2-Adamantyl tosylate

Solvolysis rate studies also indicate that there is greater stabilization by a cyclopropyl group in a bisected geomeby. In tosylate 1, the cyclopropane ring is locked into an orientation which affords a perpendicular arrangement. It reacts 300 times more slowly than the model compound 2. Tosylate 3, which corresponds to the bisected geomeby, undergoes acetolysis at least 10 times faster than the model 2-adamantyl tosylate 4. ... 

The solvolysis of isomeric 5-fluoro and 5-trimethyIstaimyl 2-adamantyl tosylates has been examined. The relative rates depend on the substituents and the stereochemistry of the reactants as shown. 

If Y is to be a valid measure of solvent ionizing power, presumably the defining reaction should proceed via the Lim (pure SnI) process. This was the basis for the original choice of r-butyl chloride. It is now believed that /-butyl chloride sol-volyzes with some solvent participation, and modern versions of Y are based on other compounds, of which 2-adamantyl tosylate (p-toluenesulfonate, OTs), 6, is the most favored." ... 

Adamantyl tosylate undergoes SnI substitution 1,000 times more slowly than tert-butyl tosylate. 

One possible explanation is that adamantyl cation, an intermediate in the reaction, is particularly unstable because it cannot accomodate a planar carbocation center (see Chapter 1, Problem 9). Examine the geometry of adamantyl cation. Does it incorporate a planar carbocation center Compare electrostatic potential maps of adamantyl cation and 2-methyl-2-propyl cation. Which cation better delocalizes the positive charge Assuming that the more delocalized cation is also the more stable cation, would you expect adamantyl tosylate to react slower or faster than tcrf-butyl tosylate Calculate the energy of the reaction. 

Rates of solvolysis of 2-adamantyl azoxytosylate were measured over a range of temperatures in ethanoic acid, methanoic acid, and various mixed solvents. For comparison solvolysis rates of 2-adamantyl tosylate were measured in several of the same solvents. The m value for 2-adamantyl azoxytosylate solvolysis is only 0.46, one of the lowest observed values for a reaction that is unambiguously 5 nI. 

Solvolysis rates of isopropyl tosylate and 2-adamantyl tosylate (28, p. 243) in 80 percent ethanol are measured with and without added azide. Define rate enhancement, R.E., as the ratio of rate with azide to rate without, and designate by /bn3 the fraction of alkyl azide in the product. Explain the significance of the fact that the isopropyl results fit the equation... 

Possibly this is due to a greater charge development in the transition states of the chromyl acetate oxidations. Solvolysis reactions at the 10-position are known to be highly unfavorable 9S). The solvolytic reactivity of 10-tricyclo [5.2.1.0 4>10] decyl tosylate is retarded relative to 1-adamantyl tosylate by a factor of approximately 106 at 70°C in acetic acid. 

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). 

In order to fit solvolytic reactions into the above framework it is important to establish the magnitude of nucleophilic solvent assistance, the extent to which nucleophilic attack by solvent assists heterolytic cleavage of the bond to the leaving group. 2-Adamantyl tosylate [6] was selected as a model for SN 1 behaviour because non-bonded interactions [7] would reduce the possibility of nucleophilic attack (Fry et al., 1970a) extensive experimental evidence... 

Hi) the relative rate of 2-adamantyl tosylate and bromide (OTs/Br ratio) is similar to that of tertiary substrates (Fry et al., 1970a) this criterion of mechanism was proposed by Hoffmann (1965), but it now appears that OTs/Br ratios in secondary and tertiary systems are determined largely by steric effects (Slutsky et al., 1974) ... 

The above evidence firmly establishes that 2-adamantyl tosylate behaves very much like a tertiary substrate in its lack of response to changes in solvent nucleophilicity. As shown by correlations using eqn (9)—Section 6, Table 9—and by the relative rates in Table 1, other secondary tosylates are more responsive to solvent nucleophilicity. Schleyeret al. (1970) proposed that the magnitude of... 

Based on an extrapolated rate constant for 2-adamantyl tosylate in ethanol at 25°C. 

Thus, as well as involving the kinetically significant nucleophilic role of solvent required by the definition of the term SN 2, these reactions show the essentially complete inversion of configuration characteristic of the classical Sn 2 mechanism. However solvolyses of more hindered secondary tosylates such as menthyl tosylate (Hirsl-Starcevic et al., 1974) and 2-adamantyl tosylate (Bone et al., 1975) appear to solvolyse with excess retention of configuration. 

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