Fulvenes

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

In addition to compounds with substituent group on the fulvene skeleton, structural analogs in this class include heteroatom replacements^[3] and variations of ring-size. Thus, methylenecyclopropene can be called triafulvene and cyclopropenone is an oxafulvene.^[4] 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.^[5]

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.^[6][7]

Modern synthesis of fulvenes employ buffer systems or other mild conditions.^[8][9]

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").^[10]

Fulvenes are common ligands and ligand precursors in organometallic chemistry.^[11] 2,3,4,5-Tetramethylfulvene, abbreviated Me₄Fv, results from the deprotonation of cationic pentamethylcyclopentadienyl complexes.^[12] Some Me₄Fv complexes are called tuck-in complexes.