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==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= [[Angewandte Chemie|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 | 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>


[[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 |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/>


[[File:Hexamethyl Dewar epoxidation-rearrangement.png|500px|center]]
[[File:Hexamethyl Dewar epoxidation-rearrangement.png|500px|center]]
Line 131: Line 131:
|footer = '''Left''': Structure of {{chem|C|6|(CH|3|)|6|2+}}, as drawn by [[Steven Bachrach]]<ref name = Bachrach /><br />'''Right''': Three-dimensional representation of the [[dication]]'s rearranged pentagonal-pyramid framework, from the crystal structure<ref name = HMB />}}
|footer = '''Left''': Structure of {{chem|C|6|(CH|3|)|6|2+}}, as drawn by [[Steven Bachrach]]<ref name = Bachrach /><br />'''Right''': Three-dimensional representation of the [[dication]]'s rearranged pentagonal-pyramid framework, from the crystal structure<ref name = HMB />}}


In 1973, the [[Hexamethylbenzene#Dication|dication of hexamethylbenzene]], {{chem|C|6|(CH|3|)|6|2+}}, was produced by Hepke Hogeveen and Peter Kwant.<ref name = HMBcatObs1>{{cite journal|last1 = Hogeveen|first1 = Hepke|last2 = Kwant|first2 = Peter W.|year = 1973|title = Direct observation of a remarkably stable dication of unusual structure: (CCH<sub>3</sub>)<sub>6</sub><sup>2⊕</sup>|journal = [[Tetrahedron Lett.]]|volume = 14|issue = 19|pages = 1665–1670|doi = 10.1016/S0040-4039(01)96023-X}}</ref><ref name = HMBcatObs2>{{cite journal|last1 = Hogeveen|first1 = Hepke|last2 = Kwant|first2 = Peter W.|last3 = Postma|first3 = J.|last4 = van Duynen|first4 = P. Th.|year = 1974|title = Electronic spectra of pyramidal dications, (CCH<sub>3</sub>)<sub>6</sub><sup>2+</sup> and (CCH)<sub>6</sub><sup>2+</sup>|journal = [[Tetrahedron Lett.]]|volume = 15|issue = 49–50|pages = 4351–4354|doi = 10.1016/S0040-4039(01)92161-6}}</ref><ref name = HMBcatObs3>{{cite journal|last1 = Hogeveen|first1 = Hepke|last2 = Kwant|first2 = Peter W.|year = 1974|title = Chemistry and spectroscopy in strongly acidic solutions. XL. (CCH<sub>3</sub>)<sub>6</sub><sup>2+</sup>, an unusual dication|journal = [[J. Am. Chem. Soc.]]|volume = 96|issue = 7|pages = 2208–2214|doi = 10.1021/ja00814a034}}</ref> This can be done by dissolving the hexamethyl Dewar bezene monoepoxide in [[magic acid]], which removes the oxygen as an anion.<ref name = HMB /> NMR had previously hinted at a pentagonal pyramidal structure in a related cation,<ref>{{cite journal|title = Protonation of hexamethyl Dewar benzene and hexamethylprismane in fluorosulfuric acid – antimony pentafluoride – sulfur dioxide|first1 = Leo A.|last1 = Paquette|first2 = Grant R.|last2 = Krow|first3 = J. Martin|last3 = Bollinger|first4 = George A.|last4 = Olah|journal = [[J. Am. Chem. Soc.]]|year = 1968|volume = 90|issue = 25|pages = 7147–7149|doi = 10.1021/ja01027a060}}</ref> with the apex carbon bonding to six other carbon atoms, and X-ray crystallographic analysis of the salt with {{chem|Sb|F|6|-}} published in 2016 showed that this was indeed the case.