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'''Dewar benzene''' or '''bicyclo[2.2.0]hexa-2,5-diene''' is a [[bicyclic]] [[isomer]] of [[benzene]] with the molecular formula C<sub>6</sub>H<sub>6</sub>. The compound is named after [[James Dewar]] who included this structure in a list of possible C<sub>6</sub>H<sub>6</sub> structures in 1867.<ref>{{cite journal|first = James|last = Dewar|authorlink = James Dewar|title = On the Oxidation af Phenyl Alcohol, and a Mechanical Arrangement adapted to illustrate Structure in the Non-saturated Hydrocarbons|journal = [[Proc. R. Soc. Edin.]]|year = 1867|volume = 6|pages = 82–86|doi = 10.1017/S0370164600045387|url=https://books.google.com/books?id=PmlUAAAAIAAJ&pg=PA82}}</ref> However, he did not propose it as the structure of benzene, and in fact he supported the correct structure previously proposed by [[August Kekulé]] in 1865.<ref name=Baker/>
'''Dewar benzene''' or '''bicyclo[2.2.0]hexa-2,5-diene''' is a [[bicyclic]] [[isomer]] of [[benzene]] with the molecular formula C<sub>6</sub>H<sub>6</sub>. The compound is named after [[James Dewar]] who included this structure in a list of possible C<sub>6</sub>H<sub>6</sub> structures in 1867.<ref name = Dewar /> However, he did not propose it as the structure of benzene, and in fact he supported the correct structure previously proposed by [[August Kekulé]] in 1865.<ref name = Baker />


==Structure and properties==
==Structure and properties==
Unlike benzene, Dewar benzene is not flat because the carbons where the rings join are bonded to four atoms rather than three. These carbons tend toward [[tetrahedral geometry]], and the two cyclobutene rings make an angle where they are ''[[Cis–trans isomerism|cis]]''-[[Ring fusion|fused]] to each other. The compound has nevertheless considerable [[strain energy]] and reverts to benzene with a [[chemical half-life]] of two days. This thermal conversion is relatively slow because it is [[symmetry forbidden]] based on orbital symmetry arguments.<ref>{{cite journal
Unlike benzene, Dewar benzene is not flat because the carbons where the rings join are bonded to four atoms rather than three. These carbons tend toward [[tetrahedral geometry]], and the two cyclobutene rings make an angle where they are ''[[Cis–trans isomerism|cis]]''-[[Ring fusion|fused]] to each other. The compound has nevertheless considerable [[strain energy]] and reverts to benzene with a [[chemical half-life]] of two days. This thermal conversion is relatively slow because it is [[symmetry forbidden]] based on orbital symmetry arguments.<ref>{{cite journal|first = James O.|last = Jensen|title = Vibrational Frequencies and Structural Determination of Dewar Benzene|journal = [[Journal of Molecular Structure: THEOCHEM|J. Mol. Struct.:THEOCHEM]]|year = 2004|volume = 680|issue = 1-3|pages = 227–236|doi = 10.1016/j.theochem.2004.03.042}}</ref>
| author=James O. Jensen
| title = Vibrational Frequencies and Structural Determination of Dewar Benzene
| journal = [[Journal of Molecular Structure: THEOCHEM|J. Mol. Struct.:THEOCHEM]]
| year = 2004
| volume = 680
| pages = 227–236
| doi = 10.1016/j.theochem.2004.03.042 }}</ref>


==Synthesis==
==Synthesis==
The compound itself was first synthesized in 1962 as a [[tert-butyl|''tert''-butyl]] [[derivative (chemistry)|derivative]]<ref>
The compound itself was first synthesized in 1962 as a [[tert-butyl|''tert''-butyl]] [[derivative (chemistry)|derivative]]<ref>
{{cite journal|first1 = Eugene E.|last1 = van Tamelen|authorlink1 = Eugene van Tamelen|first2 = S. P.|last2 = Pappas|title = Chemistry of Dewar Benzene. 1,2,5-Tri-''t''-Butylbicyclo[2.2.0]Hexa-2,5-Diene|journal = [[J. Am. Chem. Soc.]]|year = 1962|volume = 84|issue = 19|pages = 3789–3791|doi = 10.1021/ja00878a054}}</ref> and then as the unsubstituted compound by [[Eugene van Tamelen]] in 1963 by [[photolysis]] of the ''cis''-1,2-dihydro derivative of [[phthalic anhydride]] followed by [[oxidation]] with [[lead tetraacetate]].<ref name = vanTamelenPappasSynthesis>{{cite journal|first1 = Eugene E.|last1 = van Tamelen|first2 = S. P.|last2 = Pappas|authorlink1 = Eugene van Tamelen|title = Bicyclo [2.2.0]hexa-2,5-diene|journal = [[J. Am. Chem. Soc.]]|year = 1963|volume = 85|issue = 20|pages = 3297–3298|doi = 10.1021/ja00903a056}}</ref><ref>{{cite journal|first1 = Eugene E.|last1 = van Tamelen|authorlink1 = Eugene van Tamelen|first2 = S. P.|last2 = Pappas|first3 = K. L.|last3 = Kirk|title = Valence Bond Isomers of Aromatic Systems. Bicyclo[2.2.0]hexa-2,5-dienes (Dewar benzenes)|journal = [[J. Am. Chem. Soc.]]|year = 1971|volume = 93|issue = 23|pages = 6092–6101|doi = 10.1021/ja00752a021}}</ref>
{{cite journal
| author = [[E. E. van Tamelen]], S. P. Pappas
| title = Chemistry of Dewar Benzene. 1,2,5-Tri-''t''-Butylbicyclo[2.2.0]Hexa-2,5-Diene
| journal = [[J. Am. Chem. Soc.]]
| year = 1962
| volume = 84
| issue = 19
| pages = 3789–3791
| doi = 10.1021/ja00878a054}}
</ref> and then as the unsubstituted compound by [[E.E. van Tamelen]] in 1963 by [[photolysis]] of ''cis''-1,2-dihydro derivative of [[phthalic anhydride]] followed by [[oxidation]] with [[lead tetraacetate]].<ref>
{{cite journal
| author = [[E. E. van Tamelen]], S. P. Pappas
| title = Bicyclo [2.2.0]hexa-2,5-diene
| journal = [[J. Am. Chem. Soc.]]
| year = 1963
| volume = 85
| issue = 20
| pages = 3297–3298
| doi = 10.1021/ja00903a056}}
</ref><ref>{{cite journal
| author= [[E. E. van Tamelen]], S. P. Pappas, K. L. Kirk
| title = Valence Bond Isomers of Aromatic Systems. Bicyclo[2.2.0]hexa-2,5-dienes (Dewar benzenes)
| journal = [[J. Am. Chem. Soc.]]
| year = 1971
| volume = 93
| issue = 23
| pages = 6092–6101
| doi = 10.1021/ja00752a021 }}</ref>


:[[File:Dewar benzene synthesis.svg|400px|Dewar benzene synthesis E. E. Van Tamelen, S. P. Pappas
:[[File:Dewar benzene synthesis.svg|400px|Dewar benzene synthesis reported by [[E. E. van Tamelen|van Tamelen]] and Pappas<ref name = vanTamelenPappasSynthesis />]]
]]


=="Dewar benzene" and benzene==
=="Dewar benzene" and benzene==
It is sometimes incorrectly claimed that Dewar proposed his structure as the true structure of benzene. In fact, Dewar merely wrote the structure as one of seven possible isomers and believed that his experiments on benzene supported the (correct) structure that had been proposed by [[Friedrich August Kekulé von Stradonitz|Kekulé]].<ref name=Baker>
It is sometimes incorrectly claimed that Dewar proposed his structure as the true structure of benzene. In fact, Dewar merely wrote the structure as one of seven possible isomers{{cite journal|first = James|last = Dewar|authorlink = James Dewar|title = On the Oxidation af Phenyl Alcohol, and a Mechanical Arrangement adapted to illustrate Structure in the Non-saturated Hydrocarbons|journal = [[Proc. R. Soc. Edin.]]|year = 1867|volume = 6|pages = 82–86|doi = 10.1017/S0370164600045387|url = https://books.google.com/books?id=PmlUAAAAIAAJ&pg=PA82}}</ref> and believed that his experiments on benzene supported the (correct) structure that had been proposed by [[Friedrich August Kekulé von Stradonitz|Kekulé]].<ref name = Baker>{{cite journal|first1 = Wilson|last1 = Baker|authorlink1 = Wilson Baker|first2 = Dennis H.|last2 = Rouvray|title = Para-Bond or "Dewar" Benzene?|journal = [[J. Chem. Educ.]]|year = 1978|volume = 55|issue = 10|page = 645|doi = 10.1021/ed055p645}}</ref>
{{cite journal
|author1=W. Baker |author2=D. H. Rouvray | title = Para-Bond or "Dewar" Benzene?
| journal = [[J. Chem. Educ.]]
| year = 1978
| volume = 55
| issue = 10
| pages = 645
| doi = 10.1021/ed055p645}}
</ref>


