Solvolysis of 2-adamantyl tosylate

The effect of a 1-cyclopropyl substituent on the rate of solvolysis of 2-adamantyl tosylates (129) has been compared to a variety of other 1-substituents. These rate effects were attributed to electronic substituent effects on formation of a bridged ion, but steric effects are probably also involved and a detailed interpretation of the conjugative effect of cyclopropyl is not straightforward. 

Allard, B., Casadevall, A., Casadevall, E. and Largeau, C. (1980) The acid-enhancing effect of HFIP as solvent. Carbocation rearrangemebt during solvolysis of 2-adamantyl tosylate in anhydrous HFIP. Nouv. J. Chim., 4, 539-545. 

The parameter Y is defined by the solvolysis of 2-adamantyl tosylate and Figure 11 illustrates the fit to Equation (32) for the solvolysis of isopropyl tosylate. The additional term introduces a better correlation and indicates that the solvolysis has substantial nucleophilic assistance (/ = 0.4) in the transition structure as would be expected. 

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... 

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. 

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). 

Alkyl substituents at C—1 of 2-adamantyl tosylate greatly increase the yield of 4-exo-protoadamantyl solvolysis products573. l-Methyl-2-adamantyl tosylate (869) and 4-methyl-4-exo-protoadamantyl dinitrobenzoate (871) give approximately the... 

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. 

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... 

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... 

With the aim of studying a geometrically well-defined cyclopropylcarbinyl cation Baldwin and Foglesong (1968a) prepared the 8,9-dehydro-2-adamantyl 3,5-dinitrobenzoates [125 X = H, D or T]. The solvolysis of [125] in 60% aqueous acetone proceeded with considerable rate enhancement in comparison with 2-adamantyl tosylate. Scrambling of the label to the 8 and 9 positions in the solvolysis of [125 X = D] and [125 X = T] revealed a degenerate rearrangement (90) of the intermediate 8,9-dehydro-2-adamantyl cation [126]. 

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. 

The value of m from the Winstein-Grunwald equation (for 2-endo-tosylate 0.69, i.e. between 2-adamantyl tosylate, 0.91, and isopropyl tosylate, m = 0.44)also points to some participation of the solvent. Finally, as shown by Nordlander et al. kjikj, = 30 for the solvolysis of 2-endo-norbornyl tosylate. If this ester was solvolyzed with considerable steric hindrance to ionization the solvent participation would be unusually great while the ratio observed coincides with that for trans-2-methylcyclopentyl tosylate (kjikj = 30). 

First, two general papers should be mentioned. 1- and 2-adamantyl tosylates are included in a study of the effect of pressure on solvolysis. The activation volumes (see ref. 133) were found to depend in a systematic manner on substrate structure and solvent. Activation volumes for reactions with little solvent assistance were less negative than those for reactions with solvent assistance. In the case of solvent-assisted reactions the activation volume was found to be governed by the partial W. J. Hehre and P. C. Hiberty, J. Amer. Chem. Soc., 1974,96, 2665. 

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. 

Shiner showed that the y-trimethylstannyl substituent accelerated the solvolysis of 5-(trimethylstannyl)-l-adamantyl sulfonates (3-22),38 and Lambert showed that the effect was apparent in 3-trialkylstannylcyclohexyl tosylate (3-23), but only when the two groups are cis so that the Sn-C and C-0 bonds are in a W-geometry the solvolysis in 97% CF3CH2OH was then accelerated by a factor of > 2 x 103.39... 

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