<ref name = HMB>{{cite journal|title=Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C<sub>6</sub>(CH<sub>3</sub>)<sub>6</sub><sup>2+</sup>|url=http://onlinelibrary.wiley.com/doi/10.1002/anie.201608795/pdf|year=2016|doi=10.1002/anie.201608795|first1=Moritz|last1=Malischewski|first2=Konrad|last2=Seppelt|authorlink2=Konrad Seppelt|journal=[[Angewandte Chemie International Edition]]}}</ref> 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&nbsp;&times;&nbsp;0.54&nbsp;+&nbsp;1&nbsp;=&nbsp;3.7&nbsp;<&nbsp;4, and thus the species is not [[Hypervalent molecule|hypervalent]], but it is hypercoordinate.<ref name = Bachrach>{{cite web|title = A six-coordinate carbon atom|last = Bachrach|first = Steven M.|authorlink = Steven Bachrach|url = http://comporgchem.com/blog/?page_id=3929|accessdate = January 18, 2017|date = January 17, 2017|website = comporgchem.com}}</ref> From the perspective of [[organometallic chemistry]], the species can be viewed as having a carbon(IV) centre ({{chem|C|4+}}) bound to an aromatic {{nowrap|&eta;<sup>5</sup>–[[pentamethylcyclopentadienyl]]}} anion (six-electron donor) and a methyl anion (two-electron donor), thereby satisfying the [[octet rule]]<ref name = HMBviaOMetChem>{{cite journal|title = Pyramidal mono- and dications. Bridge between organic and organometallic chemistry|first1 = Hepke|last1 = Hogeveen|first2 = Peter W.|last2 = Kwant|journal = [[Acc. Chem. Res.]]|year = 1975|volume = 8|issue = 12|pages = 413–420|doi = 10.1021/ar50096a004}}</ref> and being analogous to the gas-phase [[organozinc compound|organozinc]] monomer {{nowrap|[(&eta;{{chem|5|–C|5|(CH|3|)|5|)Zn(CH|3}})],}} which has the same [[ligand]]s bound to a zinc(II) centre ({{chem|Zn|2+}}) and satisfies the [[18 electron rule]] on the metal.<ref name = Cp*ZnMe>{{cite journal|journal = [[J. Organomet. Chem.]]|volume = 153|issue = 2|year = 1978|pages = 187–192|title = The molecular structure of monomeric methyl(cyclopentadienyl)zinc, (CH<sub>3</sub>)Zn(η-C<sub>5</sub>H<sub>5</sub>), determined by gas phase electron diffraction|first1 = Arne|last1 = Haaland|authorlink1 = Arne Haaland|first2 = Svein|last2 = Samdal|first3 = Ragnhild|last3 = Seip|doi = 10.1016/S0022-328X(00)85041-X}}</ref><ref>{{cite book|url = https://books.google.com.au/books?id=B_-OCwAAQBAJ&pg=PA74|title = Organometallics|first = Christoph|last = Elschenbroich|publisher = [[John Wiley & Sons]]|year = 2006|isbn = 9783527805143|edition = 3rd|chapter = Organometallic Compounds of Groups 2 and 12|pages = 59–85}}</ref> Thus, while unprecedented,<ref name = HMB /> and having attracted comment in ''[[Chemical & Engineering News]]'',<ref name = PyramidalC&ENews>{{cite journal|title = Six bonds to carbon: Confirmed|volume = 94|issue = 49|page = 13|date = 19 December 2016|first = Stephen K.|last = Ritter|url = http://cen.acs.org/articles/94/i49/Six-bonds-carbon-Confirmed.html|journal = [[Chem. Eng. News]]}}</ref> ''[[New Scientist]]'',<ref name = PyramidalNewSci>{{cite journal|journal = [[New Scientist]]|date = 14 January 2017|title = Carbon seen bonding with six other atoms for the first time|first = Rebecca|last = Boyle|issue = 3108|url = https://www.newscientist.com/article/mg23331084-900-carbon-seen-bonding-with-six-other-atoms-for-the-first-time/|accessdate = 14 January 2017}}</ref> ''[[Science News]]'',<ref name = PyramidalSciNews>{{cite