Pentamethylcyclopentadiene

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diene with the formula , often written , where Me is . It is a colorless liquid. 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted Cp* () and read as "C P star", the "star" signifying the five methyl groups radiating from the core of the ligand. Thus, the 1,2,3,4,5-pentamethylcyclopentadiene's formula is also written CpH. In contrast to less-substituted cyclopentadiene derivatives, CpH is not prone to dimerization. Pentamethylcyclopentadiene is commercially available. It was first prepared from tiglaldehyde and 2-butenyllithium, via 2,3,4,5-tetramethylcyclopent-2-enone, with a Nazarov cyclization reaction as a key step. Alternatively, 2-butenyllithium adds to ethyl acetate followed by acid-catalyzed dehydrocyclization: CpH is a precursor to organometallic compounds containing the C5Me5− ligand, commonly called Cp−. Some representative reactions leading to such Cp*–metal complexes follow: Deprotonation with n-butyllithium: CpH + C4H9Li → CpLi + C4H10 Synthesis of (pentamethylcyclopentadienyl)titanium trichloride: CpLi + TiCl4 → CpTiCl3 + LiCl Synthesis of (pentamethylcyclopentadienyl)iron dicarbonyl dimer from iron pentacarbonyl: 2 CpH + 2 Fe(CO)5]] → [η5-CpFe(CO)2]2 + H2 + 6 CO This method is analogous to the route to the related Cp complex, see cyclopentadienyliron dicarbonyl dimer. Some Cp* complexes are prepared using silyl transfer: CpLi + Me3SiCl → CpSiMe3 + LiCl CpSiMe3 + TiCl4 → CpTiCl3 + Me3SiCl A now-obsolete route to Cp* complexes involves the use of hexamethyl Dewar benzene. This method was traditionally used for preparation of the chloro-bridged dimers [CpIrCl2]2 and [CpRhCl2]2, but has been discontinued with the increased commercial availability of Cp*H. Such syntheses rely on a hydrohalic acid induced rearrangement of hexamethyl Dewar benzene to a substituted pentamethylcyclopentadiene prior to reaction with the hydrate of either iridium(III) chloride or rhodium(III) chloride.