Fulvenes
Class of chemical compounds
From Wikipedia, the free encyclopedia
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]
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.[6][7]
Modern synthesis of fulvenes employ buffer systems or other mild conditions.[8][9]
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").[10]
Ligand in organometallic chemistry
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.
