Pentamethylcyclopentadiene

Pentamethylcyclopentadiene
Identifiers
CAS number	4045-44-7
Jmol-3D images	Image 1
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
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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.

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⁺

Organometallic derivatives

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.^[5]

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:^[6]

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 dimers [Cp*IrCl₂]₂ and [Cp*RhCl₂]₂. Such syntheses rely on a hydrohalic acid induced rearrangement of hexamethyl Dewar benzene^[7][8] to a substituted pentamethylcyclopentadiene prior to reaction with the hydrate of either iridium(III) chloride^[9] or rhodium(III) chloride.^[10]

Comparison of Cp* with Cp

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

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., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv8p0505 ; 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. ^ Yamamoto, A. Organotransition Metal Chemistry: Fundamental Concepts and Applications. (1986) p. 105
6. ^ 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. 
7. ^ Paquette, L. A.; Krow, G. R. (1968). "Electrophilic Additions to Hexamethyldewarbenzene". Tetrahedron Lett. 9 (17): 2139–2142. doi:10.1016/S0040-4039(00)89761-0. 
8. ^ Criegee, R.; Gruner, H. (1968). "Acid-catalyzed Rearrangements of Hexamethyl-prismane and Hexamethyl-Dewar-benzene". Angew. Chem. Int. Ed. Engl. 7 (6): 467–468. doi:10.1002/anie.196804672. 
9. ^ Kang, J. W.; Mosley, K.; Maitlis, P. M. (1968). "Mechanisms of Reactions of Dewar Hexamethylbenzene with Rhodium and Iridium Chlorides". Chem. Commun. (21): 1304–1305. doi:10.1039/C19680001304. 
10. ^ Kang, J. W.; Maitlis, P. M. (1968). "Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium(III) Chloride". J. Amer. Chem. Soc. 90 (12): 3259–3261. doi:10.1021/ja01014a063. 

See also

• Methylcyclopentadiene

References