry of 2-adamantyl cations [198] has been difficult to study under stable-ion conditions since they undergo facile rearrangement to the more stable tertiary 1-adamantyl cations [199]. These difficulties were... 

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)] ... 

Olah has discussed the extent of norbornyl cations and studied the l,2-diphenyl-2-norbomyl cation (102) by H and n.m.r. He concludes that it behaves as a rapidly equilibrating carbenium ion undergoing fast alkyl shift at -78°C. The cation (103) has also been prepared from several precursors in acidic media at —120 °C. Its spectra indicate rapid equilibration between carbenium forms. At — 60°C it rearranges to (104). The 8,9-dehydro-2-adamantyl cations (105 R = H or Me) have n.m.r. spectra which show that the tertiary cation is a static classical ion, whereas in the secondary case there is rapid equilibration making carbons 1, 8, and 9 equivalent. The analogous 2-substituted 2-adamantyl cations with R = Me, Et, Ph, OH, or halogen are simple ions which do not isomerize under the strong acid conditions used. " ... 

The Cs 2-adamantyl cation, for which rapid rearrangements to the thermodynamically favored 1-adamantyl cation via an intermolecular hydride transfer have long been known experimentally, has two strongly hyperconjugating C-C bonds and two only weakly hyperconjugating C-C bonds (see Figure 1). 

Another feature of systems that are subject to B-strain is their reluctance to form strained substitution products. The cationic intermediates usually escape to elimination products in preference to capture by a nucleophile. Rearrangements are also common. 2-Methyl-2-adamantyl p-nitrobenzoate gives 82% methyleneadamantane by elimination and 18% 2-methyl-2-adamantanol by substitution in aqueous acetone. Elimination accounts for 95% of the product from 2-neopentyl-2-adaman l p-nitrobenzoate. The major product (83%) from 2-r-butyl-2-adamantyl p-nitrobenzoate is the rearranged alkene 5. 

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