Adamantyl system

Reactant structure will also influence the degree of nucleophilic solvent participation. Solvation is minimized by steric hindrance. The 2-adamantyl system is regarded as being a... 

The 2-adamantyl system has been used as a model reactant for defining the characteristics of ionization without nucleophilic participation. The degree of nucleophilic participation in other reactions can then be estimated by comparison with the 2-adamantyl system. ... 

The reason the adamantyl system is much more sensitive to the substitutions of CH3 for H is that its cage structure prevents solvent participation whereas the i-propyl system has much stronger solvent participation. The internal stabilizing effect of the methyl substituent is therefore more important in the adamantyl system. 

The activation energies for the fragmentation of the carbene in CH2C12 were calculated by the B3LYP/6-31G method to be 14.6, 2.2, and —0.95 for the bicyclo[2.2.1]heptyl, bicyclo[2.2.2]octyl, and adamantyl systems, respectively. Are the product trends consistent with these computational results, which presumably reflect the relative stability of the carbocation formed by the fragmentation ... 

Grob and Sawlewicz examined the effect of /-trimethylsilyl groups using the adamantyl systems 5467. 

Substituent effects do not appear to be reliable probes for hyperconjugation, however. Even in the 2-norbornyl cation, where considerable C-C bond delocalization is generally considered to be present16S), substituents fail to indicate any electron deficiency at the 6-position of the developing 2-norbornyl cation 162,166>16 ). Methyl substituent effects may also not be expected to provide a reliable test for C-C hyperconjugation in the 1-adamantyl system. 

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). 

If one extends the series of 3-substituted adamantanes to include several more polar substituents a possible explanation for the apparent inconsistent behavior of the methyl substituent, at least in the adamantyl system, is suggested. Fig. 4 illustrates the decrease in the solvolytic reactivity of 3-substituted-l-adamantyl... 

Another adamantyl system which has recently been studied by Harris and coworkers is the endo-2,4-dehydro-5-homoadamantyl 3,5-dinitrobenzoate (86). The product from... 

Adamantyl and Homoadamantyl Cations. The 2-adamantyl system is exceptional among sec-alkyl substrates since it solvolyzes without nucleophilic solvent participation (Section 7.2.1). The extent of anchimeric assistance is more difficult to evaluate. Some evidence is consistent with a weakly bridged intermediate ... 

A similar observation was made by Farcasiu from a study of the reactions of adamantyl systems he attributed the rearrangement to steric strain relief on the basis of force-field calculations. 

The 2-adamantyl system (54) has been proposed as a model k system. Numerous studies have indicated that neither nucleophilic attack on (54)... 

Although the 1-adamantyl system seems ideal, the ionization of 2-adamantyl systems has become the most widely accepted system for separating the effects of ionizing power and nucleophilic assistance. Again, the reference solvent is 80% ethanol. It may not be imme-... 

Eq. 11.45 shows an example of an SrnI reaction. It does not seem to fit the criteria of either Sn2 or SnI. Substitution takes place on a tertiary center that has a very electron withdrawing group on it. The tertiary center impedes an Sn2 reaction, while the electron withdrawing group impedes an SnI reaction. In addition, the nucleophile seems too bulky for a standard Sn2 reaction. The SrnI mechanism allows otherwise unfavorable substitutions to occur. Even 1-adamantyl systems can undergo facile substitution, and the leaving groups can be NO2, N3, and sometimes even phenyl ... 

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