Bicyclic and polycyclic carbocations

The cyclobutyl/cyclopropylmethyl cation system (C4II7 ) has probably been the focus of more studies than any other carbocation system except the 2-norbornyl cation. Bridged cyclobutyl cations 16 are called bicyclobutonium ions. Bicyclobutonium 

However, contrary to the parent cation, the corresponding isomeric cyclopropyl-methyl cation structures 58 and 59 are no minimum structures (MP2/6-31G(d)) and do not contribute to the averaged chemical shifts. 

3-e Jo-trialk y 1 si 1 y 1 bieye 1 ob u tonium ions 62 (R = CH3 or Me Bu) could be characterized by ID 111 and 13C NMR spectroscopy including 2D NMR methods (H,H-COSY and H,C-COSY) as the first static bicyclobutonium ions. 

The endo conformation was confirmed by comparison with calculated 13C NMR chemical shifts (GIAO-MP2/TZP/DZ) of MP2/6-31G(d,p) optimized model 

The structure of the 2-norbornyl cation has been a focal point of controversy in physical organic chemistry. Experimental NMR spectroscopy and computational methods have been the decisive tools, favoring the hypercoordinated symmetric bridged structure 30, a protonated nortricyclane.79 The tricoordinated 2-norbornyl cation 31 is not a local minimum (MP2/6-31G(d)) on the energy surface.80 

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Bicyclic and polycyclic carbocations, NMR spectroscopy, 145-150 1, 2-dimethyl-2-norbornyl cation, 149 bicyclobutonium ions, 145-146, 146/ 3- /<7o-trialkylsilylbicyclobutonium ions, 147-148... 

We saw how Jra/is-annular reactions are common in carbocation chemistry. The close proximity of atoms in polycyclic systems means that, not only carbocation reactions, but also carbanion and free radical are prone to take place across the ring system. Figure 5.41 shows an anionic reaction in a bicyclic monoterpenoid. 

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