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 group 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
of a bullvalene depends on the number of unique substituents. The number of isomers with ten distinguishable substituents is 10
Factorial/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 .
Related compounds
Bullvalones
In
bullvalones one vinyl group in one of the arms in bullvalene is replaced by a
ketone 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:
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 (C
8H
8), 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 ( 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.
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.
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,
[Alex Nickon, Ernest F. Silversmith, Organic Chemistry: The Name Game: Modern Coined Terms and Their Origins, p. 133, Pergamon Press, 1987.] (1932–2006) professor of the Department of Biochemistry and Biomedical Sciences at McMaster University.
[ A tribute to professor emeritus Barbara Ferrier , McMaster University, 6 January 2006]
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,
William "Bull" Doering.
[Author Ault (2001) also suggests the name stems from bullshit because of an unimpressed grad student] According to Klärner in 2011, the weekly seminars organised by Doering were secretly called "" by PhD students and postdocs and "were feared by those who were poorly prepared".
[Klärner, F.-G. (2011), William von Eggers Doering (1917–2011). Angewandte Chemie International Edition, 50: 2885–2886. ] 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.
[Nickon, A.; Silversmith, E. F. Organic Chemistry: The Name Game; Pergamon: New York, 1972; p 131.]
