Edge-protonated nortricyclene

All data which are available on 6,2 (6,1) -H shifts are nicely compatible with an interconversion of corner-protonated nortricyclenes (bridged 2-norbornyl cations) via edge-protonated nortricyclenes. 

The 6,2-hydride shift in the primarily developing nonclassical carbocations can be assumed to occur with the formation of either a nortricyclonium ion 34 or an edge-protonated nortricyclene 35. A choice between these two possibilities has been made from the reaction stereochemistry of unsaturated acids 84 and 85 with 50 % sulphuric acid. The product in either case turned out to be lactone 86. The formation of this product clearly indicates the 6,2-hydride shifts in this system to occur by endo,endo-migrations. If the intermediate here were a nortricyclonium ion, then besides lactone 86 the formation of two other isomers of the deuterium position would be equally probable. Suche isomers, however, have not been found This makes it... 

The non-classical ion may exist as edge or face protonated nortricyclene, represented by stmcture [9], as an alternative. This species is not a resonance stmcture of [7] or [8]. If it is a discrete intermediate, one might expect to be able to detect the displaced proton via a suitably designed exchange experiment. 

These spectral changes are consistent with either (a) freezing out a stable non-classical ion, or (b) the presence of the classical ion in which the 6,2-hydride shift has been slowed down, but the Wagner-Meerwein rearrangement is still exceedingly rapid. If the ion is non-classical, the spectrum indicates that it must be a comer-protonated nortricyclene rather than the edge-protonated ion because the latter would have split into a 1 2 1 pattern. 

Quantum mechanical calcidations made by Klopman have shown the least stable of the norbornyl ions to be the classical ion 6 and the face-protonated nortricyclene 34 they are by 40kcal/mole le stable than the edge- 35 or comer-protonated 5 nortricyclenes. 

In 1972 Olah revised his ideas of the structure of the norbornyl ion and its analogues under the stable ion conditions. While earlier he practically identified the static 2-norbornyl ion with the corner-protonated nortricyclene 5 a and denoted its structure by the formula 5a in he considers two possible ways of protonating the cyclopropane ring —- on the C—C bond and on C—H bond to form, in the case of protonating the cyclopane itself, the C-cycloproponium (edge-protonated) ion with structure II or the H-cycloproponium (corner-protonated) ion with structure I. 

The solvolysis of propyl and bornyl systems and protonolysis of cyclopropane and nortricyclene are two routes to protonated cyclopropanes (Scheme 10). For sufficiently dissymmetric cyclopropanes, corner and edge attack by electrophiles at cyclopropane can be differentiated. This stereochemical division of electrophilic attack is not to be confused... 

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