2-Adamantyl tosylates

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." ... 

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... 

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. 

The above refinements of the interpretation of earlier work have been brought about by two important developments, recognition from studies in trifluoroacetic acid that acetic and formic acids are nucleophilic (Peterson et al., 1965) and the use of 2-adamantyl tosylate as a reference point for SnI reactivity (Schleyer et al., 1970). These developments have led to sensitive ways of detecting nucleophilic assistance and have shown that solvolyses of secondary substrates in acetic and formic acids may be significantly nucleophilically assisted (Table 2). Previous arguments were based on less sensitive tests such as the rate-accelerating effect of low concentrations of added nucleophiles, which must be appropriately oriented and desolvated before they are able nucleophilically to assist in the displacement reaction (Peterson et al., 1967). 

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... 

Other evidence to support the selection of 2-adamantyl tosylate as model compound has been discussed in Section 2 (p. 8), and the main conclusion can be summarized as follows. Independent evidence and the results in Fig. 15 firmly establish that nucleophilic solvent assistance cannot be appreciable for solvolyses of 2-adamantyl tosylate, since its response to solvents of widely varying nucleophilicities is almost exactly the same as 1-bicyclo [2,2,2] octyl tosylate [49] for which rearside attack is impossible if solvolyses of 2-adamantyl tosylate are anchimerically assisted, the extent of anchimeric assistance does not appear to be dependent on solvent (cf. Pritt and Whiting, 1975). In addition to these conclusions, the results in Fig. 14 suggest that internal return from intermediate contact ion pairs does not occur to a detectable extent for solvolyses of 2-adamantyl tosylate (see also Bentley and Schleyer, 1976). 

Grunwald-Winstein eqn (9) with OTs values derived from 2-adamantyl tosylate (Section 3, p. 36) and with iV0Ts values based on methyl tosylate (Section 5), Schadt et al. (1976) have correlated a variety of solvolyses of other tosylates. Whilst their work is more extensive in that Sn 2 type solvolyses were studied (Table 9), only tosylates have been examined and solvation effects caused by different leaving groups are thus minimized. However, the subtlety of solvation effects is emphasized by the dependence of the relative rates of p-bromobenzenesulphonates and tosylates on solvent electro-... 

When eqn (9) utilizes values of Afoxs [based on methyl tosylate eqn (11)] and Iots (based on 2-adamantyl tosylate), there is an... 

The results in Table 11 also illustrate the dubious basis of the generalization (e.g. Pritt and Whiting, 1975) that Sn 1 reactions have AS 0, since t-butyl chloride in water has AS = +12-2, and 2-adamantyl tosylate in 97% hexafluoropropan-2-ol/water has AS = —17 cal deg 1 mol 1. Whilst it could be argued that solvation in mixed solvents is more complex than in pure solvents, it is nevertheless clear that there is a wide range of values of AS for these Sjjl solvolyses. 

SnI reactions of secondary and tertiary substrates R3C—X e.g. 2-adamantyl tosylate [665], t-butyl heptafluorobutyrate [666]) proceed via activated complexes with high carbenium ion character to give ion-pair intermediates. They exhibit electrophilic solvent assistance in protic solvents SOH (H-bonding to the leaving group), but they are practically insensitive to changes in solvent nucleophihcity cf. Eq. (5-135). 

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