Methylbicyclobutonium ion

The 1-methylbicyclobutonium ion (56) and the l-(trimethylsilyl)bicyclobutonium ion (57) also undergo fast threefold methylene rearrangements (Scheme 3, R = CH3 or Si(CH3)3). 

For the analogous 1-methyl-substituted cation [C gCFb]"1", recent experimental investigations and quantum chemical ab initio calculations agree that the dynamics of this cation system can be adequately described by considering only one degenerate set of cations, which have the hypercoordinated puckered methylbicyclobutonium ion structure 420 without contributions from a degenerate set of (l -methylcyclopropyl)methyl cation structures 4217,166,167. 

The geometric and electronic properties of the 1-trialkylsilylbicyclobutonium cations, as concluded from a comparison of the experimental data for 422 and 424 and the calculated data for 425, are intermediate between that of the parent bicyclobutonium ion 418 and that of the 1-methylbicyclobutonium ion 420. 

From the comparison of calculated NMR shifts, the low lying species is considered to be bicyclobutonium ion 1709s However, in the case of CSH9+ 39, the low lying species are the methylbicyclobutonium ions 171 with no contribution from either... 

Similarly, both the cx-methylcyclopropyhnethyl cation 108 and the nonclassical methylbicyclobutonium ion 109 (both of G symmetry) were found to be minima on the potential energy surface of C5H/, with the latter being 5.9 kcal mol more stable [MP4(SDTO)/cc-pVTZ//MP2/cc-pVTZ-1-ZPE level]. A good correlation has been found between the calculated C NMR chemical shifts of 109 and the experimental values. 

Fig. 3. Qualitative energy profiles, energy E) vs. reaction coordinate (RC) for the rearrangement of parent bieyclobutonium ions 1 and cyclopropylmethyl cations 2, and of 1-methylbicyclobutonium ions 3 and (r-methylcyclopropyl)metbyl cations 4.

---