Bullvalenes, rearrangement

To explain the temperature-dependent behavior of the spectrum of bullvalene, chemists have determined that bullvalene rearranges through a series of isomerizations known as Cope rearrangements. 

As a proof of principle, the twin states were characterized for the semi-bullvalene rearrangement and found to possess virtually identical geometries. As shown in Figure 19, the twin excited state possesses symmetry as the symmetry of the reaction coordinate of the thermal process. And the transition state mode, bx, which is imaginary for (Ai) was shown to be real for These calculations match the intriguing findings of Quast... 

Schroder and Witt ° have reported the synthesis of crown ethers having fluctuating ring sizes which they have termed breathing crown ethers . The structures are based on the bullvalene subunit and, as the tetracyclic subunit undergoes Cope rearrangement, the size of the macroring likewise varies. The synthetic steps follow the conventional routes used for the preparation of crown ethers and are illustrated in Eq. (3.44). 

Besides simple condensation reactions, valence isomerization reactions, in particular, arc used for the synthesis of unsaturated, eight-membered-ring azaheterocycles. These isomeriz-ations mainly involve rearrangements of nitrogen-containing bicyclo[4.2.0]octatriene or semi-bullvalene systems. 

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 In the case of the compound barbaralane (108) (bullvalene in which one CH=CH has been replaced by a CH2) ... 

Given that the boat transition state 8 is unfavourable, it is at first sight surprising that the Cope rearrangements of bullvalene (14), barbaralane (15), and semibullvalene (16) should take place so readily given that the transition states (17) of these reactions are derivatives of 8. We therefore decided 3S-) to calcu-... 

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

Shortly after this prediction, Schroder (1963) isolated bullvalene. Numerous studies amply demonstrated the facile Cope rearrangement of [84] and its derivatives (see for example Schroder and Oth, 1967 Doering et al., 1967). Theory and experiment agree that, by pinching the methano bridges closer together, the rate of the Cope process increases in the sequence semibullvalene [83] > barbaralane [85] > bullvalene [84] (Dewar and Schoeller, 1971 Anastassiou et al., 1975). 

Following the suggestion that donor-acceptor (Dewar-Hoffmann) semi-bullvalenes [83a] would have a lower activation barrier for the Cope rearrangement, or even a homoaromatic ground state (Hoffmann and Stohrer, 1971 Dewar and Lo, 1971), numerous syntheses and studies of appropriately substituted semibullvalenes have been reported. In fact, this aspect of the search for homoaromatic semibullvalenes has been the most extensively investigated (for a partial summary of this work see Quast et al., 1985 Gompper et al., 1988, and references cited therein). 

In principle, the divinylcyclopropane structure discussed here is incorporated into very well known systems such as bullvalene 547, barbaralane 548 and semibullvalene 549, which very easily undergo a Cope rearrangement. 

Serratose, et al., have succeeded in converting readily available lactone 361 to semi-bullvalene. The scheme, which involves no skeletal rearrangement, is based on diazoketone cyclization chemistry within an oxygenated cyclopentenyl derivative... 

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. 

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... 

The most intriguing hydrocarbon of this molecular formula is named bullvalene, which is found in the mixture of products of the reaction given above. G. Schroder (1963, 1964, 1967) synthesized it by a thermal dimerization presumably via diradicals of cyclooctatetraene and the photolytical cleavage of a benzene molecule from this dimer. The carbon-carbon bonds of bullvalene fluctuate extremely fast by thermal Cope rearrangements. 101/3 = 1,209,600 different combinations of the carbon atoms are possible. 

Predict the relative rates for the Cope rearrangement of bullvalene A, barbaralone B, protonated barbaralone B, barbaralane C and octamethylsemibullvalene D. 

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