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Hexamethyl Dewar benzene

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Hexamethyl Dewar benzene
Names
IUPAC name
1,2,3,4,5,6-Hexamethylbicyclo[2.2.0]hexa-2,5-diene
Identifiers
3D model (JSmol)
ChemSpider
EC Number
  • 231-576-3
  • InChI=1S/C12H18/c1-7-8(2)12(6)10(4)9(3)11(7,12)5/h1-6H3
    Key: RVNQQZMIWZPGNA-UHFFFAOYSA-N
  • CC1=C(C2(C1(C(=C2C)C)C)C)C
Properties
C12H18
Molar mass 162.276 g·mol−1
Related compounds
Related compounds
 Hexamethylbenzene
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Hexamethyl Dewar benzene is a derivative of Dewar benzene with application in organometallic chemistry. It consists of the Dewar benzene core, with a methyl group substituent on each of its six carbon positions.

Synthesis

[edit]

Hexamethyl Dewar benzene has been prepared by bicyclotrimerization of dimethylacetylene with aluminium chloride.[1]

Rearrangement to cyclopentadienes

[edit]

Hexamethyl Dewar benzene undergoes a rearrangement reaction with hydrohalic acids to give a cyclopentadiene structure. A rhodium or iridium salt in methanol can be added to it to form the organometallic pentamethylcyclopentadienyl rhodium dichloride[2][3][4][5] and pentamethylcyclopentadienyl iridium dichloride dimers;[6] Consequently, it can be used as a starting material for synthesising some pentamethylcyclopentadienyl organometallic compounds[7][8] including [Cp*Rh(CO)2].[9] The hydrogen halide reaction need not be performed as a separate preliminary step, but can be triggered as a side effect of the dissolved metal-halide salt reacting with hexamethyl Dewar benzene.[6]

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

Attempting a similar reaction with potassium tetrachloroplatinate results in the formation of a pentamethylcyclopentadiene complex, [(η4-Cp*H)PtCl2], indicating that the rhodium and iridium metal centres are necessary for the step in which the aromatic anion is formed.[5]

Epoxidation

[edit]

One of the alkenes can be epoxidized using mCPBA,[10] peroxybenzoic acid,[11] or dimethyldioxirane (DMDO).[12] By varying the amount of DMDO, either the mono- or diepoxide can be formed, with the oxygen atoms exo on the bicyclic carbon framework.[12]

The epoxide products are stable when the oxidation is performed under neutral conditions, such as when using DMDO that has acetone as a byproduct. When Using a peracid (mCPBA or peroxybenzoic acid), the epoxy product quickly rearranges, catalyzed by the acid byproduct of the epoxidation.[10]

Dication

[edit]
See also: Pyramidal carbocation

In 1973, the dication of hexamethylbenzene, C
6(CH
3)2+
6, was produced by Hepke Hogeveen and Peter Kwant.[13] This can be done by dissolving the hexamethyl Dewar benzene monoepoxide in magic acid, which removes the oxygen as an anion.[14] NMR had previously hinted at a pentagonal pyramidal structure in a related cation[15] as had spectral data on the Hogeveen and Kwant dication.[16][17] The pyramidal structure having an apex carbon bonding to six other carbon atoms was confirmed by X-ray crystallographic analysis of the hexafluoroantimonate salt published in 2016.[14]

Left: Structure of C
6(CH
3)2+
6, as drawn by Steven Bachrach[18]
Right: Three-dimensional representation of the dication's rearranged pentagonal-pyramid framework, from the crystal structure[14]

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.[18] 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[19] 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.[20][21] Thus, while unprecedented,[14] and having attracted comment in Chemical & Engineering News,[22] New Scientist,[23] Science News,[24] and ZME Science,[25] the structure is consistent with the usual bonding rules of chemistry. Moritz Malischewski, who carried out the work with Konrad Seppelt,[14] commented that one the motivations for undertaking the work was to illustrate "the possibility to astonish chemists about what can be possible."[23]

