Pentamethylcyclopentadiene

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Pentamethylcyclopentadiene
Pentamethylcyclopentadiene
Identifiers
CAS number [4045-44-7]
SMILES CC1=C(C)C(C)C(C)=C1C
Properties
Molecular formula C10H16
Molar mass 136.24 g/mol
Boiling point

55–60 °C (13 mm Hg)
Solubility in water Sparingly soluble
Hazards
Flash point 114 °C
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and references

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C5Me5H (Me = CH3).[1] 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.

Contents

[edit] Synthesis

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

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+

[edit] Synthesis of Cp* complexes

Cp*-metal Complexes
Cp*2Fe yellow
Cp*TiCl3 red
[Cp*Fe(CO)2]2 red-violet
[Cp*RhCl2]2 red
Cp*Re(CO)3 colorless
Cp*Mo(CO)2CH3 orange

Some representative reactions leading to such Cp*-metal complexes follow:[5]

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 preparation of the chloro-bridged dimer [Cp*RhCl2]2.

[edit] 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.[6] 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.

[edit] See also

[edit] References

  1. ^ Overview of Cp* Compounds: Elschenbroich, C. and Salzer, A. Organometallics: a Concise Introduction (1989) p. 47
  2. ^ 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. 
  3. ^ 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. 
  4. ^ 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. 
  5. ^ R. B. King, M. B. Bisnette (1967). "Organometallic chemistry of the transition metals XXI. Some π-pentamethylcyclopentadienyl derivatives of various transition metals". Journal of Organometallic Chemistry 8: 287–297. doi:10.1016/S0022-328X(00)91042-8. 
  6. ^ Yamamoto, A. Organotransition Metal Chemistry: Fundamental Concepts and Applications. (1986) p. 105