Bullvalene
Organic molecule (C10H10)
From Wikipedia, the free encyclopedia
Bullvalene is a hydrocarbon with the chemical formula C10H10. 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 C−C and C=C bonds forming and breaking rapidly through Cope rearrangements; this property makes it a fluxional molecule.[1]
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| Names | |||
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| Preferred IUPAC name
Tricyclo[3.3.2.02,8]deca-3,6,9-triene | |||
| Other names
Bullvalen | |||
| Identifiers | |||
3D model (JSmol) |
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PubChem CID |
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CompTox Dashboard (EPA) |
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| Properties | |||
| C10H10 | |||
| Molar mass | 130.19 g/mol | ||
| Melting point | 96 °C (205 °F; 369 K) | ||
| Boiling point | decomposition at about 400 °C (752 °F; 673 K) | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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.[2] 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.[3]
Synthesis
In 1963, G. Schröder produced bullvalene by photolysis of a dimer of cyclooctatetraene. The reaction proceeds with expulsion of benzene.[4]
In 1966 W. von Eggers Doering and Joel W. Rosenthal synthesized it by the photochemical rearrangement of cis-9,10-dihydronaphthalene.[5]
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:[6]
Compound 1 in scheme 2 is not a fluxional molecule but by adding base (sodium methoxide in methanol) 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 (C8H8), 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.[7]
Semibullvalene exists only as two valence tautomers (2a and 2b in scheme 3) 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.[8] 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.
A synthetic procedure for alkylated semibullvalenes published in 2006 is based on cyclodimerisation of a substituted 1,4-dilithio-1,3-butadiene with copper(I) bromide.[9] At 140 °C the ethylated semibullvalene isomerises to the cyclooctatetraene derivative.
Barbaralane
In barbaralane, one ethylene arm is replaced by a methylene bridge and the dynamics are comparable to that of semibullvalene. There is also an intermediate ketone in bullvalene synthesis called "barbaralone". Both are named after Barbara M. Ferrier,[10] (1932–2006) professor of the Department of Biochemistry and Biomedical Sciences at McMaster University.[11]
Origin of the name
The name bullvalene is derived from the nickname of one of the scientists who predicted its properties in 1963 and the underlying concept of valence tautomerism,[12] William "Bull" Doering.[13][14] According to Klärner in 2011, the weekly seminars organised by Doering were secretly called "Bull sessions" by PhD students and postdocs and "were feared by those who were poorly prepared".[15] The name was bestowed on the molecule, in 1961, by two of Doering's Yale graduate students, Maitland Jones Jr and Ron Magid. The name celebrates Bill Doering's well-known nickname and was chosen to rhyme with fulvalene, a molecule of great interest to the research group.[16]