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 In the case of the compound barbaralane (108) (bullvalene in which one CH=CH has been replaced by a CH2) ...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

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... 

Complete lineshape analysis of the ABCj DEF2 exchange process observed in the n.m.r. spectra of 2,6-disubstituted homotropylidenes has been employed in the evaluation of the thermodynamic parameters for Cope rearrangement, The phenyl groups at the 2,6-positions of the barbaralane (155) destabilize the transition state relative to the unsubstituted hydrocarbon (154) as evidenced by the AGj g values, respectively 9.3 and 7.6 ( 0.1) kcalmol" Kinetic data for the Diels-Alder addition of acrolein to cydohexa-1,3-diene, and its reverse reaction, in the gas phase have been interpreted in favour of a biradical mechanism. Both the heat and entropy of formation of the endo-adduct have been estimated. Structural modification of 2,3-dimethylenenorbornane (156) or of 2,3-dimethylenenorbornene (158) by the introduction of oxygen in place of C-7, as in (157) and (159) respectively, leads to a decrease in rate of Diels-Alder reaction of the dienes with dimethyl acetylenedi-carboxylate. The double bond in (158) likewise resulted in deactivation relative to (156), and epoxidation of the endocyclic double bond in (158) also afforded an s-cis-diene of decreased reactivity. 

Jackman LM, Fernandes E, Heubes M et al (1998) The effects of substituents on the degenerate cope rearrangement of semibullvalenes and barbaralanes. Eur J Org Chem 10 2209-2227... 

MINDO/2 methods have again been used to determine the strain energies of semibullvalene (16), barbaralane (17), and bullvalene (18), and the activation energies for the Cope rearrangement have been calculated. 

In his efforts to lower the activation energy for the Cope rearrangement, Doerlng designed the first fluxional molecule homotropUldine (79). Barbaralane (80), bullvalene (81), and semibullvalene (68) are each related to 79 as homotropilidines locked in the appropriate Cope conformation (79b). As a conse-... 

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