Adamantyl system solvolysis

In contrast to typical mono- or acyclic substrates (e. g.,isopropyl), 2-adaman-tyl derivatives are also found to be insensitive to changes in solvent nucleophilicity. A variety of criteria, summarized in Table 13, establish this point. In all cases, the behavior of 2-adamantyl tosylate is comparable to that observed for its tertiary isomer but quite unlike that observed for the isopropyl derivative. Significant nucleophilic solvent participation is indicated in the solvolysis reactions of the isopropyl system. The 2-adamantyl system, on the other hand, appears to be a unique case of limiting solvolysis in a secondary substrate 296). The 2-adamantyl/ isopropyl ratios in various solvents therefore provide a measure of the minimum rate enhancement due to nucleophilic solvent assistance in the isopropyl system 297). 

Use of other methods has contributed further to the emerging picture of solvolysis of most secondary systems as being solvent-assisted. For example, the solvolysis rate acceleration on substituting a-hydrogen by CH3 in 2-adamantyl bromide is 107 5, much larger than that found for other secondary—tertiary pairs such as isopropyl-/-butyl. In molecules less hindered than 2-adamantyl, the secondary substrate is accelerated by nucleophilic attack of solvent.100 Rate accelerations and product distributions found on adding azide ion to solvolysis mixtures (Problem 4) also provide confirmatory evidence for these conclu-... 

A final example of the utility of adamantyl substrates as model systems in mechanistic investigations is provided by 1-adamantylcarbinyl derivatives. Detailed product and rate constant determinations of 1-adamantylcarbinyl solvolyses have been interpreted relative to the mechanism of the solvolysis reactions of neopentyl systems in general. 

Kevill and co-workers first address the much-debated issue of nucleophilic involvement in solvolysis of tert-butyl derivatives. Interestingly, the tert-butyl sulfonium salt shows more rate variation with solvent changes than does the 1-adamantyl salt. In particular, the tert-butyl salt shows a rate increase in aqueous TFEs (where both Y and N increase) that is not found for 1-adamantyl. Because a variation in Y cannot explain the result, Kevill argues that the tert-butyl derivative is receiving nucleophilic solvent assistance. On the basis of the available evidence, Harris et al. (Chapter 17) propose that tert-butyl chloride is inaccurately indicated by some probes to receive nucleophilic solvent assistance because the model system (1-adamantyl chloride) has a different susceptibility to solvent electrophilicity. Kevill and coworkers disagree with this proposal, noting that essentially the same tert-butyl to 1-adamantyl rate ratio is found for the chlorides and the sulfonium salts if solvent electrophilicity were important in one case but not the other, then the rate ratio should vary. 

Solvent Dependence of Reactivity. Solvolysis reactions were investigated to obtain structure-reactivity relationships, but these studies were complicated by the solvent dependence of relative rates (Table I). These results show a 1010 variation in relative rates of solvolyses of methyl and 2-adamantyl tosylates (2-AdOTs) in trifluoroacetic acid (TFA) compared with those of ethanolysis. Even for two secondary systems, relative rates for 2-AdOTs-(CH3)2CHOTs vary from 36 in trifluoroacetic acid to 0.0011 in ethanol (4). Hence, separate intrinsic structural effects must be separated from solvent-induced effects. 

A two-parameter scale of solvent ionizing power is the Yots scale introduced by Schleyer and co-workers. This system is based on the solvolysis of 2-adamantyl tosylate (5). The free energy relationship is... 

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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