Chemistry:Pentamethylcyclopentadiene

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Pentamethylcyclopentadiene
Skeletal formula of pentamethylcyclopentadiene
Ball-and-stick model of the pentamethylcyclopentadiene molecule
Names
Preferred IUPAC name
1,2,3,4,5-Pentamethylcyclopenta-1,3-diene
Identifiers
3D model (JSmol)
ChemSpider
UNII
Properties
C10H16
Molar mass 136.238 g·mol−1
Appearance Colorless liquid[1]
Odor Mild[1]
Density 0.87 g/cm3[2]
Boiling point 55 to 60 °C (131 to 140 °F; 328 to 333 K) at 13 mmHg (1.7 kPa)
Sparingly soluble
Hazards
Main hazards Flammable
GHS pictograms GHS02: Flammable
GHS Signal word Warning
HH226Script error: No such module "Preview warning".Category:GHS errors
Flash point 114 °C (237 °F; 387 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diene with the formula C
5(CH
3)
5H, often written C
5Me
5H, where Me is CH
3.[3] It is a colorless liquid.[1]

1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted Cp* (C
5Me
5) and read as "C P star", the "star" signifying the five methyl groups radiating from the core of the ligand. Thus, the 1,2,3,4,5-pentamethylcyclopentadiene's formula is also written Cp*H. 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 and 2-butenyllithium, via 2,3,4,5-tetramethylcyclopent-2-enone, with a Nazarov cyclization reaction as a key step.[4]

Synthesis of pentamethylcyclopentadiene from tiglaldehyde

Alternatively, 2-butenyllithium adds to ethyl acetate followed by acid-catalyzed dehydrocyclization:[5][6]

Synthesis of pentamethylcyclopentadiene from ethyl acetate
Sample of pentamethylcyclopentadiene in ampoule.

Organometallic derivatives

Cp*–metal complexes
Cp*
2Fe 		yellow
Cp*TiCl
3 		red
[Cp*Fe(CO)
2]
2 		red-violet
[Cp*RhCl
2]
2 		red
[Cp*IrCl
2]
2 		orange
Cp*Re(CO)
3> 		colorless
Cp*Mo(CO)
2CH
3 		orange

Cp*H is a precursor to organometallic compounds containing the C
5Me−
5 ligand, commonly called Cp*−
.[7] Some representative reactions leading to such Cp*–metal complexes follow:[8] Deprotonation with n-butyllithium:

Cp*H + C
4H
9Li → Cp*Li + C
4H
10

Synthesis of (pentamethylcyclopentadienyl)titanium trichloride:

Cp*Li + TiCl
4 → Cp*TiCl
3 + LiCl

Synthesis of (pentamethylcyclopentadienyl)iron dicarbonyl dimer from iron pentacarbonyl:

2 Cp*H + 2 Fe(CO)
5< → [η5−Cp*Fe(CO)
2]
2 + H
2 + 6 CO

This method is analogous to the route to the related Cp complex, see cyclopentadienyliron dicarbonyl dimer.

Some Cp* complexes are prepared using silyl transfer:

Cp*Li + Me
3SiCl → Cp*SiMe
3 + LiCl
Cp*SiMe
3 + TiCl
4 → Cp*TiCl
3 + Me
3SiCl

A now-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*IrCl
2]
2 and [Cp*RhCl
2]
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]

Synthesis of the iridium(III) dimer [Cp*IrCl
2]
2 using hexamethyl Dewar benzene

The methyl group in Cp* complexes can undergo C–H activation leading to "tuck-in complexes".

Comparison to other Cp ligands

Structure of t
Bu
3C
5H
3, a prototypical bulky cyclopentadienyl ligand

Complexes of pentamethylcyclopentadienyl differ in several ways from the more common cyclopentadienyl (Cp) derivatives. Being more electron-rich, Cp*−
 is a stronger donor and dissociation, like ring-slippage, is more difficult with Cp* than with Cp.[13] Its complexes tend to be more soluble in non-polar solvents.

