Bullvalene Explained

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 on the NMR timescale; this property makes it a fluxional molecule.[1]

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 with ten distinguishable positions 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.[3]

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]

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.[6]

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.

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.

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]

Notes and References

  1. Addison Ault . The Bullvalene Story. The Conception of Bullvalene, a Molecule That Has No Permanent Structure . Journal of Chemical Education . 78. 7 . 2001 . 924 . 10.1021/ed078p924. 2001JChEd..78..924A.
  2. Oth . J. . Mullen . K. . Gilles . J. . Schröder . G. . 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 . 1974 . 57 . 5 . 1415–1433 . 10.1002/hlca.19740570518.
  3. Schröder . Gerhard . Preparation and Properties of Tricyclo[3,3,2,04,6]deca-2,7,9-triene (Bullvalene) ]. Angewandte Chemie International Edition in English . 1963 . en . 2 . 8 . 481–482 . 10.1002/anie.196304814 . 0570-0833.
  4. 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. 10.1021/ja00961a061.
  5. 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. 10.1021/ja063900+. 17105247.
  6. 10.1021/ja00953a045 . 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.
  7. 10.1021/ja00991a064 . Zimmerman. H. E.. Binkley. R. W.. Givens. R. S.. Sherwin. M. A.. 1967. Mechanistic Organic Photochemistry. XXIV. The Mechanism of the Conversion of Barrelene to Semibullvalene. A General Photochemical Process. J. Am. Chem. Soc.. 89. 15. 3932–3933.
  8. Wang. C.. Yuan. J.. Li. G.. Wang. Z.. Zhang. S.. Xi. Z.. 2006. Metal-Mediated Efficient Synthesis, Structural Characterization, and Skeletal Rearrangement of Octasubstituted Semibullvalenes. J. Am. Chem. Soc.. 128. 14. 4564–4565. 10.1021/ja0579208. 16594680.
  9. Alex Nickon, Ernest F. Silversmith, Organic Chemistry: The Name Game: Modern Coined Terms and Their Origins, p. 133, Pergamon Press, 1987.
  10. http://fhs.mcmaster.ca/main/news/news_archives/ferrier.htm A tribute to professor emeritus Barbara Ferrier
  11. 10.1016/S0040-4020(01)99207-5 . Doering. W. von E.. Roth. W. R.. 1963. A Rapidly Reversible Degenerate Cope Rearrangement : Bicyclo[5.1.0]octa-2,5-diene. Tetrahedron. 19. 5. 715–737.
  12. 10.1021/ed078p924 . Ault . Addison . 2001 . The Bullvalene Story. The Conception of Bullvalene, a Molecule That Has No Permanent Structure . . 78 . 7 . 924 . 2001JChEd..78..924A .
  13. Author Ault (2001) also suggests the name stems from BS because of an unimpressed grad student
  14. Klärner, F.-G. (2011), William von Eggers Doering (1917–2011). Angewandte Chemie International Edition, 50: 2885–2886.
  15. Nickon, A.; Silversmith, E. F. Organic Chemistry: The Name Game; Pergamon: New York, 1972; p 131.