2.2- Dimethylcyclopropylcarbinyl cation

The nmr spectrum of the 4,4-dimethylcyclopropylcarbinyl cation in superacid is only in agreement with the bisected structure of the ion (78). In Structure 78 the two methyl groups are not equivalent—one is cis to the cyclopropyl ring... 

Similar to the a,a-dimethylcyclopropylcarbinyl cation 20, the 1-cyclopropylcyclopentyl and 1 -cyclopropylcyclohexy 1 cations 21 and 22 show restricted rotation of the cyclopropyl... 

Olah and coworkers obtained the a,a-dimethylcyclopropylcarbinyl cation 20 from the corresponding alcohol by ionization with SbF5/S02ClF. The cation shows distinct absorptions for the two methyl groups in the proton and NMR. The cw-methyl group is shielded ((53.57) compared to the trans-methyl (3.68 ppm). The NMR shows the following absorptions 279.9 (C), 59.1 (CH), 53.1 (CH2), 39.1 (exo CH3), 30.1 (endo... 

The 9-Cyclopropyl-9-xanthyl cation 33 is so stable that the positive charge is hardly delocalized into the cyclopropyl group. The alpha and beta hydrogens of the cyclopropyl group show absorptions at 2.96,1.38 and 1.96, comparable to that of cyclopropylam-monium ion ( H 2.93 and 0.93), rather than a typical cyclopropylcarbinyl cation, such as dimethylcyclopropylcarbinyl cation ( H 3.4,4.0). ... 

Dimethylcyclopropylcarbinyl cation 274 8,9-Dehydro-2-adamantyl and related cations 275... 

Considerable support for Roberts suggestion has come from later studies, notably those involving NMR studies in superacid media. Thus, while the spectrum of the a,a-dimethylcyclopropylcarbinyl cation is best rationalized in terms of a classical structure that for the parent system is consistent with neither static nor rapidly equilibrating classical structures The parent system seems best to be described as a mixture of the equilibrating bicyclobutonium ions and, less than 4 kcal mol higher in energy another structure that might be the classical cyclopropylcarbinyl cation ... 

An early success of NMR studies in superacid media dealt with the determination of the preferred conformation of cyclopropylcarbinyl cations. The NMR spectrum of dimethylcyclopropylcarbinyl cation (entry 14) exhibits signals for two nonequivalent methyl groups, clearly supporting the bisected conformation. An alternative conformation, the perpendicular conformation, would exhibit a single peak for two equivalent methyl groups. Subsequent work established the rotational... 

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