Degenerate rearrangements, bullvalene

Table 4. Calculated (MINDO/2) and Observed Activation Energies for Degenerate Rearrangements in the Bullvalene Series...

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... 

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... 

Bullvalene is said to be fluxional, meaning that it undergoes rapid degenerate rearrangements (reference 17). 

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... 

No other system, however, comes anywhere near the multiple degeneracy possible in the molecule bullvalene (59). This system was predicted to be capable of a 1 209 600-fold degenerate rearrangement (Doering and Roth,... 

Another compound for which degenerate Cope rearrangements result in equivalence for all the carbons is hypostrophene W1). In the case of the compound barbaralane (108) (bullvalene in which one CH=CH has been replaced by a CH2) ... 

Doering and Roth, at the time of their first investigations of 86 in 1963, proposed that the structure 90, which they named bullvalene, should undergo degenerate Cope rearrangements that would make each of the ten CH groups equivalent.179 Equation 12.109 illustrates just a few of these transformations.180... 

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

---