Bullvalene
Bullvalene is a hydrocarbon with the chemical formula. The molecule has a cage-like structure formed by the fusion of one cyclopropane and three cyclohepta-1,4-diene rings. Bullvalene is unusual as an organic molecule due to the and bonds forming and breaking rapidly through Cope rearrangements; this property makes it a fluxional molecule.
Stereodynamics
The bullvalene molecule is a cyclopropane platform with three vinylene arms conjoined at a methine group. This arrangement enables a degenerate Cope rearrangement with the result that all carbon atoms and hydrogen atoms appear equivalent on the NMR timescale. At room temperature the 1H NMR signals average to a rounded peak at 5.76 ppm. At lower temperatures the peak broadens into a mound-like appearance, and at very low temperatures the fluxional behavior of bullvalene is reduced, allowing for 4 total signals to be seen. This pattern is consistent with an exchange process whose rate k is close to the frequency separation of the four contributing resonances. The number of possible valence tautomers of a bullvalene depends on the number of unique substituents. The number of isomers with ten distinguishable substituents is 10!/3 = 1,209,600.Synthesis
In 1963, G. Schröder produced bullvalene by photolysis of a dimer of cyclooctatetraene. The reaction proceeds with expulsion of benzene.In 1966 W. von Eggers Doering and Joel W. Rosenthal synthesized it by the photochemical rearrangement of cis-9,10-dihydronaphthalene.
Related compounds
Bullvalones
In bullvalones one vinyl group in one of the arms in bullvalene is replaced by a keto group on a methylene bridge. In this way it is possible to activate the fluxional state by adding base and deactivate it again by removing the base:Image:Bullvalone.png|left|thumb|500px|Scheme 2. A bullvalone
Compound 1 in scheme 2 is not a fluxional molecule but by adding base the ketone converts to the enolate 2 and the fluxional state is switched on. Deuterium labeling is possible forming first 3 a then a complex mixture with up to 7 deuterium atoms, compound 4 being just one of them.
Semibullvalene
In semibullvalene, one ethylene arm is replaced by a single bond. The compound was first prepared by photolysis of barrelene in isopentane with acetone as a photosensitizer in 1966.Semibullvalene exists only as two valence tautomers but in this molecule the Cope rearrangement takes place even at −110 °C, a temperature at which this type of reaction is ordinarily not possible.
One insight into the reaction mechanism for this photoreaction is given by an isotope scrambling experiment. The 6 vinylic protons in barrelene 1 are more acidic than the two bridgehead protons and therefore they can be replaced by deuterium with N-deuteriocyclohexylamide. Photolysis of 2 results in the initial formation of a biradical intermediate with a cyclopropane ring formed. This product rearranges to a second intermediate with a more favorable allylic radical as two mesomers. Intersystem crossing and radical recombination results in equal quantities of semibullvalenes 3 and 4. The new proton distribution with allylic, vinylic and cyclopropanyl protons determined with proton NMR confirms this model. As noted, the conversion of barrelene to semibullvalene is a di-π-methane rearrangement.
Image:Semibullvalene mechanism.png|thumb|left|600px|Scheme 4. Barrelene photolysis mechanism
A synthetic procedure for alkylated semibullvalenes published in 2006 is based on cyclodimerisation of a substituted 1,4-dilithio-1,3-butadiene with copper bromide. At 140 °C the ethylated semibullvalene isomerises to the cyclooctatetraene derivative.
Image:NewSemibullvaleneSynthesis.png|thumb|left|600px|Scheme 5. New semibullvalene synthesis