journal|journal = [[Science News]]|first = Laurel|last = Hamers|url = https://www.sciencenews.org/article/carbon-can-exceed-four-bond-limit|title = Carbon can exceed four-bond limit|volume = 190|issue = 13|date = 24 December 2016|page = 17}}</ref> and ZME Science,<ref>{{cite web|title = Exotic carbon molecule has six bonds, breaking the four-bond limit|url = http://www.zmescience.com/science/news-science/carbon-six-bond-molecule/|first = Tibi|last = Puiu|date = 5 January 2017|accessdate = 14 January 2017|website = zmescience.com|publisher = [[ZME Science]]}}</ref> the structure is consistent with the usual bonding rules of chemistry. Moritz Malischewski, who carried out the work with [[Konrad Seppelt]],<ref name = HMB /> commented that one the motivations for undertaking the work was to illustrate "the possibility to astonish chemists about what can be possible."<ref name = PyramidalNewSci />
In 1973, the [[Hexamethylbenzene#Dication|dication of hexamethylbenzene]], {{chem|C|6|(CH|3|)|6|2+}}, was produced by Hepke Hogeveen and Peter Kwant.<ref name = HMBcatObs1>{{cite journal|last1 = Hogeveen|first1 = Hepke|last2 = Kwant|first2 = Peter W.|year = 1973|title = Direct observation of a remarkably stable dication of unusual structure: (CCH<sub>3</sub>)<sub>6</sub><sup>2⊕</sup>|journal = [[Tetrahedron Lett.]]|volume = 14|issue = 19|pages = 1665–1670|doi = 10.1016/S0040-4039(01)96023-X}}</ref><ref name = HMBcatObs2>{{cite journal|last1 = Hogeveen|first1 = Hepke|last2 = Kwant|first2 = Peter W.|last3 = Postma|first3 = J.|last4 = van Duynen|first4 = P. Th.|year = 1974|title = Electronic spectra of pyramidal dications, (CCH<sub>3</sub>)<sub>6</sub><sup>2+</sup> and (CCH)<sub>6</sub><sup>2+</sup>|journal = [[Tetrahedron Lett.]]|volume = 15|issue = 49–50|pages = 4351–4354|doi = 10.1016/S0040-4039(01)92161-6}}</ref><ref name = HMBcatObs3>{{cite journal|last1 = Hogeveen|first1 = Hepke|last2 = Kwant|first2 = Peter W.|year = 1974|title = Chemistry and spectroscopy in strongly acidic solutions. XL. (CCH<sub>3</sub>)<sub>6</sub><sup>2+</sup>, an unusual dication|journal = [[J. Am. Chem. Soc.]]|volume = 96|issue = 7|pages = 2208–2214|doi = 10.1021/ja00814a034}}</ref> This can be done by dissolving the hexamethyl Dewar bezene monoepoxide in [[magic acid]], which removes the oxygen as an anion.<ref name = HMB /> NMR had previously hinted at a pentagonal pyramidal structure in a related cation,<ref>{{cite journal|title = Protonation of hexamethyl Dewar benzene and hexamethylprismane in fluorosulfuric acid – antimony pentafluoride – sulfur dioxide|first1 = Leo A.|last1 = Paquette|first2 = Grant R.|last2 = Krow|first3 = J. Martin|last3 = Bollinger|first4 = George A.|last4 = Olah|journal = [[J. Am. Chem. Soc.]]|year = 1968|volume = 90|issue = 25|pages = 7147–7149|doi = 10.1021/ja01027a060}}</ref> with the apex carbon bonding to six other carbon atoms, and X-ray crystallographic analysis of the salt with {{chem|Sb|F|6|-}} published in 2016 showed that this was indeed the case.<ref name = HMB>{{cite journal|title=Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C<sub>6</sub>(CH<sub>3</sub>)<sub>6</sub><sup>2+</sup>|url=http://onlinelibrary.wiley.com/doi/10.1002/anie.201608795/pdf|year=2016|doi=10.1002/anie.201608795|first1=Moritz|last1=Malischewski|first2=Konrad|last2=Seppelt|authorlink2=Konrad Seppelt|journal=[[Angew. Chem. Int. Ed.]]