Carbonium ions adamantyl

Vinyl cations also have been invoked as intermediates in the addition of carbonium ions generated in strong acid to acetylene (50-53). Sasaki et al (50) observed 1-adamantyl methyl ketone, 25, as the sole product in the reaction of acetylene with 1-bromoadamantane in concentrated H2SO4 at 5°. Bott (51), on the other hand, reported a mixture consisting of 75% 1-adamantylacetaldehyde,... 

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). 

The nmr spectrum of 1-chloroadamantane in SbFs-S02 s°) or of adaman-tane in SbFs-FS03H 157) is attributed to the 1-adamantyl cation. The essential characteristics of the spectrum are summarized in Table 4. Contrary to the normal observation where the hydrogens adjacent to the carbonium ion site experience the largest deshielding effect, the 7 (bridgehead) hydrogens of the... 

Since the solvolysis reactions of bridgehead substrates are mechanistically uncomplicated (i.e. competing elimination is highly unlikely187 ns-H9) an(j backside solvent participation is impossible), they provide ideal models for limiting carbonium ion behavior. Adamantyl derivatives have become the substrates of choice for this purpose due to their availability and convenient reactivity. 

Substituted adamantanes have been employed in the study of several reaction mechanisms. The Kolbe electrolyses of 1- and 2-adamantane carboxylic acids and 1- and 2-adamantyl acetic acids have been compared in detail. Car-bonium ion-derived products are observed from the electrolyses of both 1-adamantyl carboxylic acid and 1-adamantyl acetic acid. A mixture of radical and carbonium ion-derived products are obtained from 2-adamantyl carboxylic acid and mainly radical-derived products are obtained from 2-adamantyl acetic acid. These results parallel the known or expected stabilities of the intermediate carbonium ions 366 ... 

As described in the case of the 1-adamantyl cation, protosolvation of carbon-carbon and carbon-hydrogen sigma bonds can lead to cationic 2e-3c bonding and the formation of carbonium ion centers. The role of this interaction in the chemistry of superelectrophiles is typihed by protosolvation of the tert-butyl cation (1) in superacid to provide the dicationic... 

What can we deduce as to the nature of the transition state from these values of /, x Since we are dealing with symmetrical reactions the transition state must lie on a =. Hence pj[x will measure the tightness or looseness of the transition state. If the transition state is close to the carbonium ion, then as argued above for the adamantyl case,... 

The apocamphyl case is an extreme one, and bridgehead carbonium ions in other systems are more accessible. Inclusion of more atoms in the bridge gives a more flexible molecule and allows carbonium ion formation to proceed with a somewhat lower activation energy. Thus, the relative solvolysis rates of the bridgehead bromides 1-bromoadamantane, l-bromobicyclo[2.2.2]octane, and 1-bromobicyclo[2.2.1]heptane in 80% ethanol at 25°C are 1,10", and Under the same conditions, the rate of solvolysis of tert-butyl bromide is 1000 times that of 1-bromoadamantane. The 1-adamantyl cation is sufficiently stable to be generated in antimony pentafluoride in concentrations sufficient for observation by NMR. ... 

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