In the Classroom
The Bullvalene Story. The Conception of Bullvalene, a Molecule That Has No Permanent Structure Addison Ault Department of Chemistry, Cornell College, Mount Vernon, IA 52314;
[email protected] Benzene, C 6H6, and cyclooctatetraene, C8H8 (1, 2), provide simple and familiar examples of how the properties of a substance force us to draw important conclusions about molecular structure. Thus the great stability of benzene and the high reactivity of cyclooctatetraene lead us to think very differently about the molecular natures of these substances. Although each is customarily represented by a cyclic array of alternating double and single bonds, we interpret these representations quite differently, speaking of benzene as being “aromatic” and cyclooctatetraene as being “a polyene”.
benzene stable “aromatic”
∆E ‡ = 259 kJ/mol The difference of 88 kJ/mol is attributed to stabilization of the allylic radical by delocalization of the p electrons. Doering and Roth then suggested that one might expect an activation energy of 347 – 88 – 88 = 171 kJ/mol for the breaking of the central bond of 1,5-hexadiene.
cyclooctatetraene reactive “a polyene”
But there is yet another molecule of the general formula (CH)n that provides a beautiful example of how the unusual properties of a substance lead to unprecedented conclusions about molecular structure. This molecule, of molecular formula C10H10, is usually called “bullvalene”. Its molecular structure has the astonishing feature of not being a structure in the conventional sense. That is, bullvalene has no permanent carbon– carbon bonds! All carbon atoms are bonded, or not bonded, to the same extent with every other carbon atom in the molecule. C10H10 bullvalene no permanent carbon–carbon bonds “a fluxional molecule”
Not only is this true without any possibility of doubt, but the existence of such a molecule was predicted (3). Surely this was one of the most risky predictions ever made in the history of chemistry! Unfortunately, none of this story is presented in any of the “sophomore-level” organic texts on my shelf. One text mentions bullvalene in a problem, and another reviews the fluxional nature of the molecule but does not touch on the thinking that led to the prediction of the existence of a molecule of this unprecedented nature. This prediction was made in 1963 by William Doering and Wolfgang Roth (3), and this paper reviews the logical steps that formed the basis for it, presenting the most important insights and some of the most persuasive examples. The Story Doering and Roth started with the assumption that the bond dissociation energy of the carbon–carbon single bond of ethane is 347 kJ/mol. H3C–CH3 → H3C + CH3
924
They then cited an activation energy of 259 kJ/mol for the homolysis of 1-butene to form a methyl radical and an allyl radical. CH2=CH–CH2–CH3 → CH2=CH–CH2 + CH3
∆H = 347 kJ/mol
∆E ‡ = 171 kJ/mol When 1,5-hexadiene is heated above 200 °C, however, it does not dissociate into radicals but undergoes a Cope rearrangement. 210-250 °C
1,5-hexadiene
1,5-hexadiene
Since the product of this particular Cope rearrangement cannot be distinguished from the starting material, it is called a degenerate rearrangement. The fact that the reaction occurs, however, is revealed by the observation that when the 1,1-dideutero analog is similarly treated, the deuterium atoms are scrambled between the 1 and 3 positions. CD2
210-250 °C
CH2
1,1-dideutero-1,5-hexadiene
CD2 CH2
3,3-dideutero-1,5-hexadiene
The rate of scrambling equals the rate of the reaction, and the corresponding energy of activation, ∆E ‡ , was determined to be about 146 kJ/mol (4 ). The value of 146 kJ/mol for the Cope rearrangement indicates that the Cope transition state is stabilized by about 171 – 146 = 25 kJ/mol. Since this value is below that estimated for dissociation of 1,5-hexadiene into a pair of allyl radicals, the transition state for the Cope rearrangement appears to be stabilized somewhat by interaction of the allyl fragments. Doering and Roth also realized that a carbon–carbon single bond could be broken more easily if it was part of a cyclopropane ring, since the enthalpy of activation for the conversion of cis-1,2-dideuterocyclopropane to trans-1,2-dideuterocyclopropane, assumed to occur by carbon–carbon bond breaking, rotation, and re-closure, was about 267 kJ/mol.
