Cyclobutylmethyl cations, rearrangement

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... 

Considerable interest has recently been devoted to the cyclobutylmethyl to pent-l-enyl cation rearrangement (i.e., 32 -> 33). This intriguing transformation is illustrated below. 

Treatment with dimethyl sulfoxide in the presence of phenyl dichlorophosphate or phosphoryl chloride converts ( + )-a-pinene (39) to (6/ ,4.S)-4-isopropenyl-l-methyl-6-chlorocyclohexene (51) in virtually quantitative yield, presumably also via the cyclobutylmethyl to pent-l-enyl cation rearrangement.164 Similarly, / -pinene (45) is converted to 4-isopropenyl-l-chloromethylcyclohexene (52), also in quantitative yield.164... 

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... 

Further examples of the cyclobutylmethyl to cyclopentyl cation rearrangement via protonation of an alkene are given in Table 1. 

Table 1. Methanesulfonic Acid Induced Cyclobutylmethyl to Cyclopentyl Cation Rearrangements...

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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