Barbaralanes rearrangement

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 In the case of the compound barbaralane (108) (bullvalene in which one CH=CH has been replaced by a CH2) ...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

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). 

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. 

A number of semibullvalenes and some barbaralanes, which undergo a facile Cope rearrangement via bicyclooctadienyl diradicals, produce various cyclic peroxides on exposure to triplet oxygen ". Thus, cyclopentano semibullvalene 82a is peroxidated with air to give a mixture of 5-membered 83a and 6-membered endoperoxide 84a in a 1 2 ratio, whereas the cyclohexano derivative 82b yields only the 6-membered cyclic peroxide 84b (Scheme 15). The energy in the highly strained mono(Dewar benzene) isomer of... 

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

H. Quast, M. Seefelder, Angew. Chem. Int. Ed. Engl. 38, 1064 (1999). The Equilibrium between Localized and Delocalized States of Thermochromic Semibullvalenes and Barbaralanes—Direct Observation of Transition States of Degenerate Cope Rearrangements. 

Guenther, H., Runsink, J., Schmickler, H., Schmitt, P. Activation parameters forthe degenerate Cope rearrangement of barbaralane and 3,7-disubstituted barbaralanes. J. Org. Chem. 1985, 50, 289-293. 

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