1- Methylcyclobutyl cation

The 1-methylcyclobutyl cation 8, which can also be considered as equilibrating 1-methylcyclopropylcarbinyl cations, was shown to have a nonclassical structure by 13C NMR spectroscopic studies in conjunction with Saunders isotopic perturbation of equilibria techniques. 

The ion could be easily prepared from either 1-methylcyclobutanol or 1-methylcyclo-propylmethyl alcohol as well as from their halide derivatives (equation 13). At above -25 °C the cation rearranges to cyclopropylmethylcarbinyl cation (vide infra) with an activation barrier of about 20 kcalmol. At equilibrium, the 1-methylcyclobutyl cation is found to be only about 2% (Keq = 50) showing that the latter cation is about 2 kcalmol 1 more stable than the 1-methylcyclobutyl cation38 (equation 14). 

The ff-delocalized structures are nonplanar, although H NMR shows only one absorption for all the methylene protons. Saunders and Krause have given unequivocal evidence for the nonplanarity of the 1 -methylcyclobutyl cation based on the isotopic perturbation of the equilibrium among nonplanar structures using stereospecifically labeled a,/3,a -trideuteriated 1-methylcyclobutyl cation 11 (equation 17)44. The cation showed two peaks... 

All the obtained evidence gathered thus far clearly shows the nonclassical nature of the 1-methylcyclobutyl cation. 

Gas-phase protonation of spiropentane 19 using 3HeT+ or D3+ as protonating agents provided initially the comer protonated spiropentane, which rearranged into 1 -methylcy-clobutyl cation (i.e. bicyclobutonium ion 9)45. Under mild protonating conditions, cis- and trows-2-methylcyclopropylcarbinyl cations 17 and 18 were observed. These cations were presumably formed through the isomerization of the 1-methylcyclobutyl cation 9 into the... 

Ab initio MO calculations at the MP2(FC)/6-31G level of theory gave estimates of the activation barrier of 1 -methylcyclobutyl cation to the a-methylcyclopropylcarbinyl cation of about 20.0 kcal mol1. At MP2/6-31G + ZPE, the trans-isomer is 2 kcal mol 1 more stable than the cw-isomer, whereas the bent 1-methylcyclobutyl cation 9 is less stable than the trans-1 -methylcyclopropylcarbinyl cation 18 by 0.8 kcal mol"1 (vide supra). The MP2/6-31G optimized bond lengths (in A) for the cis and trans isomeric cations are as shown ... 

The parent secondary cyclobutyl cation 47 undergoes threefold degenerate rearrangement viao-bond delocalization involving nonclassical bicyclobutonium ion-like system171,172 (see Section 3.5.2.5). Similar behavior is also observed for the 1-methylcyclobutyl cation 48a.173 176 The 1-phenylcyclobutyl cation 48b, on the other... 

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