Cyclobutadiene

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Cyclobutadiene
Cyclobutadiene
Cyclobutadiene
Names
Preferred IUPAC name
Cyclobuta-1,3-diene
Other names
1,3-Cyclobutadiene
Cyclobutadiene
[4]Annulene
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
  • InChI=1S/C4H4/c1-2-4-3-1/h1-4H checkY
    Key: HWEQKSVYKBUIIK-UHFFFAOYSA-N checkY
  • InChI=1/C4H4/c1-2-4-3-1/h1-4H
    Key: HWEQKSVYKBUIIK-UHFFFAOYAI
  • C1=CC=C1
  • C1=CC=C1
Properties
C4H4
Molar mass 52.076 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Cyclobutadiene is an organic compound with the formula Template:Carbon4Template:Hydrogen4. It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is quite stable. Since the compound degrades by a bimolecular process, the species can be observed by matrix isolation techniques at temperatures below 35 K. It is thought to adopt a rectangular structure.[1][2]

Structure and reactivity

The compound is the prototypical antiaromatic hydrocarbon with 4 π-electrons. It is the smallest [n]-annulene ([4]-annulene). Its rectangular structure is the result of the Jahn–Teller effect, which distorts the molecule, converting the triplet to a singlet ground state.[3] The electronic states of cyclobutadiene have been explored with a variety of computational methods.[4] The rectangular structure is consistent with the existence of two different 1,2-dideutero-1,3-cyclobutadiene stereoisomers. This distortion indicates that the pi electrons are localized, in agreement with Hückel's rule which predicts that a π-system of 4 electrons is not aromatic.

In principle, another situation is possible. Namely, cyclobutadiene could assume an undistorted square geometry, if it adopts a triplet spin state. While a theoretical possibility, the triplet form of the parent cyclobutadiene and its substituted derivatives remained elusive for decades. However, in 2017, the square triplet excited state of 1,2,3,4-tetrakis(trimethylsilyl)-1,3-cyclobutadiene was observed spectroscopically, and a singlet-triplet gap of EST = 13.9 kcal/mol was measured for this compound.[5]

Synthesis

Several cyclobutadiene derivatives have been isolated with steric bulky substituents. Orange tetrakis(tert-butyl)cyclobutadiene arises by thermolysis of its isomer tetra-tert-butyltetrahedrane. Although the cyclobutadiene derivative is stable (with respect to dimerization), it decomposes upon contact with O2.[6][7]

Trapping

Samples of cyclobutadiene are unstable since the compound dimerizes at temperatures above 35 K by a Diels-Alder reaction.[8] By suppressing bimolecular decomposition pathways, cyclobutadiene is well-behaved. Thus it has been generated in a hemicarceplex.[2] The inclusion compound is generated by photodecarboxylation of bicyclopyran-2-one.[9] When released from the host–guest complex, cyclobutadiene dimerizes and then converts to cyclooctatetraene.

After numerous attempts, cyclobutadiene was first generated by oxidative degradation of cyclobutadieneiron tricarbonyl with ammonium cerium(IV) nitrate.[10][11] When liberated from the iron complex, cyclobutadiene reacts with electron-deficient alkynes to form a Dewar benzene:[12]

Cyclobutadiene to Dewarbenzene conversion

The Dewar benzene converts to dimethyl phthalate on heating at 90 °C.

One cyclobutadiene derivative is also accessible through a [2+2]cycloaddition of a di-alkyne. In this particular reaction the trapping reagent is 2,3,4,5-tetraphenylcyclopenta-2,4-dienone and one of the final products (after expulsion of carbon monoxide) is a cyclooctatetraene:[13]

Acetylene-Acetylene [2 + 2] Cycloadditions Chung-Chieh Lee 2006

See also

References

  1. ^ Kollmar, H.; Staemmler, V. (1977). "A theoretical study of the structure of cyclobutadiene H. Kollmar, V. Staemmler; J. Am. Chem. Soc". Journal of the American Chemical Society. 99 (11): 3583–3587. doi:10.1021/ja00453a009.
  2. ^ a b Cram, Donald J.; Tanner, Martin E.; Thomas, Robert (1991). "The Taming of Cyclobutadiene Donald J. Cram, Martin E. Tanner, Robert Thomas". Angewandte Chemie International Edition in English. 30 (8): 1024–1027. doi:10.1002/anie.199110241.
  3. ^ Peter Senn (1992). "A Simple Quantum Mechanical Model that Illustrates the Jahn-Teller Effect". J. Chem. Educ. 69 (10): 819. doi:10.1021/ed069p819.
  4. ^ Balkova, A.; Bartlett, R. J. J. Chem. Phys. 1994, 101, 8972–8987.
  5. ^ Kostenko, Arseni; Tumanskii, Boris; Kobayashi, Yuzuru; Nakamoto, Masaaki; Sekiguchi, Akira; Apeloig, Yitzhak (2017-07-03). "Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene". Angewandte Chemie International Edition. 56 (34): 10183–10187. doi:10.1002/anie.201705228. ISSN 1433-7851. PMID 28635054.
  6. ^ "Tetra-tert-butyltetrahedrane". Angew. Chem. Int. Ed. Engl. 17 (7): 520. 1978. doi:10.1002/anie.197805201. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
  7. ^ "Structure of Tetra-tert-butylcyclobutadiene". Angewandte Chemie International Edition in English. 19 (3): 211–212. 1980. doi:10.1002/anie.198002111. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
  8. ^ Carey, Francis A.; Sundberg, Richard J. (2007). Advanced Organic Chemistry: Part A: Structure and Mechanisms (5th ed.). Springer. p. 725. ISBN 978-0-387-44897-8.
  9. ^ E. J. Corey, Jacques Streith (1964). "Internal Photoaddtion Reactions of 2-Pyrone and N-Methyl-2-pyridone: A New Synthetic Approach to Cyclobutadiene". J. Am. Chem. Soc. 86 (5): 950–951. doi:10.1021/ja01059a059.
  10. ^ "Cyclobutadiene- and Benzocyclobutadiene-Iron Tricarbonyl Complexes". J. Am. Chem. Soc. 87: 131–133. 1965. doi:10.1021/ja01079a032. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
  11. ^ "Cyclobutadieneiron tricarbonyl". Organic Syntheses. 50: 21. 1970. doi:10.15227/orgsyn.050.0021. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
  12. ^ "Cyclobutadiene". J. Am. Chem. Soc. 87 (14): 3253–3254. 1965. doi:10.1021/ja01092a049. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
  13. ^ Chung-Chieh Lee; Man-kit Leung; Gene-Hsiang Lee; Yi-Hung Liu; Shie-Ming Peng (2006). "Revisit of the Dessy-White Intramolecular Acetylene-Acetylene [2 + 2] Cycloadditions". J. Org. Chem. 71 (22): 8417–8423. doi:10.1021/jo061334v. PMID 17064014.
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