Adamantyl derivatives, solvolysis

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. 

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). 

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. 

Winstein (1957c). They argued that during solvolysis of neophyl substrates [46, X = Cl, Br] internal return from the proposed phenonium ion-pair intermediate [47] would yield the tertiary derivative [48] (Fig. 13) and this would solvolyse rapidly. Thus, the titrimetric rate constants should correspond to the ionization rate constant k j (Fig. 2). Later neophyl tosylate [46, X = OTs] and its p-methoxy derivative were used (Smith et al., 1961 Diaz et al., 1968b Yamataka et al., 1973), and recently Schadt et al. (1976) defined as a scale of solvent ionizing power for tosylates, designated F2-AdOTs or Yqt, using eqn (5) but based on 2-adamantyl tosylate [6] instead of t-butyl chloride. A correlation of the rates of solvolysis of neophyl tosylate with F0 Xs (Fig. 14) is satisfactory (correlation coefficient... 

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. 

Accurate determination of the a for 1-AdCl requires solution of the full SCE equation. Unfortunately, this solution is difficult to accomplish because determining rates in weakly nucleophilic, nonhydroxylic solvents is a complex matter. An example is the study of Kevill and Kim (15) on the solvolysis of 1-adamantyl arenesulfonates in acetonitrile. These workers found that the reaction proceeded to give an early equilibrium between the reactant and the nitrilium ion formed by acetonitrile acting as nucleophile. So that the equilibrium could be removed and good kinetics obtained, trapping of the cation with azide ion to give a tetrazole product was necessary. Presumably, other solvolyses of 1-Ad derivatives in other weakly nucleophilic solvents would be similarly complex. Having rates in these nonhydroxylic solvents is critical if the complete equation is to be solved because of colinearity between certain of the independent variables in hydroxylic solvents. 

There is very good evidence that if the cyclopropyl group of a cyclo-propylcarbinyl derivative can interact with the developing cationic center in a bisected conformation a very large rate enhancement results, whereas if this conformation is prevented sterically, there is no rate enhancement. Thus on the basis of the rate of solvolysis of 8,9-dehydro-2-adamantyl 3,5-dinitrobenzoate in aqueous acetone it was estimated that the corresponding toluene-p-sulfonate (43) would undergo acetolysis about 8 powers of 10 faster than 2-adamantyl toluene-p-sulfonate (44). It was also esti-... 

Grunwald-Winstein-type analyses (using ici) for the solvolysis of extremely crowded tertiary alkyl chlorides having a neopentyl or a (l-adamantyl)methyl group at the reaction centre show that they behave as fcc substrates. The solvolytic behaviour of various crowded alkyl derivatives and the origins of dispersions of data points of aqueous organic solvents in Grunwald-Winstein type correlations are discussed in detail. A possible role of hydrophobic solvation is tentatively raised. 

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