Methylcyclopropylmethyl cations

In contrast to the classical secondary and tertiary systems, primary cyclopropylmethyl cations are delocalized and the nonclassical nature of both the parent cyclopropylmethyl cation 101 and the 1-methylcyclopropylmethyl cation 102 is now firmly established (Scheme 5.6) Ion 101 can be generated from alkenyl, cyclobutyl, and cyclopropylmethyl precursors under stable ion conditions. Even at the lowest temperatures studied by NMR spectroscopy, that is, -155°C, ion 101 gives rise to a spectrum indicating a structure of threefold symmetry or a set of rapidly interconverting structures with average threefold symmetry as shown by Olah et al. ... 

It should be noted that analogous secondary a-methylcyclopropylmethyl cations are observable species in superacidic media at low temperatures.21 Observation of the secondary cyclopropylmethyl carbocations under superacidic conditions indicates the superior charge delocalization into the cyclopropyl ring as compared to the cyclobutyl ring. 

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