After the development in 1928 of the [[valence bond]] theory, the three possible Dewar structures were considered as minor [[Resonance (chemistry)|resonance]] contributors in the overall description of benzene. The major resonance contributors are of course the two possible Kekulé structures.
After the development of [[valence bond theory]] in 1928, benzene was described primarily using its two major [[Resonance (chemistry)|resonance]] contributors, the two Kekulé structures. The three possible Dewar structures were considered as minor resonance contributors in the overall description of benzene, alongside other classic structures such as the isomers [[prismane]], [[benzvalene]] and [[Claus' benzene]]. Prismane and benzvalene were synthesized in the 1970s; Claus' benzene is impossible to synthesize.<ref>{{cite journal|doi = 10.1002/anie.200705775|pmid = 18418829|year = 2008|last1 = Hoffmann|first1 = Roald|authorlink1 = Roald Hoffmann|last2 = Hopf|first2 = Henning|title = Learning from Molecules in Distress|volume = 47|issue = 24|pages = 4474–4481|journal = [[Angew. Chem. Inter. Ed.]]}}</ref>

Other classic structures that have been considered as possible benzene isomers are [[prismane]], [[benzvalene]] and [[Claus' benzene]]. Prismane and benzvalene were synthesized in the 1970s; Claus' benzene is impossible to synthesize.<ref>{{Cite journal| doi = 10.1002/anie.200705775| pmid = 18418829| year = 2008| last1 = Hoffmann | first1 = R.| last2 = Hopf | first2 = H.| title = Learning from molecules in distress| volume = 47| issue = 24| pages = 4474–4481| journal = Angewandte Chemie International Edition in English }}</ref>


