Degenerate rearrangement of bullvalene

Figure 12.6. a) Cope rearrangement of 1,5-hexadiene (6) oxy-Cope rearrangement (c) divinyl-cyclopropane rearrangement (d) degenerate rearrangements of bullvalene. 

The a bonding orbital may also be raised by incorporation into a 3- or 4-membered ring. The divinylcyclopropane to cyclohepta-1,4-diene (Figure 12.6c) is an example, as is the rapid degenerate rearrangement of bullvalene (Figure 12.6d) and related compounds. 

From the variable temperature H-NMR spectrum Saunders found = 11.8 kcalmol" and A = 12.3 s for the degenerate rearrangement of bullvalene. The remarkably small probability and entropy barriers to the rearrangements that involve both carbon-carbon bond breaking and making were ascribed both to extremely favourable geometry and to weakness of the bond . These values were amended slightly... 

Figure 3. One of the three possible pathways in the degenerate Cope rearrangement of bullvalene.

As rapid as the Cope rearrangement of bullvalene is, and blessed as it is with an esthetically pleasing level of symmetry, other degenerate rearrangements have been discovered that are even faster. Barbaralane (37), for example, rearranges to itself... 

Conformations and Dynamic Situations.— The degenerate Cope rearrangement of bullvalene (35) is the subject of three papers which report values for the thermodynamic parameters (Table 1) obtained from lineshape analysis of pulsed Fourier Transform C spectra at different temperatures. The communication by Nakanishi and Yamamoto analyses data at 35 temperatures using modified Bloch equations. The margin of error stated is fairly high. A very thorough and well explained analysis... 

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... 

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