References

[edit]
  1. ^ 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.
  2. ^ 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.
  3. ^ 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.
  4. ^ Herrmann, Wolfgang A.; Zybill, Christian (1996). "Bis{(μ-chloro)[chloro(η-pentamethylcyclopentadienyl)rhodium]} — {Rh(μ-Cl)Cl[η-C5(CH3)5]}2". In Herrmann, Wolfgang A.; Salzer, Albrecht (eds.). Synthetic Methods of Organometallic and Inorganic Chemistry – Volume 1: Literature, Laboratory Techniques, and Common Starting Materials. Georg Thieme Verlag. pp. 148–149. ISBN 9783131791610.
  5. ^ a b Heck, Richard F. (1974). "Reactions of Dienes Trienes and Tetraenes with Transition Metal Compounds". Organotransition Metal Chemistry: A Mechanistic Approach. Academic Press. pp. 116–117. ISBN 9780323154703.
  6. ^ a b Kang, Jung W.; Moseley, K.; Maitlis, Peter M. (1969). "Pentamethylcyclopentadienylrhodium and -iridium halides. I. Synthesis and properties". J. Am. Chem. Soc. 91 (22): 5970–5977. Bibcode:1969JAChS..91.5970K. doi:10.1021/ja01050a008.
  7. ^ 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.
  8. ^ Kang, J. W.; Maitlis, Peter M. (1968). "Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium(III) Chloride". J. Am. Chem. Soc. 90 (12): 3259–3261. Bibcode:1968JAChS..90.3259K. doi:10.1021/ja01014a063.
  9. ^ Herrmann, Wolfgang A.; Zybill, Christian (1996). "Dicarbonyl(η-pentamethylcyclopentadienyl)rhodium — Rh[η-C5(CH3)5](CO)2". In Herrmann, Wolfgang A.; Salzer, Albrecht (eds.). Synthetic Methods of Organometallic and Inorganic Chemistry – Volume 1: Literature, Laboratory Techniques, and Common Starting Materials. Georg Thieme Verlag. pp. 147–148. ISBN 9783131791610.
  10. ^ a b King, R. B.; Douglas, W. M.; Efraty, A. (1977). "5-Acetyl-1,2,3,4,5-pentamethylcyclopentadiene". Organic Syntheses. 56: 1. doi:10.15227/orgsyn.056.0001; Collected Volumes, vol. 6, p. 39.
  11. ^ 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 (= Hexamethyl Dewar Benzene)]. Chem. Ber. (in German). 100 (8): 2508–2514. doi:10.1002/cber.19671000807.
  12. ^ 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.
  13. ^ 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.
  14. ^ 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. 56 (1): 368–370. doi:10.1002/anie.201608795. PMID 27885766.
  15. ^ 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. Bibcode:1968JAChS..90.7147P. doi:10.1021/ja01027a060.
  16. ^ 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.
  17. ^ 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. Bibcode:1974JAChS..96.2208H. doi:10.1021/ja00814a034.
  18. ^ a b Bachrach, Steven M. (January 17, 2017). "A six-coordinate carbon atom". comporgchem.com. Archived from the original on January 19, 2017. Retrieved January 18, 2017.
  19. ^ 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.
  20. ^ 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.
  21. ^ Elschenbroich, Christoph (2006). "Organometallic Compounds of Groups 2 and 12". Organometallics (3rd ed.). John Wiley & Sons. pp. 59–85. ISBN 9783527805143.
  22. ^ Ritter, Stephen K. (December 19, 2016). "Six bonds to carbon: Confirmed". Chem. Eng. News. 94 (49): 13. Archived from the original on January 9, 2017.
  23. ^ a b Boyle, Rebecca (January 14, 2017). "Carbon seen bonding with six other atoms for the first time". New Scientist (3108). Archived from the original on January 16, 2017. Retrieved January 14, 2017.
  24. ^ Hamers, Laurel (December 24, 2016). "Carbon can exceed four-bond limit". Science News. 190 (13): 17. Archived from the original on February 3, 2017.
  25. ^ Puiu, Tibi (January 5, 2017). "Exotic carbon molecule has six bonds, breaking the four-bond limit". zmescience.com. ZME Science. Archived from the original on January 16, 2017. Retrieved January 14, 2017.
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