Cyclobutylmethyl cations

An experimental and calculational NMR investigation of dicyclopropyl substituted cyclobutylmethyl cation (19)45 has shown that IGLO/DZ//B3LYP/6-31G(d) calculated 13C NMR chemical shifts facilitate the assignment of the spectra. 

Chapter 5 by M. Fujita and T. Okuyama examines the ringopening reactions of alkylidenecyclopropanone acetals for solvolytic generation and trapping of alkylideneallyl cations (resonance hybrids of 1-vinyl-substituted vinyl cations). In Chapter 6 by V. P. Reddy et al. stable ion and computational studies of cyclobutylmethyl cations are discussed. In Chapter 7, G. I. Borodkin and V. G. Shubin discuss and... 

The cyclopropylmethyl ion 11 is unusually stable and has a 14kcal/mol barrier to rotation about the cyclopropyl-carbocation bond.69 In contrast, the corresponding cyclobutylmcthyl ion 12 quickly rearranges to a cyclopentyl cation. Here, some strain relief occurs in the rearrangement, but this is opposed by the conversion of the stable tertiary carbocation to the less stable secondary ion. Although rearrangement is the normal process for cyclobutylmethyl cations, there is one case 13 in which rearrangement does not occur, and a small rate acceleration is observed.70... 

Another method by which a carbocation can be generated is by protonation of either an alkene or a carbonyl group. Thus, cyclobutylmethyl cations are formed by treatment of a ketone or an alkene with acid the cyclobutylmethyl cations subsequently rearrange to give cyclopentyl cations. 

The final step in an assemblage of the (+ )-isocomene skeleton also requires the cyclobutyl-methyl to cyclopentyl cation rearrangement. However, the desired cyclobutylmethyl cation was generated by protonation of an alkene. Thus, p-toluenesulfonic acid induced rearrangement converted 2,6,8-trimethyl-5-methylenetricyclo[6.3.0.016]undccane to isocomene (6).20-21... 

An alternative approach to form a cyclobutylmethyl cation from a vinylcyclobutane uses organopalladium chemistry.25 For example, exposure of l-methyl-7-vinylbicyclo[4.2.0]octan-7-ol to 5mol% of bis(benzonitrile)palladium dichloride and 1,4-benzoquinone in boiling tetrahydrofuran gave the desired 6,9-dimcthylbicyclo[4.3.0]non-l(9)-en-8-one (7) in 67% yield.25 The mechanistic pathway of this transformation is shown. Further examples, showing the utility of the reaction, are shown for the rearrangement to 8.23... 

The examples discussed in this subsection are miscellaneous reactions which are initiated by the rearrangement of cyclobutylmethyl cations to cyclopentyl cations, and are then followed by further bond migrations to produce more stable cations. As a result, molecules with unexpected skeletal architectures are usually obtained. 

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... 

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