}}</ref> 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&nbsp;&times;&nbsp;0.54&nbsp;+&nbsp;1&nbsp;=&nbsp;3.7&nbsp;<&nbsp;4, and thus the species is not [[Hypervalent molecule|hypervalent]], but it is hypercoordinate.<ref name = Bachrach>{{cite web|title = A six-coordinate carbon atom|last = Bachrach|first = Steven M.|authorlink = Steven Bachrach|url = http://comporgchem.com/blog/?page_id=3929|accessdate = January 18, 2017|date = January 17, 2017|website = comporgchem.com}}</ref> From the perspective of [[organometallic chemistry]], the species can be viewed as having a carbon(IV) centre ({{chem|C|4+}}) bound to an aromatic {{nowrap|&eta;<sup>5</sup>–[[pentamethylcyclopentadienyl]]}} anion (six-electron donor) and a methyl anion (two-electron donor), thereby satisfying the [[octet rule]]<ref name = HMBviaOMetChem>{{cite journal|title = Pyramidal mono- and dications. Bridge between organic and organometallic chemistry|first1 = Hepke|last1 = Hogeveen|first2 = Peter W.|last2 = Kwant|journal = [[Acc. Chem. Res.]]|year = 1975|volume = 8|issue = 12|pages = 413–420|doi = 10.1021/ar50096a004}}</ref> and being analogous to the gas-phase [[organozinc compound|organozinc]] monomer {{nowrap|[(&eta;{{chem|5|–C|5|(CH|3|)|5|)Zn(CH|3}})],}} which has the same [[ligand]]s bound to a zinc(II) centre ({{chem|Zn|2+}}) and satisfies the [[18 electron rule]] on the metal.<ref name = Cp*ZnMe>{{cite journal|journal = [[J. Organomet. Chem.]]|volume = 153|issue = 2|year = 1978|pages = 187–192|title = The molecular structure of monomeric methyl(cyclopentadienyl)zinc, (CH<sub>3</sub>)Zn(η-C<sub>5</sub>H<sub>5</sub>), determined by gas phase electron diffraction|first1 = Arne|last1 = Haaland|authorlink1 = Arne Haaland|first2 = Svein|last2 = Samdal|first3 = Ragnhild|last3 = Seip|doi = 10.1016/S0022-328X(00)85041-X}}</ref><ref>{{cite book|url = https://books.google.com.au/books?id=B_-OCwAAQBAJ&pg=PA74|title = Organometallics|first = Christoph|last = Elschenbroich|publisher = [[John Wiley & Sons]]|year = 2006|isbn = 9783527805143|edition = 3rd|chapter = Organometallic Compounds of Groups 2 and 12|pages = 59–85}}</ref> Thus, while unprecedented,<ref name = HMB /> and having attracted comment in ''[[Chemical & Engineering News]]'',<ref name = PyramidalC&ENews>{{cite journal|title = Six bonds to carbon: Confirmed|volume = 94|issue = 49|page = 13|date = 19 December 2016|first = Stephen K.|last = Ritter|url = http://cen.acs.org/articles/94/i49/Six-bonds-carbon-Confirmed.html|journal = [[Chem. Eng. News]]}}</ref> ''[[New Scientist]]'',<ref name = PyramidalNewSci>{{cite journal|journal = [[New Scientist]]|date = 14 January 2017|title = Carbon seen bonding with six other atoms for the first time|first = Rebecca|last = Boyle|issue = 3108|url = https://www.newscientist.com/article/mg23331084-900-carbon-seen-bonding-with-six-other-atoms-for-the-first-time/|accessdate = 14 January 2017}}</ref> ''[[Science News]]'',<ref name = PyramidalSciNews>{{cite journal|journal = [[Science News]]|first = Laurel|last = Hamers|url = https://www.sciencenews.org/article/carbon-can-exceed-four-bond-limit|title = Carbon can exceed four-bond limit|volume = 