Adamantyl cation stability

The trishomoaromatic 1,3-dehydro-5-adamantyl cation [52] has been invoked to explain the extremely high reactivity of the bromide [53] (Scott and Pincock, 1973). Recent calculations suggest that [52] does indeed enjoy homoaromatic stabilization to the extent of 25.4 kcal mol1 relative to the 1-adamantyl cation and 1,3-dehydroadamantane (Bremer et al., 1987). 

To study the possible stabilizing effect of [3-silyl cations, Olah and co-workers334 prepared the 2- [(1 -trimethylsilyl)vinyl]-2-adamantyl cation 132 [Eq. (3.43)] as well as the parent silicon-free carbocation. In contrast to the above observations, NMR data [the (Cl ), (C2), and (C2 ) carbons are more deshielded in 132 than in the parent ion] showed that cation 132 is destabilized compared with the silicon-free analog. Furthermore, at — 100°C the C(l) and C(3) carbons were found to be equivalent, whereas in the parent ion they were nonequivalent. This indicates a rapid rotation about the C(l)-C(3) bond in 132, which can be rationalized by assuming the intermediacy of the [3-silyl-stabilized cation 133. The difference between cation 132 and those having [3-silyl-stabilization discussed above may be the orthogonal arrangement of the [3-C-Si bond and the p-orbital of the carbocation center. 

The stability and symmetry of the 8,9-dehydro-2-adamantyl cation have been investigated and comparison has been made with the cyclopropyl car-binyl cation itself, 3S). For a complete discussion see Section V.B.l. 

It IS well known that adamantane or its 1-halo derivatives, for example 50, can be easily transformed into a stable tertiary 1-adamantyl cation 26. As was already mentioned vide supra), this cation owes its increased stability to the unique stabilization effects involving the participation of the remote centers in the charge delocalization. The pattern of NMR spectra may be used as a sensitive probe to the charge delocalization effects. Thus for the series of aliphatic tertiary carbenium ions, the presence of the positive charge induces a downfield shift of the H NMR signals (relative to those of the parent covalent precursor) at the adjacent centers. The magnitude of this effect decreases in the order S> y In the case of 26, a substantial downfield shift of -protons is also observed, but this effect is pronounced even stronger for the y-protons... 

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). 

The stability of this ion (Chart 2) was obtained by direct bromide exchange with 3-noradamantyl cation, prepared in turn by DPA of the 3-bromonoradamantane. For the sake of consistency with the treatment in Refs. 57 and 58, Af//° (25) reported in Table 6 was anchored to that for the 1-adamantyl cation, using for the... 

In discussing the elFect of structure on the stabilization of alkyl cations on the basis of the carbonylation-decarbonylation equilibrium constants, it is assumed that—to a first approximation—the stabilization of the alkyloxocarbonium ions does not depend on the structure of the alkyl group. The stabilization of the positive charge in the alkyloxocarbonium ion is mainly due to the resonance RC = 0 <-> RC = 0+, and the elFect of R on this stabilization is only of minor importance. It has been shown by Brouwer (1968a) that even in the case of (tertiary) alkylcarbonium ions, which would be much more sensitive to variation of R attached to the electron-deficient centre, the stabilization is practically independent of the structure of the alkyl groups. Another argument is found in the fact that the equilibrium concentrations of isomeric alkyloxocarbonium ions differ by at most a factor of 2-3 from each other (Section III). Therefore, the value of K provides a quantitative measure of the stabilization of an alkyl cation. In the case of R = t-adamantyl this equilibrium constant is 30 times larger than when R = t-butyl or t-pentyl, which means that the non-planar t-adamantyl ion is RT In 30= 2-1 kcal... 

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