Pentamethylcyclopentadiene

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.

[7] Some representative reactions leading to such Cp*–metal complexes follow:[8] Deprotonation with n-butyllithium: Synthesis of (pentamethylcyclopentadienyl)titanium trichloride: Synthesis of (pentamethylcyclopentadienyl)iron dicarbonyl dimer from iron pentacarbonyl: This method is analogous to the route to the related Cp complex, see cyclopentadienyliron dicarbonyl dimer.

This method was traditionally used for preparation of the chloro-bridged dimers [Cp*IrCl2]2 and [Cp*RhCl2]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[9][10] to a substituted pentamethylcyclopentadiene prior to reaction with the hydrate of either iridium(III) chloride[11] or rhodium(III) chloride.

[12] Complexes of pentamethylcyclopentadienyl differ in several ways from the more common cyclopentadienyl (Cp) derivatives.

Its bulk also attenuates intermolecular interactions, decreasing the tendency to form polymeric structures.

Skeletal formula of pentamethylcyclopentadiene
Ball-and-stick model of the pentamethylcyclopentadiene molecule
Structure of t Bu 3 C 5 H 3 , a prototypical bulky cyclopentadiene