Journal of Chemical Education • Vol. 78 No. 7 July 2001 • JChemEd.chem.wisc.edu
In the Classroom
D
D H
H
D 440 °C
H
Doering and Roth would have liked to compare the properties of the cis isomer of 1,2-divinylcyclopropane to those of the trans isomer, but the cis isomer of this compound was not a known compound.
D H
H
H
H
H
cis-1,2-dideuterocyclopropane
D
H
D
H
H H
H D
H
D
cis-1,2-divinylcyclopropane
H
H
∆E ‡ = 267 kJ/mol
trans-1,2-dideuterocyclopropane
This value of 267 kJ/mol is 80 kJ/mol below the 347 kJ/mol bond dissociation energy of the carbon–carbon bond of ethane, and it represents 2/3 of the strain energy, 113 kJ/mol, of cyclopropane. Doering and Roth then considered the consequence of combining the effects of allylic stabilization and the strain of a cyclopropane ring in molecules such as the cis and trans isomers of 1,2-divinylcyclopropane.
Vogel, Ott, and Gajek (6 ) had attempted to prepare cis-1,2-divinylcyclopropane by the Hofmann elimination of trimethylamine from the cis isomer of 1,2-bis(β-dimethylaminoethyl)cyclopropane, compound 1, but the only hydrocarbon product that could be isolated was cyclohepta-1,4-diene. + N
+ N
NaOH 80 °C
1
cyclohepta-1,4-diene
The corresponding amine oxide gave the same result, and both of these were interpreted as an initial elimination to give cis-1,2-divinylcyclopropane, followed by Cope rearrangement to cyclohepta-1,4-diene.
divinylcyclopropane
The trans isomer of divinylcyclopropane was a known substance and was reported to rearrange to cyclohepta-1,4-diene upon heating to 190 °C (5).
cis-1,2-divinylcyclopropane
cyclohepta-1,4-diene
Doering and Roth attempted to prepare cis-1,2-divinylcyclopropane in another way, by the cyclopropanation of cis-1,3,5-hexatriene.
190 °C
trans-1,2-divinylcyclopropane
Cope
cyclohepta-1,4-diene
The reaction probably takes place by homolysis of the 1,2bond of cyclopropane to give a pair of resonance-stabilized allylic radicals, which undergo a conformational change and then ring closure to form the product.
cis-1,3,5-hexatriene
Although several cyclopropane derivatives were formed, they could detect no cis-1,2-divinylcyclopropane in the product mixture. CH2 -45 °C
initial conformation
subsequent conformation
You can estimate that the activation energy for this process should be equal to that for the homolysis of the carbon–carbon bond of ethane minus twice the energy of stabilization of an allyl radical minus the partial relief of the strain of a cyclopropane ring, or 347 – 88 – 88 – 80 = 91 kJ/mol, a little low for a process that takes place at a reasonable rate at 190 °C. Doering and Roth suggested that perhaps rotation into the conformation required for ring closure requires additional energy.
+
+
+
Instead of cis-1,2-divinylcyclopropane, there appeared cyclohepta-1,4-diene plus the products of its cyclopropanation. Thus it again seemed that cis-1,2-divinylcyclopropane, once formed, quickly isomerizes via a Cope rearrangement to cyclohepta-1,4-diene. However, Doering and Roth were able to isolate trans-1,2-divinylcyclopropane from the mixture of hydrocarbons that is formed in the cyclopropanation of trans1,3,5-hexatriene. At this point they realized that if the rearrangement of cis-1,2-divinylcyclopropane is indeed fast at or near room temperature, inclusion of this feature in a ring system might increase the speed of the process, possibly making the isomerization fast enough to be studied by NMR methods. A molecule that meets these requirement is bicyclo[5.1.0]octa-2,5diene, or 3,4-homotropilidene.
