Fulvenes

Fulvenes are the class of hydrocarbon obtained by formally cross-conjugating one ring and methylidene through a common exocyclicdouble 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]

フルベンは有機金属化学において一般的な配位子および配位子前駆体である。^{[ 11 ]} 2,3,4,5-テトラメチルフルベン（略称Me4Fv _{）は、カチオン性}ペンタメチルシクロペンタジエニル錯体の脱プロトン化によって生成する。^{[ 12 ]} _Me4Fv錯体の中にはタックイン錯体と呼ばれるものもある。