190|issue = 13|date = 24 December 2016|page = 17}}</ref> and ZME Science,<ref>{{cite web|title = Exotic carbon molecule has six bonds, breaking the four-bond limit|url = http://www.zmescience.com/science/news-science/carbon-six-bond-molecule/|first = Tibi|last = Puiu|date = 5 January 2017|accessdate = 14 January 2017|website = zmescience.com|publisher = [[ZME Science]]}}</ref> the structure is consistent with the usual bonding rules of chemistry. Moritz Malischewski, who carried out the work with [[Konrad Seppelt]],<ref name = HMB /> commented that one the motivations for undertaking the work was to illustrate "the possibility to astonish chemists about what can be possible."<ref name = PyramidalNewSci />


==References==
==References==

Revision as of 04:52, 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 E.E. van Tamelen in 1963 by photolysis of cis-1,2-dihydro derivative of phthalic anhydride followed by oxidation with lead tetraacetate.[5][6]

Dewar benzene synthesis E. E. Van Tamelen, S. P. Pappas

"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 Kekulé.[2]

After the development in 1928 of the valence bond theory, the three possible Dewar structures were considered as minor resonance contributors in the overall description of benzene. The major resonance contributors are of course the two possible Kekulé structures.

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.[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. ^ J. Dewar (1867). "On the Oxidation af Phenyl Alcohol, and a Mechanical Arrangement adapted to illustrate Structure in the Non-saturated Hydrocarbons". Proc. Royal Soc. Edinburgh. 6: 82–86.
  2. ^ a b W. Baker; D. H. Rouvray (1978). "Para-Bond or "Dewar" Benzene?". J. Chem. Educ. 55 (10): 645. doi:10.1021/ed055p645.
  3. ^ James O. Jensen (2004). "Vibrational Frequencies and Structural Determination of Dewar Benzene". J. Mol. Struct.:THEOCHEM. 680: 227–236. doi:10.1016/j.theochem.2004.03.042.
  4. ^ E. E. van Tamelen, S. P. Pappas (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. ^ E. E. van Tamelen, S. P. Pappas (1963). "Bicyclo [2.2.0]hexa-2,5-diene". J. Am. Chem. Soc. 85 (20): 3297–3298. doi:10.1021/ja00903a056.
  6. ^ E. E. van Tamelen, S. P. Pappas, K. L. Kirk (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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ Hoffmann, R.; Hopf, H. (2008). "Learning from molecules in distress". Angewandte Chemie International Edition in English. 47 (24): 4474–4481. doi:10.1002/anie.200705775. PMID 18418829.
  8. ^ Sami A. Shama and Carl C. Wamser (1990). "Hexamethyl Dewar Benzene". Organic Syntheses; Collected Volumes, vol. 7, p. 256.
  9. ^ Paquette, L. A.; Krow, G. R. (1968). "Electrophilic Additions to Hexamethyldewarbenzene". Tetrahedron Lett. 9 (17): 2139–2142. doi:10.1016/S0040-4039(00)89761-0.
  10. ^ Criegee, R.; Gruner, 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, P. 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, P. M. (1968). "Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium(III) Chloride". J. Am. Chem. Soc. 90 (12): 3259–3261. doi:10.1021/ja01014a063.
  13. ^ a b King, R. B.; Douglas, W. M.; Efraty, A. (1977). "5-Acetyl-1,2,3,4,5-pentamethylcyclopentadiene". Organic Syntheses. 56: 1.