Bulky cyclopentadienyl ligands and 1,2,3-trisubstituted Cp ligands are also known.[14] (Trifluoromethyl)tetramethylcyclopentadienyl (C
5Me
4CF
3) has the steric properties of Cp* and the electronic properties of Cp.[15]

See also

References

  1. ↑ 1.0 1.1 1.2 "1,2,3,4,5-Pentamethylcyclopentadiene". https://pubchem.ncbi.nlm.nih.gov/compound/1_2_3_4_5-Pentamethylcyclopentadiene. 
  2. ↑ "Safety data sheet - 1,2,3,4,5-Pentamethylcyclopentadiene". https://www.sigmaaldrich.com/GB/en/sds/aldrich/214027. 
  3. ↑ Elschenbroich, C.; Salzer, A. (1989). Organometallics: A Concise Introduction. VCH. p. 47. ISBN 9783527278183. 
  4. ↑ De Vries, L. (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 (10): 1838. doi:10.1021/jo01080a623. 
  5. ↑ Threlkel, S.; Bercaw, J. E.; Seidler, P. F.; Stryker, J. M.; Bergman, R. G. (1993). "1,2,3,4,5-Pentamethylcyclopentadiene". Organic Syntheses. http://www.orgsyn.org/demo.aspx?prep=cv8p0505. ; Collective Volume, 8, pp. 505 
  6. ↑ Fendrick, C. M.; Schertz, L. D.; Mintz, E. A.; Marks, T. J. (1992). "Large-Scale Synthesis of 1,2,3,4,5-Penta-Methylcyclopentadiene". Inorganic Syntheses. 29. pp. 193–198. doi:10.1002/9780470132609.ch47. ISBN 978-0-470-13260-9. 
  7. ↑ Yamamoto, A. (1986). Organotransition Metal Chemistry: Fundamental Concepts and Applications. Wiley-Interscience. p. 105. ISBN 9780471891710. https://archive.org/details/organotransition00yama/page/105. 
  8. ↑ King, R. B.; Bisnette, M. B. (1967). "Organometallic chemistry of the transition metals XXI. Some π-pentamethylcyclopentadienyl derivatives of various transition metals". J. Organomet. Chem. 8 (2): 287–297. doi:10.1016/S0022-328X(00)91042-8. 
  9. ↑ 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. 
  10. ↑ 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. 
  11. ↑ 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. 
  12. ↑ Kang, J. W.; Maitlis, P. M. (1968). "Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium(III) Chloride". J. Am. Chem. Soc. 90 (12): 3259–3261. doi:10.1021/ja01014a063. Bibcode: 1968JAChS..90.3259K. 
  13. ↑ Kuwabara, Takuya; Tezuka, Ryogen; Ishikawa, Mikiya; Yamazaki, Takuya; Kodama, Shintaro; Ishii, Youichi (2018-06-25). "Ring Slippage and Dissociation of Pentamethylcyclopentadienyl Ligand in an (η 5 -Cp*)Ir Complex with a κ 3 - O , C , O Tridentate Calix[4]arene Ligand under Mild Conditions" (in en). Organometallics 37 (12): 1829–1832. doi:10.1021/acs.organomet.8b00257. ISSN 0276-7333. 
  14. ↑ Van Den Bossche, Bram; Cramer, Nicolai (2026-01-09). "Unified Synthesis Platform for 1,2,3-Trisubstituted Cyclopentadienyl Ligands Decouples Sterics from Electronics". Journal of the American Chemical Society. doi:10.1021/jacs.5c20631. 
  15. ↑ Gassman, Paul G.; Mickelson, John W.; Sowa, John R. (1992-08-01). "1,2,3,4-Tetramethyl-5-(trifluoromethyl)cyclopentadienide: a unique ligand with the steric properties of pentamethylcyclopentadienide and the electronic properties of cyclopentadienide". Journal of the American Chemical Society 114 (17): 6942–6944. doi:10.1021/ja00043a065. ISSN 0002-7863. Bibcode: 1992JAChS.114.6942G. 



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