Pentamethylcyclopentadiene

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C5Me5H (Me = CH3). 1,2,3,4,5-Pentamethylcyclopentadiene is the organic precursor to the 1,2,3,4,5-pentamethylcyclopentadienyl-ligand (often denoted as Cp* to signify the five methyl groups radiating from the periphery of this ligand as in a five-pointed star).

Comparison Cp* with Cp
Cp* is an important ligand in organometallic compounds arising from the binding of the five ring-carbon atoms in C5Me5-, or Cp*-, to metals. Relative to the more common cyclopentadiene (Cp) ligand, pentamethylcyclopentadiene (Cp*) offers certain features that are often advantageous. Being more electron-rich, Cp* is a stronger donor and is less easily removed from the metal. Consequently its complexes exhibit increased thermal stability. Its steric bulk allows the isolation of complexes with fragile ligands. Its bulk also attenuates intermolecular interactions, decreasing the tendency to form polymeric structures. Its complexes also tend to be highly soluble in non-polar solvents.

Synthesis of 1,2,3,4,5-Pentamethylcyclopentadiene
A traditional route to Cp*H involves the addition of 2-butenyllithium to ethylacetate followed by acid-catalyzed dehydrocyclization:
 * MeCH=C(Li)Me +  MeC(O)OEt     →   (MeCH=C(Me))2C(OLi)Me  +  LIOEt
 * (MeCH=C(Me))2C(OLi)Me +  H+    →   Cp*H  +  H2O  +  Li+

Synthesis of Cp* complexes
Some representative reactions leading to such Cp*-metal complexes follow:
 * Cp*H  +  C4H9Li  →   Cp*Li  +  C4H10
 * Cp*Li +  TiCl4  →   Cp*TiCl3  +  LiCl


 * 2 Cp*H +  2 Fe(CO)5   →   [Cp*Fe(CO)2]2  +  H2

For the related Cp complex, see cyclopentadienyliron dicarbonyl dimer.

An instructive but obsolete route to Cp* complexes involves the use of hexamethyl Dewar benzene. This method was traditionally used for [RhCp*Cl2]2.