==Hexamethyl Dewar benzene==
==Hexamethyl Dewar benzene==
'''Hexamethyl Dewar benzene''' has been prepared by bicyclotrimerization of [[dimethylacetylene]] with [[aluminium chloride]].<ref>{{OrgSynth | title = Hexamethyl Dewar Benzene | author = Sami A. Shama and Carl C. Wamser | collvol = 7 | collvolpages = 256 | year = 1990 | prep = cv7p0256}}</ref> It undergoes an unusual rearrangement reaction with [[hydrohalic acid]]s to form a [[pentamethylcyclopentadiene]] derivative,<ref>{{cite journal |last1= Paquette|first1= L. A.|last2= Krow|first2= G. R.|year= 1968|title= Electrophilic Additions to Hexamethyldewarbenzene|journal= [[Tetrahedron Letters|Tetrahedron Lett.]]|volume= 9|issue= 17|pages= 2139–2142|url= |doi= 10.1016/S0040-4039(00)89761-0}}</ref><ref>{{cite journal |last1= Criegee|first1= R.|last2= Gruner|first2= H.|year= 1968|title= Acid-catalyzed Rearrangements of Hexamethyl-prismane and Hexamethyl-Dewar-benzene|journal= [[Angew. Chem. Int. Ed.]]|volume= 7|issue= 6|pages= 467–468|url= |doi= 10.1002/anie.196804672}}</ref> and consequently can be used as a starting material for synthesising some [[Pentamethylcyclopentadiene#Synthesis of Cp.2A complexes|pentamethylcyclopentadienyl]] [[organometallic compound]]s.<ref>{{cite journal |last1= Kang|first1= J. W.|last2= Mosley|first2= K.|last3= Maitlis|first3= P. M.|authorlink3=Peter Maitlis|year= 1968|title= Mechanisms of Reactions of Dewar Hexamethylbenzene with Rhodium and Iridium Chlorides|journal= [[Chemical Communications|Chem. Commun.]]|issue= 21|pages= 1304–1305|url= |doi= 10.1039/C19680001304}}</ref><ref>{{cite journal |last1= Kang|first1= J. W.|last2= Maitlis|first2= P. M.|authorlink2=Peter Maitlis|year= 1968|title= Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium(III) Chloride|journal= [[J. Am. Chem. Soc.]]|volume= 90|issue= 12|pages= 3259–3261|url= |doi= 10.1021/ja01014a063}}</ref>
'''Hexamethyl Dewar benzene''' has been prepared by bicyclotrimerization of [[dimethylacetylene]] with [[aluminium chloride]].<ref>{{OrgSynth|title = Hexamethyl Dewar Benzene|first1 = Sami A.|last1 = Shama|first2 = Carl C.|last2 = Wamser|collvol = 7|collvolpages = 256|year = 1990|prep = cv7p0256|doi = 10.15227/orgsyn.061.0062|volume = 61|page = 62}}</ref> It undergoes an unusual rearrangement reaction with [[hydrohalic acid]]s to form a [[pentamethylcyclopentadiene]] derivative,<ref>{{cite journal|last1 = Paquette|first1 = Leo A.|authorlink1 = Leo Paquette|last2 = Krow|first2 = Grant R.|year = 1968|title = Electrophilic Additions to Hexamethyldewarbenzene|journal = [[Tetrahedron Lett.]]|volume = 9|issue = 17|pages = 2139–2142|doi = 10.1016/S0040-4039(00)89761-0}}</ref><ref>{{cite journal|last1 = Criegee|first1 = Rudolf|authorlink1 = Rudolf Criegee|last2 = Grüner|first2 = H.|year = 1968|title = Acid-catalyzed Rearrangements of Hexamethyl-prismane and Hexamethyl-Dewar-benzene|journal = [[Angew. Chem. Int. Ed.]]|volume = 7|issue = 6|pages = 467–468|doi = 10.1002/anie.196804672}}</ref> and consequently can be used as a starting material for synthesising some [[Pentamethylcyclopentadiene#Synthesis of Cp.2A complexes|pentamethylcyclopentadienyl]] [[organometallic compound]]s.<ref>{{cite journal|last1 = Kang|first1 = J. W.|last2 = Mosley|first2 = K.|last3 = Maitlis|first3 = Peter M.|authorlink3 = Peter Maitlis|year = 1968|title = Mechanisms of Reactions of Dewar Hexamethylbenzene with Rhodium and Iridium Chlorides|journal = [[Chem. Commun.]]|issue = 21|pages = 1304–1305|doi = 10.1039/C19680001304}}</ref><ref>{{cite journal|last1 = Kang|first1 = J. W.|last2 = Maitlis|first2 = Peter M.|authorlink2 = Peter Maitlis|year = 1968|title = Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium(III) Chloride|journal = [[J. Am. Chem. Soc.]]|volume = 90|issue = 12|pages = 3259–3261|doi = 10.1021/ja01014a063}}</ref>


[[Image:Hexamethyl Dewar benzene reacting with rhodium chloride under acidic conditions.PNG|center|Synthesis of the rhodium(III) dimer [Cp*RhCl<sub>2</sub>]<sub>2</sub> from hexamethyl Dewar benzene]]
[[Image:Hexamethyl Dewar benzene reacting with rhodium chloride under acidic conditions.PNG|center|Synthesis of the rhodium(III) dimer [Cp*RhCl<sub>2</sub>]<sub>2</sub> from hexamethyl Dewar benzene]]


