Bullvalene
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2008-05-02T05:05:37Z
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[[Image:Bullvalene.svg|thumb|right|Scheme 1. Bullvalene]]
'''Bullvalene''' is a [[hydrocarbon]] with the [[chemical formula]] C<sub>10</sub>H<sub>10</sub> with the unusual property that the [[chemical bond]]s making up the molecule are constantly rearranging as in [[fluxional molecule]]s. 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" [[William von Eggers Doering|Doering]]<ref>''The Bullvalene Story. The Conception of Bullvalene, a Molecule That Has No Permanent Structure'' Ault, Addison. [[J. Chem. Educ.]] '''2001''' 78 924. [http://www.jce.divched.org/Journal/Issues/2001/Jul/abs924.html Abstract]</ref><ref><small>Author Ault (2001) also suggests the name stems from [[bullshit|BS]] because of an unimpressed grad student </small></ref> 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 [[methine]] group. Within a specific temperature range the molecule is subject to rapid degenerate [[Cope rearrangement]]s 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 tautomer]]s is 10[[Factorial|!]]/3 = 1,209,600. The rearrangements displayed in ''scheme 2'' are only 5 of them.
[[Image:Bullvalene rearrangements.png|center|600px|Scheme 2. Some bullvalene rearrangements]]
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:<ref>''Synthesis of Oligosubstituted Bullvalones: Shapeshifting Molecules Under Basic Conditions'' Alex R. Lippert, Juthanat Kaeobamrung, and Jeffrey W. Bode [[J. Am. Chem. Soc.]]; '''2006'''; 128(46) pp 14738 - 14739; (Communication) {{DOI|10.1021/ja063900+}}</ref>
[[Image:Bullvalone.png|center|500px|Scheme 2b. A bullvalone]]
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''' (C<sub>8</sub>H<sub>8</sub>), 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]]<ref>''The Chemistry of Barrelene. III. A Unique Photoisomerization to Semibullvalene'' Howard E. Zimmerman, Gary L. Grunewald [[J. Am. Chem. Soc.]]; '''1966'''; 88(1); 183-184. [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1966/88/i01/f-pdf/f_ja00953a045.pdf Abstract]</ref> in 1966.
[[Image:Semibullvalene synthesis.png|center|400px|Scheme 3. Semibullvalene synthesis]]
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.<ref>''Mechanistic organic photochemistry. XXIV. The mechanism of the conversion of barrelene to semibullvalene. A general photochemical process'' Howard E. Zimmerman, Roger W. Binkley, Richard S. Givens, Maynard A. Sherwin [[J. Am. Chem. Soc.]]; '''1967'''; 89(15); 3932-3933. [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1967/89/i15/f-pdf/f_ja00991a064.pdf Abstract]</ref> The 6 vinylic protons in barrelene '''1''' are more [[acidity|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 [[allyl]]ic radical as two [[mesomer]]s. [[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.
[[Image:Semibullvalene mechanism.png|center|600px|Scheme 4. Barrelene photolysis mechanism]]
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]].<ref>''Metal-Mediated Efficient Synthesis, Structural Characterization, and Skeletal Rearrangement of Octasubstituted Semibullvalenes'' Chao Wang, Jian Yuan, Guotao Li, Zitao Wang, Shiwei Zhang, and Zhenfeng Xi
[[J. Am. Chem. Soc.]]; '''2006'''; 128(14) pp 4564 - 4565; [http://dx.doi.org/10.1021/ja0579208 Abstract]</ref> At 140°C the ethylated semibullvalene isomerises to the [[cyclooctatetraene]].
[[Image:NewSemibullvaleneSynthesis.png|center|600px|Scheme 5. new semivullvalene synthesis ref. Wang 2006]]
==Barbaralane==
In '''barbaralane''' one ethylene arm is replaced by a [[methylene]] group and the dynamics are comparable to that of semibullvalene.
==References==
<references/>
==External links==
[[Category:hydrocarbons]]
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