Protonated nortricyclene

The corner-protonated cyclopropane configuration 45 is analogous to the non-classical norbornyl ion at the center of controversy for the past three decades. The non-classical cation concept had its origin with Wilson and coworkers in 1939 who depicted structure 58 as a possible intermediate in the camphene hydrochloride-isobornyl chloride 

The problems associated with theoretical calculations of the energies of protonated cyclopropane are readily apparent and the norbornyl skeleton increases the complexity of computation. Early quantum mechanical calculations of the MINDO/3, STO-3G (MINDO/3 geometry)and ab initio STO-3G type show the corner-protonated norbornyl cation less stable (- 9143, i44 (g i3)i45. 5 9144 4.9 kcal mol  

2 kcal mol ). At higher temperatures again C(3) hydrogen migration to C(2) occurs with an energy barrier of 10.8 0.6 kcal mol and this process transfers the 

SCHEME 12. Interaction with H Is with degenerate HOMOs of A may cause lengthening of both adjacent bonds but with the bond anti to the proton increasing the greater. 

---

The stable 2-norbornyl cation has recently been shown to be a non-classical, unusually stabilized species. Olah et al. (1970) proved spectroscopically that this ion is a comer-protonated nortricyclene with a pentavalent carbon atom. The value for the carbonylation-decarbonyla-tion equilibrium constant K (= of the 2-norbomyl ion illustrates... 

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. 

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 rearrangements of the 2-norbornyl cation are unexceptional. All 1,2-alkyl shifts, even those in acyclic systems (Section 7.6.1) conform to essentially the same pattern. Individual differences exist in the rates of alkyl shift, relative to the rates of capture by solvent. In that respect the 2-norbornyl cation is superior to acyclic analogs because it can approach the geometry of corner-protonated nortricyclene with only small distortions of the nuclear positions. (A more sophisticated treatment in terms of relaxation theory has been published548. ) Other bicyclic and polycyclic systems provide similarly favorable conditions for 1,2-alkyl shifts. Some of them will be discussed in the following Sections. 

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... 

Olah believes that the only structure corresponding to the data of Raman laser and NMR C spectra is that of protonated nortricyclene. At —70 C the cyclo-propyl carbons C , and C are averaged as a result of rapid 6,1,2-hydride shift (E = 5.9 + 0.2kcal/mole A = 10 s ).Ifthe protonated nortricyclene structure is comer-protonated, the C, and C are averaged as follows ... 

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. 

In a similar way the 2-norbomyl ion as a static particle (t — 150 °C) 5a represents an analogue of ion III (methylene-bridged), and the 6,1,2-hydride shifts occur via different forms of protonated nortricyclene the classical ion 6 is similar to the 1-propenium ion IV. 

---