| 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 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 |
Synthesis
Pentamethylcyclopentadiene is commercially available. It was first prepared from tiglaldehyde via 2,3,4,5-tetramethylcyclopent-2-enone.[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+
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*IrCl2]2 | orange |
| Cp*Re(CO)3 | colorless |
| Cp*Mo(CO)2CH3 | orange |
Some representative reactions leading to such Cp*-metal complexes follow:[5]
- 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.
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.
See also
References
- ^ Overview of Cp* Compounds: Elschenbroich, C. and Salzer, A. Organometallics: a Concise Introduction (1989) p. 47
- ^ 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:.
- ^ S. Threlkel, J. E. Bercaw, P. F. Seidler, J. M. Stryker, R. G. Bergman (1993), "1,2,3,4,5-Pentamethylcyclopentadiene", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv8p0505; Coll. Vol. 8: 505
- ^ 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:.
- ^ 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:.
- ^ Yamamoto, A. Organotransition Metal Chemistry: Fundamental Concepts and Applications. (1986) p. 105
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