Threefold degenerate rearrangement

The twofold degenerate rearrangement of 51 may involve unpopulated puckered 1,2,4-trimethylcyclobutyl cation 52 as an intermediate. In the threefold degenerate rearrangement of 53, isomeric 54 may be involved as an NMR unpopulated intermediate80. 

The 3-homonortricyclyl cation 70 was prepared by the isomerization of bicyclo[3.2.1]oct-3-en-2-yl cation 71 at 20 °C in SbFs/SC ClF solution83. The ion shows a threefold degenerate rearrangement between -85 °C to 20 °C. At 20 °C the C4, C6, C8 and Cl, C3, C7 carbons become equivalent with an average of 36.19 ppm and 135.8 ppm, respectively (equation 43). Below -80 °C the cation is a static secondary cyclopropylcarbinyl cation with the cationic center chemical shift at <5I3C 234.1. 

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... 

Siehl and co-workers949 950 have used the matrix co-condensation technique to generate the l-(trimethylsilyl)bicyclobutonium ion 528 (Scheme 3.23). The 1H and 13 C NMR spectra of ion 528 show averaged methylene signals, which is in accord with a fast threefold degenerate rearrangement and a puckered hypercoordinate structure. 

Fig. 2. Threefold degenerate rearrangement of bicyclobutonium ions via cyclopropylmethyl cation structures.

Scheme 3 Rearrangement of [C4H6R] + cations, for R = H a threefold degenerate interconversion of bicyclobutonium ion (54) and cyclopropylmethyl cations (55).

Olah and coworkers NMR study of the 8,9-dehydro-2-adamantyl cation, obtained from the corresponding alcohol in superacidic medium, showed the equivalence of C2, C8 and C9 carbons ((5 C 157.0) in agreement with the conclusions of solvolytic studies Even at -120 °C the structure of the cation could not be frozen out to a static cation, showing the extremely fast equilibration of the threefold degenerate cyclopropylcarbinyl cation 74. An identical NMR spectrum was obtained from the ionization of the 2,5-dehydro-4-protoadamantanol, which prompted the suggestion of the intermediacy of the 2,5-dehydro-4-protoadamantyl cation 75. The ion rearranges to an ally lie cation 76 at -78 (equation 45). 

X 10 s at 25° At 10° C, the H-NMR spectrum of bullvalene exhibits a single peak at 4.22 ppm, which indicates the fluxional nature of the molecule. Owing to the threefold axis of symmetry present in bullvalene, the degenerate rearrangement results in all of the carbons having an identical averaged environment. This is illustrated in the... 

Summary The l-(trimethylsilyl)bicyclobutonium ion and the 3-e c fe>-(/er/-butyldi-methylsilyl)bicyclobutonium ion were investigated by NMR spectroscopy in superacid solution and by quantum chemical ab initio calculations. The l-(trimethyl-silyl)bicyclobutonium ion undergoes a threefold degenerate methylene rearrangement. The 3-e /o-(/ert-butyldimethylsilyl)bicyclobutonium ion is the first static bicyclo-butonium ion. The NMR spectra of this carbocation are a direct proof for the hypercoordinated and puckered structure of bicyclobutonium ions. 

The l-(trimethylsilyl)cyclobutyl cation is obtained by reaction of [l -(trimethylsilyl)cyclopropyl]-methanol with SbFs at -130°C. It has a hypercoordinated puckered l-(trimethylsilyl)-bicyclobutonium structure 5. Cation 5 undergoes a fast threefold degenerate methylene rearrangement (compare Fig. 2 with R = SiMe3). Averaged NMR signals are observed for the... 

But now perhaps you notice the threefold rotational axis in bullvalene. Remove the color and the three double bonds become indistinguishable. In fact, there is a Cope rearrangement possible in each of the three faces of the molecule, and every Cope rearrangement is degenerate each re-forms the same molecule (Rg. 20.67). This point is much easier to see if you use a model. 

FIGURE 20.67 Bullvalene has a threefold rotational axis AH three double bonds are equivalent. The Cope rearrangement can occur in each of the three faces of the molecule and is degenerate in every case. 

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