JChemEd.chem.wisc.edu • Vol. 78 No. 7 July 2001 • Journal of Chemical Education
925
In the Classroom
bicyclo[5.1.0]octa-2,5-diene 3,4-homotropilidene
The Cope rearrangement of this molecule would merely regenerate the starting material and would lead to no other constitutional isomers, just as was true of the Cope rearrangement of 1,5-hexadiene.
molecule just as a fast camera can capture the instantaneous positions of the spokes of a wheel. In contrast, NMR spectrometry can capture only a somewhat averaged spectrum of a molecule, just a slow camera can show only an averaged blur for the positions of the spokes of a wheel. Doering and Roth then went on to point out that 3,4homotropilidine probably exists as rapidly interconverting chair-like and boat-like conformational isomers.
Cope
3,4-homotropilidene
chair-like lower energy
3,4-homotropilidene
Cyclopropanation of tropilidene produced the desired substance, 3,4-homotropilidene, as well as the isomeric product of monocyclopropanation, 2. CH2
2
3,4-homotropilidene
H2
H6 H6 H4
They also proposed that if this is true, the Cope rearrangement must take place by way of the higher energy boat-like isomer, as indicated here. Cope
The spectral properties of 3,4-homotropilidene are indeed consistent with the notion that it can easily undergo a degenerate Cope rearrangement. At room temperature, the proton NMR spectrum of 3,4-homotropilidine shows a featureless peak in the vinyl region and an extremely broad hump between δ = 4 and δ = 0. At ᎑ 50 °C and +180 °C, however, the spectra are quite different, and each shows relatively sharp multiplets. When the hot sample is again cooled, the original lowtemperature NMR spectra can again be observed. In contrast, the infrared spectrum is the same at all temperatures. These observations can be explained in this way. At ᎑ 50 °C, apparently, the Cope process is slow and the NMR spectrum is that of individual forms of the molecule, all of which are the same. On the other hand, at +180 °C, the Cope process is fast, and the proton NMR spectrum is that of an average form of the molecule, as indicated here, in which protons at positions 3 and 5 have the same, average, chemical shift, the methylene protons at positions 1 and 7 are equivalent, the methylene protons at positions 2 and 6 are equivalent, and there are only 2 vinyl protons (at position 4). H1
conformational isomers of 3,4-homotropilidene
+
tropilidene
H2 H1
H7
H5
H3
Cope
boat-like higher energy
H7
H4
“boat-like”
“boat-like”
conformational change
“chair-like”
conformational change
“chair-like”
Thus the activation energy for the Cope rearrangement would include the difference in energy between the two conformational isomers as well as the free energy of activation of the Cope process. Doering and Roth then proposed that a molecule that was locked into the boat-like conformation by a third ethylene bridge between the cyclopropane ring and the carbon at the other end of the molecule would undergo even more facile degenerate Cope rearrangements. This molecule of molecular formula C10H10, called tricyclo[3.3.2.0]deca-2,7,9-triene or “bullvalene”, is shown here.1
H5
H3
H5
H3
H3
H5 H4
H4
3,4-homotropilidene
Since, however, the only species present at all temperatures is 3,4-homo-tropilidine, the infrared spectrum does not change with a change in temperature. This is a beautiful example of a concept that is often mentioned but rarely illustrated: the difference in “shutter speed” between IR spectrometry and NMR spectrometry. IR spectrometry can capture an “instantaneous” spectrum of a
926
tricyclo[3.3.2.0]deca-2,7,9-triene bullvalene
