Cyclopropane 2-norbornyl cation

The complete Fourier transform C-nmr spectra (with all coupling constants and multiplicities) were obtained for the 2-norbornyl cation at —70°C and -150°C by Olah et al. (1973a). At -70°C in SbFs-SO ClF-SOjF the ion undergoes rapid equilibration via 6,2-hydride shifts and only one resonance was observed for cyclopropane-like carbons 101.8 (ref CSa, quintet, /isch = 53.3 Hz). This peak was split into two at —150°C. One peak... 

These observations, when compared with spectra of the 7-norbornenyl and 7-norbornadienyl cations, confirmed the protonated cyclopropane structure [4] for the norbornyl cation. Average chemical shifts and coupling constants estimated for C(l), C(2) using equilibrating classical models were not in accord with the experimental i C-nmr data (Olah, 1976). 

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" ... 

All results obtained on equilibrium isotope effects are consistent and support conclusively that the 2-norbornyl cation has a single energy minimum and the symmetrical protonated cyclopropane structure [99]. 

Mechanism a involves a corner-protonated cyclopropane (28) we have already seen examples of such ions in the 2-norbornyl and 7-norbornenyl cations (pp. 453, 460). Mechanism b involves an edge-protonated cyclopropane (29). Mechanism c... 

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... 

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