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 with ten distinguishable positions is 10!/3 = 1,209,600 not counting enantiomers.
In 1966 W. von Eggers Doering and Joel W. Rosenthal synthesized it by the photochemical rearrangement of cis-9,10-dihydronaphthalene.[4]
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:[5]
Scheme 2. A bullvalone
Compound 1 in scheme 2 is not a fluxional molecule but by adding base (sodium methoxide in methanol) the ketone converts to the enolate2 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 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.[7] 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.
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(I) bromide.[8] At 140 °C the ethylated semibullvalene isomerises to the cyclooctatetraene derivative.
Scheme 5. New semibullvalene synthesis
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,[9] (1932–2006) professor of the Department of Biochemistry and Biomedical Sciences at McMaster University.[10]
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,[11]William "Bull" Doering.[12][13] 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".[14] 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.[15]
References
^Addison Ault (2001). "The Bullvalene Story. The Conception of Bullvalene, a Molecule That Has No Permanent Structure". Journal of Chemical Education. 78 (7): 924. Bibcode:2001JChEd..78..924A. doi:10.1021/ed078p924.
^Oth, J.; Mullen, K.; Gilles, J.; Schröder, G. (1974). "Comparison of 13C- and 1H- magnetic resonance spectroscopy as techniques for the quantitative investigation of dynamic processes. The Cope rearrangement in bullvalene". Helv Chim Acta. 57 (5): 1415–1433. doi:10.1002/hlca.19740570518.
^Von Eggers Doering, W.; Rosenthal, Joel W. (1966). "9,10-Dihydronaphthalene. Formation from Bullvalene and Nenitzescu's Hydrocarbon, Thermal Reorganization, and Photorearrangement to Bullvalene". J. Am. Chem. Soc. 88 (9): 2078–2079. doi:10.1021/ja00961a061.
^Lippert, A. R.; Kaeobamrung, J.; Bode, J. W. (2006). "Synthesis of Oligosubstituted Bullvalones: Shapeshifting Molecules Under Basic Conditions". J. Am. Chem. Soc.128 (46): 14738–14739. doi:10.1021/ja063900+. PMID17105247.
^
Zimmerman, H. E.; Grunewald, G. L. (1966). "The Chemistry of Barrelene. III. A Unique Photoisomerization to Semibullvalene". J. Am. Chem. Soc.88 (1): 183–184. doi:10.1021/ja00953a045.
