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 precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted as 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.

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Synthesis

Pentamethylcyclopentadiene is commercially available. It was was first prepared from tiglaldehyde via 1,2,3,4,5-pentamethylcyclopent-2-eneone.^[1] Alternatively 2-butenyllithium adds to ethylacetate followed by acid-catalyzed dehydrocyclization:^[2][3]

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.

Comparison of Cp* with Cp

Cp*H is an important precursor to organometallic compounds arising from the binding of the five ring-carbon atoms in C₅Me₅^-, or Cp*^-, to metals.^[4] Relative to the more common cyclopentadienyl (Cp) ligand, pentamethylcyclopentadienyl (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.

See also

• Methylcyclopentadiene

References

1. ^ L. de Vries (1960). "= Preparation of 1,2,3,4,5-Pentamethyl-cyclopentadiene, 1,2,3,4,5,5-Hexamethyl-cyclopentadiene, and 1,2,3,4,5-Pentamethyl-cyclopentadienylcarbinol". J. Org. Chem. 25: 1838. doi:10.1021/jo01080a623.
2. ^ S. Threlkel, J. E. Bercaw, P. F. Seidler, J. M. Stryker, R. G. Bergman (1993). "1,2,3,4,5-Pentamethylcyclopentadiene". Org. Synth.; Coll. Vol. 8: 505. 
3. ^ Fendrick, C. M.; Schertz, L. D.; Mintz, E. A.; Marks, T. J. (1992). "Large-Scale Synthesis of 1,2,3,4,5-Pentamethylcyclopentadiene". Inorganic Syntheses 29: 193-198. doi:10.1002/9780470132609.ch47.
4. ^ Yamamoto, A. Organotransition Metal Chemistry: Fundamental Concepts and Applications. (1986) p. 105

• Overview of Cp* Compounds: Elschenbroich, C. and Salzer, A. Organometallics: a Concise Introduction (1989) p. 47
• Initial examples of the synthesis of Cp*-metal complexes: R. B. King, M. B. Bisnette, Journal of

Organometallic Chemistry volume, 8 (1967) pp. 287-297.