One of the [[alkene]]s can be [[epoxidation|epoxidized]] using [[mCPBA|''m''CPBA]],<ref name=OSEpoxidation>{{OrgSynth |first1= R. B. |last1= King |first2= W. M. |last2= Douglas |first3= A. |last3= Efraty |year=1977 |title=5-Acetyl-1,2,3,4,5-pentamethylcyclopentadiene |volume=56 |pages=1 |prep=CV6P0039}}</ref> [[peroxybenzoic acid]],<ref name=ChemBerEpoxidation>{{cite journal |last1= Junker |first1= Hans-Nikolaus |last2= Schäfer |first2= Wolfgang |last3= Niedenbrück |first3= Hans |title= Oxydationsreaktionen mit Hexamethyl-bicyclo[2.2.0]-hexadien-(2.5) (= Hexamethyl-Dewar-Benzol) |trans-title= Oxidation reactions with hexamethylbicyclo[2.2.0]-hexa-2,5-diene |journal= [[Chem. Ber.]]|language= DE |volume= 100 |issue= 8 |doi= 10.1002/cber.19671000807 |pages= 2508–2514 |year= 1967 }}</ref> or [[dimethyldioxirane]] (DMDO).<ref name=DMDOEpoxidation>{{cite journal |journal= [[Tetrahedron Lett.]] |volume= 41 |issue= 4 |year= 2000 |pages= 539–542 |title= Regioselective and diastereoselective dimethyldioxirane epoxidation of substituted norbornenes and hexamethyl Dewar benzene |first1= Amalia |last1= Asouti |first2= Lazaros P. |last2= Hadjiarapoglou |doi= 10.1016/S0040-4039(99)02113-9 }}</ref> Using a peracid (''m''CPBA or peroxybenzoic acid), the epoxy product quickly rearranges, catalyzed by the acid byproduct of the epoxidation.<ref name=OSEpoxidation/>
One of the [[alkene]]s can be [[epoxidation|epoxidized]] using [[mCPBA|''m''CPBA]],<ref name = OSEpoxidation>{{OrgSynth|first1 = R. B. |last1 = King|first2 = W. M.|last2 = Douglas|first3 = A.|last3 = Efraty|year = 1977|title = 5-Acetyl-1,2,3,4,5-pentamethylcyclopentadiene|volume = 56|page = 1|prep = CV6P0039|doi = 10.15227/orgsyn.056.0001|collvol = 6|collvolpages = 39}}</ref> [[peroxybenzoic acid]],<ref name = ChemBerEpoxidation>{{cite journal|last1 = Junker|first1 = Hans-Nikolaus|last2 = Schäfer|first2 = Wolfgang|last3 = Niedenbrück|first3 = Hans|title = Oxydationsreaktionen mit Hexamethyl-bicyclo[2.2.0]-hexadien-(2.5) (= Hexamethyl-Dewar-Benzol)|trans-title = Oxidation reactions with hexamethylbicyclo[2.2.0]-hexa-2,5-diene (= Hexamethyl Dewar Benzene)|journal = [[Chem. Ber.]]|language = DE|volume = 100|issue = 8|doi = 10.1002/cber.19671000807|pages = 2508–2514|year = 1967}}</ref> or [[dimethyldioxirane]] (DMDO).<ref name = DMDOEpoxidation>{{cite journal|journal = [[Tetrahedron Lett.]]|volume = 41|issue = 4|year = 2000|pages = 539–542|title = Regioselective and diastereoselective dimethyldioxirane epoxidation of substituted norbornenes and hexamethyl Dewar benzene|first1 = Amalia|last1 = Asouti|first2 = Lazaros P.|last2 = Hadjiarapoglou|doi= 10.1016/S0040-4039(99)02113-9}}</ref> Using a peracid (''m''CPBA or peroxybenzoic acid), the epoxy product quickly rearranges, catalyzed by the acid byproduct of the epoxidation.<ref name=OSEpoxidation/>


[[File:Hexamethyl Dewar epoxidation-rearrangement.png|500px|center]]
[[File:Hexamethyl Dewar epoxidation-rearrangement.png|500px|center]]

Revision as of 08:35, 22 April 2017

Dewar benzene
Skeletal formula
The conjoined cyclobutene rings of Dewar benzene form an obtuse angle.
Ball-and-stick model
Names
IUPAC name
Bicyclo[2.2.0]hexa-2,5-diene
Identifiers
3D model (JSmol)
ChemSpider
  • InChI=1S/C6H6/c1-2-6-4-3-5(1)6/h1-6H checkY
    Key: CTLSARLLLBZBRV-UHFFFAOYSA-N checkY
  • InChI=1/C6H6/c1-2-6-4-3-5(1)6/h1-6H
    Key: CTLSARLLLBZBRV-UHFFFAOYAO
  • C\1=C\C2/C=C\C/12
Properties
C6H6
Molar mass 78.1 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Dewar benzene or bicyclo[2.2.0]hexa-2,5-diene is a bicyclic isomer of benzene with the molecular formula C6H6. The compound is named after James Dewar who included this structure in a list of possible C6H6 structures in 1867.[1] However, he did not propose it as the structure of benzene, and in fact he supported the correct structure previously proposed by August Kekulé in 1865.[2]