The astonishing, yet inevitable, consequence of rapid Cope rearrangements would be that all 10 hydrogen atoms and all 10 carbon atoms of the molecule would be equivalent, with the result that the high-temperature proton NMR spectrum of bullvalene would consist of a single line! Since 10
Journal of Chemical Education • Vol. 78 No. 7 July 2001 • JChemEd.chem.wisc.edu
In the Classroom
points cannot be symmetrically placed on the surface of a sphere, the 10 carbon atoms of bullvalene would not be stationary, but would move about on the surface of a sphere in such a way that their relationships to one another would be, on the average, the same. Each carbon atom would be bonded (or not bonded) to every other carbon atom to the same extent. Some of the 10!/3 or 1,209,600 contributors to this unique, unprecedented, and at that time unknown “fluxional” structure are shown here. 9
10
9
8
Summary
10
8 1
4
3
2
1
5
7
6
2
6
7
−(1-7) +(3-5) 9
−(10-4) +(2-8)
10
8
8 1
5
4
3
4
3
2
−(7-8) +(5-10)
5
7
9 10
1
2
3
4
7
5 6
6 −(1-8) +(3-10)
Bullvalene, a solid of melting point 96 °C, decomposes only at temperatures approaching 400 °C. And sure enough, whereas at 15 °C its proton NMR spectrum shows one extremely broad hump, at 120 °C the proton NMR spectrum consists of exactly one sharp singlet at δ = 4.2! Similarly, at 141 °C the 1H-noise-decoupled 13C spectrum of bullvalene shows only one sharp singlet at δ = 86.4, and at ᎑60 °C the spectrum consists of four sharp singlets at δ = 128.5, 128.3, 31.0, and 21.0 with relative intensities of 3:3:1:3 (8).
−(4-5) +(2-7)
In this paper I have reviewed and summarized the logical process through which Doering and Roth conceived of the possible existence of an isomer of molecular formula C10H10 that has a “fluxional” structure; that is, a structure in which every carbon atom is equally bonded, or not bonded, to every other carbon atom. Although in the early part of 1963 this molecule existed only as a concept in the minds of Doering and Roth, it was prepared before the year had ended. The evidence for its “fluxional” structure is that it gives a sharp singlet in its proton NMR spectrum. In order for the 10 protons, each bonded to a carbon atom, to be chemical-shift equivalent, “all ten carbon atoms [must] inevitably wander over the surface of a sphere in ever changing relationship to each other” (3). Acknowledgment
8
8
9 10
7
1
2
I thank the reviewer who drew my attention to ref 8.
1 2
3
6
9
4
3
Note 5
7
5
10
4
6
Within the year, Gerhard Schröder (7), who had determined the structures of two of the dimers of cyclooctatetraene (C16H16), announced the formation of bullvalene (C10H10) and benzene (C6H6) by photolysis of the dimer of cyclooctatetraene that melts at 76 °C.
1. The origin of the name bullvalene is not known for sure. Some say that the name is based on Professor Doering’s nickname, “Bull” Doering; others say that the name was supplied by an irreverent and skeptical graduate student: Bull-valene.
Literature Cited 1. 2. 3. 4.
light
5. 6. 7. 8. mp 76 °C
bullvalene and benzene
Samet, C. J. Chem. Educ. 1993, 70, 291. Ault, A. J. Chem. Educ. 2000, 77, 55. Doering, W. von E.; Roth, W. Tetrahedron 1963, 19, 715. Doering, W. von E.; Gilbert, J. C. Tetrahedron Supplement 1966, 7, 397. Vogel, E. Angew. Chem. 1960, 74, 4. Vogel, E.; Ott, K.-H.; Gajek, K. Liebigs Ann. 1961, 644, 172. Schröder, G. Angew. Chem., Int. Ed. Eng. 1963, 2, 481. Oth, J. F. M.; Müllen, K.; Gilles, J.-M.; Schröder, G. Helv. Chim. Acta 1974, 57, 1415.
JChemEd.chem.wisc.edu • Vol. 78 No. 7 July 2001 • Journal of Chemical Education
927
