Pentamethylcyclopentadiene

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diene with the formula, often written, where Me is [Methyl group|]. 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 Cp*H. In contrast to less-substituted cyclopentadiene derivatives, Cp*H is not prone to dimerization.

Synthesis

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:

Organometallic derivatives

Cp*H is a precursor to organometallic compounds containing the ligand, commonly called. Some representative reactions leading to such Cp*–metal complexes follow:
Deprotonation with n-butyllithium:
Synthesis of titanium trichloride:
Synthesis of iron dicarbonyl dimer from iron pentacarbonyl:
This method is analogous to the route to the related Cp complex, see cyclopentadienyliron dicarbonyl dimer.
Some Cp* complexes are prepared using silyl transfer:
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 and, 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.
The methyl group in Cp* complexes can undergo C–H activation leading to "tuck-in complexes".

Comparison to other Cp ligands

Complexes of pentamethylcyclopentadienyl differ in several ways from the more common cyclopentadienyl derivatives. Being more electron-rich, is a stronger donor and dissociation, like ring-slippage, is more difficult with Cp* than with Cp. Its complexes tend to be more soluble in non-polar solvents.
Bulky cyclopentadienyl ligands and 1,2,3-trisubstituted Cp ligands are also known. tetramethylcyclopentadienyl has the steric properties of Cp* and the electronic properties of Cp.