Cyclopropylidene cation

As discussed in recent reviews the cyclopropylidene cation 116 is significantly stabilized by 7r-donation from cyclopropyl, as the ring bisects the vacant p orbital in this cation so that 7r-donation by the ring should be quite efficient. 

The GIAO-MP2/TZP calculated 13C NMR chemical shifts of the cyclopropylidene substituted dienyl cation 27 show for almost all carbon positions larger deviations from the experimental shifts than the other cations 22-26. The GIAO-MP2/TZP method overestimates the influence of cr-delocalization of the positive charge into the cyclopropane subunit on the chemical shifts. Electron correlation corrections for cyclopropylidenemethyl cations such as 27 and 28 are too large to be adequately described by the GIAO-MP2 perturbation theory method and higher hierarchies of approximations such as coupled cluster models are required to rectify the problem. 

Siehl and coworkers have prepared cyclopropylcyclopropylidenemethyl cation 37, by protonation of bis(cyclopropylidene)methane with FS03H-SbF5 at liquid nitrogen temperature (equation 31 )67. The vinyl cationic center is stabilized by the a-cyclopropyl group. 

Other cyclopropylidene-substituted vinyl cations, 39 and 40, have also been prepared70. 

The reactive cyclopropylidene cyclopropylcarbinyl cation 37 was generated from the corresponding bromide and silver hexafluoroantimonate, and was reacted in situ with olefins such as cyclohexene to form the addition products (equation 33). On the other hand, under the same conditions, the isopropylidene cyclopropylcarbinyl cation 41 rearranged spontaneously by ring expansion (equation 34)71. 

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