Bullvalene



Bullvalene is a hydrocarbon with the chemical formula C10H10 with the unusual property that the chemical bonds making up the molecule are constantly rearranging as in fluxional molecules. For this reason bullvalene is extensively studied in organic chemistry. The name bullvalene is derived from the nickname of one the scientists who predicted its properties in 1963, William "bull" Doering  and the underlying concept of valence tautomerism. This tautomerism is also the reason for its extraordinary stability at low temperatures (about 100 Kelvin).

Bullvalene
The bullvalene molecule can be envisioned as a cyclopropane platform with three vinyl arms connecting with a methyl group. Within a specific temperature range the molecule is subject to rapid degenerate Cope rearrangements with the result that all carbon atoms and hydrogen atoms are equivalent and that none of the carbon-carbon bonds are permanent. The number of possible valence tautomers is 10!/3 = 1,209,600. The rearrangements displayed in scheme 2 are only 5 of them.



The compound was first synthesised in 1963 by G. Schröder by photolysis of a dimer of cyclooctatetraene with expulsion of benzene.

The unusual dynamic properties of bullvalene show up in proton NMR. In a spectrum taken at room temperature a broad peak is visible representing many different protons. The spectrum at 120°C on the other hand shows just one peak at 4.2 ppm. At this temperature the bullvalane protons are rearranging faster than the NMR can resolve resonances, this is largely due to the Heisenberg Uncertainty Principle (energy/time formulation).

Bullvalones
In bullvalones one vinyl group in one of the arms in bullvalene is replaced by a ketone group. In this way it is possible to activate the fluxional state by adding base and deactivate it again by removing the base:

Compound 1 in scheme 2b 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 toluene with acetone as a photosensitizer in 1966.



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. 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 an 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.



A new synthetic procedure for alkylated semibullvalenes published in 2006 is based on cyclodimerization of a 1,4-dilithio-1,3-diene with copper(I) bromide. At 140°C the ethylated semibullvalene isomerises to the cyclooctatetraene.



Barbaralane
In barbaralane one ethylene arm is replaced by a methylene group and the dynamics are comparable to that of semibullvalene.