Pentamethylcyclopentadiene

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic dialkene with the formula C5Me5H (Me = CH3). 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted Cp* (to signify the five methyl groups radiating from the periphery of this ligand as in a five-pointed star). In contrast to less-substituted cyclopentadiene derivatives, Cp*H is not prone to dimerization. 1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic dialkene with the formula C5Me5H (Me = CH3). 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted Cp* (to signify the five methyl groups radiating from the periphery of this ligand as in a five-pointed star). In contrast to less-substituted cyclopentadiene derivatives, Cp*H is not prone to dimerization. Pentamethylcyclopentadiene is commercially available. It was first prepared from tiglaldehyde via 2,3,4,5-tetramethylcyclopent-2-enone. Alternatively 2-butenyllithium adds to ethyl acetate followed by acid-catalyzed dehydrocyclization: Cp*H is a precursor to organometallic compounds containing the C5Me−5 ligand, commonly called Cp*−. Some representative reactions leading to such Cp*–metal complexes follow: