Cyclobutylmethyl cations, nonclassical

Dauben and coworkers observed unusual rate accelerations and ring-expansion rearrangements in the solvolysis of bicyclo[2.2.0]hexane-l-methyl / -nitrobenzoate, in agreement with Winstein s proposed nonclassical cyclobutylmethyl cations (Figure 3). ° Thus, the rate of the solvolysis of bicyclo[2.2.0]hexane-l-methyl p-nitrobenzoate is 7 x 10 times faster than that of the corresponding extrapolated rate for neopentyl derivative. The lack of scrambling of the label in the solvolysis of the 0-labeled... 

One very interesting structure that might at first glance appear to be a 1° carbocation is the cyclopropylmethyl cation. Calculations indicated, however, that this species is best described by a pair of rapidly equilibrating nonclassical structures (62 and 63, Figure 5.46) that are approximately equal in energyAttempts to prepare the 1° cyclobutylmethyl cation were unsuccessful, however, with cyclopentyl cations being formed instead. ... 

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