Fulvenes are the class of hydrocarbon obtained by formally cross-conjugating one ring and methylidene through a common exocyclic double bond. In addition to compounds with substituent group on the fulvene skeleton, structural analogs in this class include heteroatom replacements and variations of ring-size. Thus, methylenecyclopropene can be called triafulvene and cyclopropenone is an oxafulvene. Analogs in which certain ring carbons are replaced by other elements, sometimes entailing changes of the double-bonding pattern of the heterocyclic core as in dithiafulvene, lead to changes in the electronic properties of the ring.
Preparation Fulvenes are readily prepared by the condensation of cyclopentadiene with various aldehydes and ketones under strongly basic conditions, via a cyclopentadienyl anion intermediate:
C5H6 + R2C=O → C4H4C=CR2 + H2O Johannes Thiele is credited with discovering this reaction. Modern synthesis of fulvenes employ buffer systems or other mild conditions.
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").
Ligand in organometallic chemistry Fulvenes are common ligands and ligand precursors in organometallic chemistry. 2,3,4,5-Tetramethylfulvene, abbreviated Me4Fv, results from the deprotonation of cationic pentamethylcyclopentadienyl complexes. Some Me4Fv complexes are called tuck-in complexes.
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