Structure and properties

Unlike benzene, Dewar benzene is not flat because the carbons where the rings join are bonded to four atoms rather than three. These carbons tend toward tetrahedral geometry, and the two cyclobutene rings make an angle where they are cis-fused to each other. The compound has nevertheless considerable strain energy and reverts to benzene with a chemical half-life of two days. This thermal conversion is relatively slow because it is symmetry forbidden based on orbital symmetry arguments.[3]

Synthesis

The compound itself was first synthesized in 1962 as a tert-butyl derivative[4] and then as the unsubstituted compound by Eugene van Tamelen in 1963 by photolysis of the cis-1,2-dihydro derivative of phthalic anhydride followed by oxidation with lead tetraacetate.[5][6]

Dewar benzene synthesis reported by van Tamelen and Pappas[5]

"Dewar benzene" and benzene

It is sometimes incorrectly claimed that Dewar proposed his structure as the true structure of benzene. In fact, Dewar merely wrote the structure as one of seven possible isomersDewar, James (1867). "On the Oxidation af Phenyl Alcohol, and a Mechanical Arrangement adapted to illustrate Structure in the Non-saturated Hydrocarbons". Proc. R. Soc. Edin. 6: 82–86. doi:10.1017/S0370164600045387.</ref> and believed that his experiments on benzene supported the (correct) structure that had been proposed by Kekulé.[2]

After the development of valence bond theory in 1928, benzene was described primarily using its two major resonance contributors, the two Kekulé structures. The three possible Dewar structures were considered as minor resonance contributors in the overall description of benzene, alongside other classic structures such as the isomers prismane, benzvalene and Claus' benzene. Prismane and benzvalene were synthesized in the 1970s; Claus' benzene is impossible to synthesize.[7]

Hexamethyl Dewar benzene

Hexamethyl Dewar benzene has been prepared by bicyclotrimerization of dimethylacetylene with aluminium chloride.[8] It undergoes an unusual rearrangement reaction with hydrohalic acids to form a pentamethylcyclopentadiene derivative,[9][10] and consequently can be used as a starting material for synthesising some pentamethylcyclopentadienyl organometallic compounds.[11][12]

Synthesis of the rhodium(III) dimer [Cp*RhCl2]2 from hexamethyl Dewar benzene
Synthesis of the rhodium(III) dimer [Cp*RhCl2]2 from hexamethyl Dewar benzene

One of the alkenes can be epoxidized using mCPBA,[13] peroxybenzoic acid,[14] or dimethyldioxirane (DMDO).[15] Using a peracid (mCPBA or peroxybenzoic acid), the epoxy product quickly rearranges, catalyzed by the acid byproduct of the epoxidation.[13]

Using DMDO gives the epoxide as a stable product—the byproduct of the epoxidation is neutral acetone. By varying the amount of DMDO, either the mono- or diepoxide can be formed, with the oxygen atoms exo on the bicyclic carbon framework.[15]

Left: Structure of C
6(CH
3)2+
6, as drawn by Steven Bachrach[16]
Right: Three-dimensional representation of the dication's rearranged pentagonal-pyramid framework, from the crystal structure[17]

