Valence isomer

In organic chemistry, two molecules are valence isomers when they are constitutional isomers that can interconvert through pericyclic reactions.[1][2]

Benzene

There are many valence isomers one can draw for the C6H6 formula benzene. Some were originally proposed for benzene itself before the actual structure of benzene was known. Others were later synthesized in lab. Some have been observed to isomerize to benzene, whereas others tend to undergo other reactions instead, or isomerize by ways other than pericyclic reactions.

Cyclooctatetraene

The valence isomers are not restricted to isomers of benzene. Valence isomers are also seen in the series (CH)8. Due to the larger number of units, the number of possible valence isomers is also greater and at least 21:

  • Valence isomers of cyclooctatetraene
  • Cyclooctatetraene (COT)
  • Barrelene
  • Cubane
  • Cuneane
  • Semibullvalene
  • 1,5-dihydropentalene
  • 2a,2b,4a,4b-Tetrahydrocyclopropa[cd]pentalene
  • Bicyclo[4.2.0]octa-2,4,7-triene. Tautomer with COT by thermal 6e process or photochemical 4e process [3][4][5]
  • Tricyclo[3,3,0,02,6]octa-3,8-diene. Isomerises to semibullvalene at room temperature, stable at −60 °C [6][7][8][9]
  • Tricyclo[4,2,0,02,5]octa-3,7-diene. The dimer of cyclobutadiene occurs as a cis isomer and a trans isomer. Both isomers convert to COT (symmetry forbidden hence stable) with a half-life of 20 minutes at 140 °C [10]
  • Tetracyclo[3,3,0,02,4,03,6] octa-7-ene is only known as its 4-carbomethoxy derivative.[11]
  • Tetracyclo[4,2,0,02,4,03,5] octa-7-ene has been prepared from benzvalene and isomerises to COT [12]
  • Pentacyclo[5.1.0.02,4. 03,5.06,8]octane (octabisvalene) is the third saturated valence isomer. The (Z)-3,7-phenylsulfonyl derivative is stable up to 200 °C.[13]
  • Tricyclo[5.1.0.02,8]octa-3,5-diene (octavalene) was reported synthesised from homobenzvalene and converts to COT at 50 °C [14][15]

Naphthalene and azulene

Perhaps no pair of valence isomers differ more strongly in appearance than colourless naphthalene and the intensely violet azulene.

Benzene oxide and oxepin
Benzene oxide exists in dynamic equilibrium with its valence isomer oxepin.[16]

References
  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (1994) "Valence isomer". doi:10.1351/goldbook.V06590
  2. Rearrangements and interconversions of compounds of the formula (CH)n Lawrence T. Scott, Maitland. Jones Chem. Rev., 1972, 72 (2), pp 181–202 doi:10.1021/cr60276a004
  3. Huisgen, R.; Mietzsch, F. (1964). "The Valence Tautomerism of Cyclooctatetraene". Angewandte Chemie International Edition in English. 3 (2): 83. doi:10.1002/anie.196400831.
  4. Bicyclo[4,2,0]octa-2,4,7-triene Emanuel Vogel, H. Kiefer, W. R. Roth Volume 3, Issue 6, pages 442–443, June 1964 doi:10.1002/anie.196404422
  5. Huisgen, Rolf.; Konz, Will E.; Gream, George E. (1970). "Evidence for different valence tautomers of bromocyclooctatetraene". Journal of the American Chemical Society. 92 (13): 4105. doi:10.1021/ja00716a048.
  6. Meinwald, Jerrold; Tsuruta, Haruki (1969). "Tricyclo[3.3.0.02,6]octa-3,7-diene". Journal of the American Chemical Society. 91 (21): 5877. doi:10.1021/ja01049a034.
  7. Meinwald, Jerrold; Schmidt, Douglass (1969). "Semibullvalene from tricyclo[3.3.0.02,6]octane". Journal of the American Chemical Society. 91 (21): 5877. doi:10.1021/ja01049a033.
  8. Zimmerman, Howard Elliot; Robbins, Jeffrey D.; Schantl, Joachim (1969). "C8H8 interconversions. An unusual rearrangement providing a new route to semibullvalene". Journal of the American Chemical Society. 91 (21): 5878. doi:10.1021/ja01049a035.
  9. Meinwald, Jerrold.; Tsuruta, Haruki. (1970). "(CH)8 hydrocarbons. Photochemistry of tricyclo[3.3.0.02,6]octa-3,7-diene". Journal of the American Chemical Society. 92 (8): 2579. doi:10.1021/ja00711a078.
  10. Untersuchungen in der Cyclobutanreihe, XII. Zwei stereoisomere Dimere des Cyclobutadiens Margarete Avram, Ilie G. Dinulescu, Elise Marica, Georg Mateescu, Elvira Sliam, Costin D. Nenitzescu Chemische Berichte Volume 97, Issue 2, pages 382–389, February 1964 doi:10.1002/cber.19640970210
  11. Methyl tetracyclo[3.3.0.02,4O3,6]cot-7-ene-4-carboxylate Gerhard W. Klumpp, W. G. J. Rietman, J. J. Vrielink J. Am. Chem. Soc., 1970, 92 (17), pp 5266–5267 doi:10.1021/ja00720a071
  12. Synthesis and reactions of tetracyclo[4.2.0.02,4.03,5]octanes Leverett R. Smith, George E. Gream, Jerrold Meinwald J. Org. Chem., 1977, 42 (6), pp 927–936 doi:10.1021/jo00426a001
  13. (Z)-3,7 Bis(phenylsulfonyl)pentacyclo[5.1.0.02,4.03,5.06,8]octane, an Octabisvalene Derivative (1985) Angewandte Chemie International Edition in English Volume 24, Issue 5, pages 411–412 doi:10.1002/anie.198504111
  14. The synthesis of octavalene (tricyclo[5.1.0.02,8]octa-3,5-diene) and several substituted octavalenes Tetrahedron Volume 42, Issue 6, 1986, Pages 1585-1596 Manfred Christl, Reinhard Lang and Clemens Herzog doi:10.1016/S0040-4020(01)87575-X
  15. Electronic structure of octavalene. Photoelectron spectroscopic investigations Rolf Gleiter, Peter Bischof, Manfred Christl J. Org. Chem., 1986, 51 (15), pp 2895–2898 doi:10.1021/jo00365a007
  16. E. Vogel, H. Günther (1967). "Benzene Oxide-Oxepin Valence Tautomerism". Angewandte Chemie International Edition in English. 6 (5): 385–401. doi:10.1002/anie.196703851.
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