7- Aryl-7-norbornyl cations

However, 2-aryl-2-norbornyl cations 510 show deviations from linearity in such chemical shift plots with electron-withdrawing substituents indicative of the onset of nonclassical o-delocalization fully supporting the nonclassical nature of the parent secondary cation (Figure 3.23). These conclusions were criticized by Brown et al.883 In a subsequent paper, Olah et al.67 have shown major flaws in such criticisms. 

Figure 3.23. Plot of the 13C NMR chemical shifts of the cationic center of 2-aryl-2-norbornyl cations versus those of model 1-aryl-l-cyclopentyl cations.880...

Fig. 7, A plot of SC vs 5C chemical shifts of 2-aryl-2-norbornyl cations...

Cyclobutyl, substituted cyclobutyl, and related carbocations were reviewed.23 A study of the photophysical properties and photochemistry of 1-adamantyl aryl ethers in methanol solvent showed that, although the majority of the ethers underwent photolysis by homolytic pathways, irradiation of the 4-cyanophenol ether resulted, in part, in the methyl ether, implicating an ionic mechanism with the 1-adamantyl cation as an intermediate.24 Labelling experiments demonstrated that the fragmentation of 7-norbomyloxychlorocarbene to 7-norbornyl chloride proceeds with both retention and inversion.25 While an ion pair [Cl CO R+] is a possible intermediate, computational studies suggest that the fragmentations proceed via transition states that lead to either retention or inversion. 

In the NMR spectra of stable 7-aryl-7-norbornyl ions Olah has formed an excellent linear correlation (r = 0.998) between 6C of these ions and 5C of the respective 1-aryl-1-cyclopentyl ions — the model classical cations These data testify to the classical nature of 7-aryl-7-norbornyl ions. 

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