  14. ^ Junker, Hans-Nikolaus; Schäfer, Wolfgang; Niedenbrück, Hans (1967). "Oxydationsreaktionen mit Hexamethyl-bicyclo[2.2.0]-hexadien-(2.5) (= Hexamethyl-Dewar-Benzol)" [Oxidation reactions with hexamethylbicyclo[2.2.0]-hexa-2,5-diene]. Chem. Ber. (in German). 100 (8): 2508–2514. doi:10.1002/cber.19671000807.
  15. ^ a b Asouti, Amalia; Hadjiarapoglou, Lazaros P. (2000). "Regioselective and diastereoselective dimethyldioxirane epoxidation of substituted norbornenes and hexamethyl Dewar benzene". Tetrahedron Lett. 41 (4): 539–542. doi:10.1016/S0040-4039(99)02113-9.
  16. ^ a b Bachrach, Steven M. (January 17, 2017). "A six-coordinate carbon atom". comporgchem.com. Retrieved January 18, 2017.
  17. ^ a b c d e Malischewski, Moritz; Seppelt, Konrad (2016). "Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C6(CH3)62+". Angew. Chem. Int. Ed. doi:10.1002/anie.201608795.
  18. ^ Hogeveen, Hepke; Kwant, Peter W. (1973). "Direct observation of a remarkably stable dication of unusual structure: (CCH3)62⊕". Tetrahedron Lett. 14 (19): 1665–1670. doi:10.1016/S0040-4039(01)96023-X.
  19. ^ Hogeveen, Hepke; Kwant, Peter W.; Postma, J.; van Duynen, P. Th. (1974). "Electronic spectra of pyramidal dications, (CCH3)62+ and (CCH)62+". Tetrahedron Lett. 15 (49–50): 4351–4354. doi:10.1016/S0040-4039(01)92161-6.
  20. ^ Hogeveen, Hepke; Kwant, Peter W. (1974). "Chemistry and spectroscopy in strongly acidic solutions. XL. (CCH3)62+, an unusual dication". J. Am. Chem. Soc. 96 (7): 2208–2214. doi:10.1021/ja00814a034.
  21. ^ Paquette, Leo A.; Krow, Grant R.; Bollinger, J. Martin; Olah, George A. (1968). "Protonation of hexamethyl Dewar benzene and hexamethylprismane in fluorosulfuric acid – antimony pentafluoride – sulfur dioxide". J. Am. Chem. Soc. 90 (25): 7147–7149. doi:10.1021/ja01027a060.
  22. ^ Hogeveen, Hepke; Kwant, Peter W. (1975). "Pyramidal mono- and dications. Bridge between organic and organometallic chemistry". Acc. Chem. Res. 8 (12): 413–420. doi:10.1021/ar50096a004.
  23. ^ Haaland, Arne; Samdal, Svein; Seip, Ragnhild (1978). "The molecular structure of monomeric methyl(cyclopentadienyl)zinc, (CH3)Zn(η-C5H5), determined by gas phase electron diffraction". J. Organomet. Chem. 153 (2): 187–192. doi:10.1016/S0022-328X(00)85041-X.
  24. ^ Elschenbroich, Christoph (2006). "Organometallic Compounds of Groups 2 and 12". Organometallics (3rd ed.). John Wiley & Sons. pp. 59–85. ISBN 9783527805143.
  25. ^ Ritter, Stephen K. (19 December 2016). "Six bonds to carbon: Confirmed". Chem. Eng. News. 94 (49): 13.
  26. ^ a b Boyle, Rebecca (14 January 2017). "Carbon seen bonding with six other atoms for the first time". New Scientist (3108). Retrieved 14 January 2017.
  27. ^ Hamers, Laurel (24 December 2016). "Carbon can exceed four-bond limit". Science News. 190 (13): 17.
  28. ^ Puiu, Tibi (5 January 2017). "Exotic carbon molecule has six bonds, breaking the four-bond limit". zmescience.com. ZME Science. Retrieved 14 January 2017.
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