fulvenes
File:Fulvene_with_hydrogens.svg]]
Fulvenes are the class of hydrocarbon obtained by formally cross-conjugating one ring and methylidene through a common exocyclic double bond.{{citation | last = Agranat | first = Israel | title = Ground-State Versus Excited-State Polarity of Triafulvenes: A Study of Solvent Effects on Molecular Electronic Spectra | journal = The Jerusalem Symposia on Quantum Chemistry and Biochemistry | year = 2012 | volume = 8 | pages = 573–583 | doi = 10.1007/978-94-010-1837-1_36 | isbn = 978-94-010-1839-5}}{{citation | last = Neuenschwander | first = Markus | title = Synthetic and NMR spectroscopic investigations of fulvenes and fulvalenes | journal = Pure Appl. Chem. | year = 1986 | volume = 58 | issue = 1 | pages = 55–66 | doi = 10.1351/pac198658010055|url = http://pac.iupac.org/publications/pac/pdf/1986/pdf/5801x0055.pdf}}
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.{{GoldBookRef|file=F02550|title=Fulvenes}}
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.{{cite journal
| author = Thiele, J.
| title = Ueber Ketonreactionen bei dem Cyclopentadiën
| journal = Chemische Berichte
| volume = 33
| pages = 666–673
| year = 1900
| doi = 10.1002/cber.190003301113| url = https://zenodo.org/record/1425954
Modern synthesis of fulvenes employ buffer systems.{{Cite journal|last1=Coşkun|first1=Necdet|last2=Erden|first2=Ihsan|date=2011-11-11|title=An efficient catalytic method for fulvene synthesis|journal=Tetrahedron|volume=67|issue=45|pages=8607–8614|doi=10.1016/j.tet.2011.09.036|issn=0040-4020|pmc=3196336|pmid=22021940}}{{Cite journal|last1=Sieverding|first1=Paul|last2=Osterbrink|first2=Johanna|last3=Besson|first3=Claire|last4=Kögerler|first4=Paul|date=2019-01-18|title=Kinetics and mechanism of pyrrolidine buffer-catalyzed fulvene formation|journal=J. Org. Chem.|volume=84|issue=2|pages=486–494|doi=10.1021/acs.joc.8b01660|pmid=30540466|issn=0022-3263}}
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").{{cite book|doi=10.1002/9780470772256.ch4|title=The Chemistry of Double-Bonded Functional Groups|volume=Supplement A, Part 2|editor-first=Saul|editor-last=Patai|publisher=Wiley|series=The Chemistry of Functional Groups|year=1989|chapter=Fulvenes|first=M.|last=Neuenschwander|pages=1132–1136|isbn=978-0-470-77225-6}}
Ligand in organometallic chemistry
Fulvenes are common ligands and ligand precursors in organometallic chemistry.{{cite journal |doi=10.1039/B707310K|title=Bioorganometallic fulvene-derived titanocene anti-cancer drugs|year=2008|last1=Strohfeldt|first1=Katja|last2=Tacke|first2=Matthias|journal=Chemical Society Reviews|volume=37|issue=6|pages=1174–87|pmid=18497930}} 2,3,4,5-Tetramethylfulvene, abbreviated Me4Fv, results from the deprotonation of cationic pentamethylcyclopentadienyl complexes.{{cite journal
|author1=Kreindlin, A. Z. |author2=Rybinskaya, M. A. | title = Cationic and Neutral Transition Metal Complexes with a Tetramethylfulvene or Trimethylallyldiene Ligand
| journal = Russian Chemical Reviews
| volume = 73
| pages = 417–432
| year = 2004
| doi = 10.1070/RC2004v073n05ABEH000842
| issue = 5|bibcode=2004RuCRv..73..417K }} Some Me4Fv complexes are called tuck-in complexes.
File:FulveneCmpxs.png and η6-fulvene complexes]]{{clear left}}
