Fulvenes

Chemical structure of fulvene

Fulvenes are the class of hydrocarbon obtained by formally cross-conjugating one ring and methylidene through a common exocyclic double bond.[1][2]

The name is derived from fulvene, which has one pentagonal ring. Other examples include methylenecyclopropene (triafulvene) and heptafulvene.

Fulvenes are generally named based on the number of ring atoms. Thus methylenecyclopropene is "triafulvene", methylenecyclopentadiene is "pentafulvene", etc.[3]

Preparation

Fulvenes are readily prepared by the condensation of cyclopentadiene and aldehydes and ketones:

C5H6 + R2C=O → C4H4C=CR2 + H2O

Johannes Thiele is credited with discovering this reaction.[4][5]

Modern synthesis of fulvenes employ buffer systems.[6][7]

Properties

The cross-conjugation generally destabilizes the exocyclic double bond, as (per Hückel's rules) polarization of the π electrons would lead to an aromatic ring ion. Consequently, fulvenes add nucleo- and electrophiles easily. They also have a small HOMO-LUMO gap, typically leading to the eponymous visible coloration ("fulvus" is Latin for "yellow").[8]

Ligand in organometallic chemistry

Fulvenes are common ligands and ligand precursors in organometallic chemistry.[9] 2,3,4,5-Tetramethylfulvene, abbreviated Me4Fv, results from the deprotonation of cationic pentamethylcyclopentadienyl complexes.[10] Some Me4Fv complexes are called tuck-in complexes.

η4- and η6-fulvene complexes

References

  1. ^ Agranat, Israel (2012), "Ground-State Versus Excited-State Polarity of Triafulvenes: A Study of Solvent Effects on Molecular Electronic Spectra", The Jerusalem Symposia on Quantum Chemistry and Biochemistry, 8: 573–583, doi:10.1007/978-94-010-1837-1_36, ISBN 978-94-010-1839-5
  2. ^ Neuenschwander, Markus (1986), "Synthetic and NMR spectroscopic investigations of fulvenes and fulvalenes" (PDF), Pure Appl. Chem., 58 (1): 55–66, doi:10.1351/pac198658010055
  3. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Fulvenes". doi:10.1351/goldbook.F02550
  4. ^ Thiele, J. (1900). "Ueber Ketonreactionen bei dem Cyclopentadiën". Chemische Berichte. 33: 666–673. doi:10.1002/cber.190003301113.
  5. ^ Hafner, K.; Vöpel, K. H.; Ploss, G.; König, C. (1967). "6-(Dimethylamino)Fulvene". Organic Syntheses. 47: 52. doi:10.15227/orgsyn.047.0052.
  6. ^ Coşkun, Necdet; Erden, Ihsan (2011-11-11). "An efficient catalytic method for fulvene synthesis". Tetrahedron. 67 (45): 8607–8614. doi:10.1016/j.tet.2011.09.036. ISSN 0040-4020. PMC 3196336. PMID 22021940.
  7. ^ Sieverding, Paul; Osterbrink, Johanna; Besson, Claire; Kögerler, Paul (2019-01-18). "Kinetics and mechanism of pyrrolidine buffer-catalyzed fulvene formation". J. Org. Chem. 84 (2): 486–494. doi:10.1021/acs.joc.8b01660. ISSN 0022-3263. PMID 30540466.
  8. ^ Neuenschwander, M. (1989). "Fulvenes". In Patai, Saul (ed.). The Chemistry of Double-Bonded Functional Groups. The Chemistry of Functional Groups. Vol. Supplement A, Part 2. Wiley. pp. 1132–1136. doi:10.1002/9780470772256.ch4. ISBN 978-0-470-77225-6.
  9. ^ Strohfeldt, Katja; Tacke, Matthias (2008). "Bioorganometallic fulvene-derived titanocene anti-cancer drugs". Chemical Society Reviews. 37 (6): 1174–87. doi:10.1039/B707310K. PMID 18497930.
  10. ^ Kreindlin, A. Z.; Rybinskaya, M. A. (2004). "Cationic and Neutral Transition Metal Complexes with a Tetramethylfulvene or Trimethylallyldiene Ligand". Russian Chemical Reviews. 73 (5): 417–432. Bibcode:2004RuCRv..73..417K. doi:10.1070/RC2004v073n05ABEH000842.