In 1973, the dication of hexamethylbenzene, C
6(CH
3)2+
6, was produced by Hepke Hogeveen and Peter Kwant.[18][19][20] This can be done by dissolving the hexamethyl Dewar bezene monoepoxide in magic acid, which removes the oxygen as an anion.[17] NMR had previously hinted at a pentagonal pyramidal structure in a related cation,[21] with the apex carbon bonding to six other carbon atoms, and X-ray crystallographic analysis of the salt with SbF−
6 published in 2016 showed that this was indeed the case.[17] Computational organic chemist Steven Bachrach discussed the dication, noting that the weak bonds forming the upright edges of the pyramid, shown as dashed lines in the structure he drew, have a Wiberg bond order of about 0.54; it follows that the total bond order for the apical carbon is 5 × 0.54 + 1 = 3.7 < 4, and thus the species is not hypervalent, but it is hypercoordinate.[16] From the perspective of organometallic chemistry, the species can be viewed as having a carbon(IV) centre (C4+
) bound to an aromatic η5–pentamethylcyclopentadienyl anion (six-electron donor) and a methyl anion (two-electron donor), thereby satisfying the octet rule[22] and being analogous to the gas-phase organozinc monomer [(η5
–C
5(CH
3)
5)Zn(CH
3)], which has the same ligands bound to a zinc(II) centre (Zn2+
) and satisfies the 18 electron rule on the metal.[23][24] Thus, while unprecedented,[17] and having attracted comment in Chemical & Engineering News,[25] New Scientist,[26] Science News,[27] and ZME Science,[28] the structure is consistent with the usual bonding rules of chemistry. Moritz Malischewski, who carried out the work with Konrad Seppelt,[17] commented that one the motivations for undertaking the work was to illustrate "the possibility to astonish chemists about what can be possible."[26]

References

  1. ^ Cite error: The named reference Dewar was invoked but never defined (see the help page).
  2. ^ a b Baker, Wilson; Rouvray, Dennis H. (1978). "Para-Bond or "Dewar" Benzene?". J. Chem. Educ. 55 (10): 645. doi:10.1021/ed055p645.
  3. ^ Jensen, James O. (2004). "Vibrational Frequencies and Structural Determination of Dewar Benzene". J. Mol. Struct.:THEOCHEM. 680 (1–3): 227–236. doi:10.1016/j.theochem.2004.03.042.
  4. ^ van Tamelen, Eugene E.; Pappas, S. P. (1962). "Chemistry of Dewar Benzene. 1,2,5-Tri-t-Butylbicyclo[2.2.0]Hexa-2,5-Diene". J. Am. Chem. Soc. 84 (19): 3789–3791. doi:10.1021/ja00878a054.
  5. ^ a b van Tamelen, Eugene E.; Pappas, S. P. (1963). "Bicyclo [2.2.0]hexa-2,5-diene". J. Am. Chem. Soc. 85 (20): 3297–3298. doi:10.1021/ja00903a056.
  6. ^ van Tamelen, Eugene E.; Pappas, S. P.; Kirk, K. L. (1971). "Valence Bond Isomers of Aromatic Systems. Bicyclo[2.2.0]hexa-2,5-dienes (Dewar benzenes)". J. Am. Chem. Soc. 93 (23): 6092–6101. doi:10.1021/ja00752a021.
  7. ^ Hoffmann, Roald; Hopf, Henning (2008). "Learning from Molecules in Distress". Angew. Chem. Inter. Ed. 47 (24): 4474–4481. doi:10.1002/anie.200705775. PMID 18418829.
  8. ^ Shama, Sami A.; Wamser, Carl C. (1990). "Hexamethyl Dewar Benzene". Organic Syntheses. 61: 62. doi:10.15227/orgsyn.061.0062; Collected Volumes, vol. 7, p. 256.
  9. ^ Paquette, Leo A.; Krow, Grant R. (1968). "Electrophilic Additions to Hexamethyldewarbenzene". Tetrahedron Lett. 9 (17): 2139–2142. doi:10.1016/S0040-4039(00)89761-0.
  10. ^ Criegee, Rudolf; Grüner, H. (1968). "Acid-catalyzed Rearrangements of Hexamethyl-prismane and Hexamethyl-Dewar-benzene". Angew. Chem. Int. Ed. 7 (6): 467–468. doi:10.1002/anie.196804672.
  11. ^ Kang, J. W.; Mosley, K.; Maitlis, Peter M. (1968). "Mechanisms of Reactions of Dewar Hexamethylbenzene with Rhodium and Iridium Chlorides". Chem. Commun. (21): 1304–1305. doi:10.1039/C19680001304.
  12. ^ Kang, J. W.; Maitlis, Peter M. (1968). "Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium(III) Chloride". J. Am. Chem. Soc. 90 (12): 3259–3261. doi:10.1021/ja01014a063.
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