Pentamethylcyclopentadiene

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Pentamethylcyclopentadiene
Pentamethylcyclopentadien.svg
Pentamethylcyclopentadiene molecule ball.png
Names
Preferred IUPAC name
1,2,3,4,5-Pentamethylcyclopenta-1,3-diene
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.021.586 OOjs UI icon edit-ltr-progressive.svg
PubChem CID
UNII
  • InChI=1S/C10H16/c1-6-7(2)9(4)10(5)8(6)3/h6H,1-5H3 X mark.svgN
    Key: WQIQNKQYEUMPBM-UHFFFAOYSA-N X mark.svgN
  • InChI=1/C10H16/c1-6-7(2)9(4)10(5)8(6)3/h6H,1-5H3
    Key: WQIQNKQYEUMPBM-UHFFFAOYAI
  • CC1=C(C)C(C)C(C)=C1C
Properties
C10H16
Molar mass 136.238 g·mol−1
AppearanceColorless 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
Occupational safety and health (OHS/OSH):
Main hazards
 Flammable
GHS labelling:
GHS-pictogram-flamme.svg
Warning
H226
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).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diene with the formula C5(CH3)5H, often written C5Me5H, where Me is CH3. [3] It is a colorless liquid. [1]

Contents

1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted Cp* (C5Me5) 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]

Cp*H Synthesis 1.svg

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

Cp*H Synthesis 2.svg
Cp*Hampoule.jpg

Organometallic derivatives

Cp*–metal complexes
Cp*2Fe yellow
Cp*TiCl3red
[Cp*Fe(CO)2]2red-violet
[Cp*RhCl2]2red
[Cp*IrCl2]2 orange
Cp*Re(CO)3>colorless
Cp*Mo(CO)2CH3orange

Cp*H is a precursor to organometallic compounds containing the C5Me−5 ligand, commonly called Cp*−. [7] Some representative reactions leading to such Cp*–metal complexes follow: [8] Deprotonation with n-butyllithium:

Cp*H + C4H9Li → Cp*Li + C4H10

Synthesis of (pentamethylcyclopentadienyl)titanium trichloride:

Cp*Li + TiCl4 → Cp*TiCl3 + LiCl

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

2 Cp*H + 2 Fe(CO)5< → [η5-Cp*Fe(CO)2]2 + H2 + 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 + Me3SiCl → Cp*SiMe3 + LiCl
Cp*SiMe3 + TiCl4 → Cp*TiCl3 + Me3SiCl

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*IrCl2]2 and [Cp*RhCl2]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*IrCl2]2 using hexamethyl Dewar benzene C5Me5Ir2Cl2viaMe6DewarBen.png
Synthesis of the iridium(III) dimer [Cp*IrCl2]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 Bu3C5H3, a prototypical bulky cyclopentadienyl ligand (t-Bu)3C5H3.png
Structure of Bu3C5H3, 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 (C5Me4CF3) has the steric properties of Cp* and the electronic properties of Cp. [15]

See also

References

  1. 1 2 3 "1,2,3,4,5-Pentamethylcyclopentadiene".
  2. ↑ "Safety data sheet - 1,2,3,4,5-Pentamethylcyclopentadiene".
  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 ; Collected Volumes, vol. 8, p. 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. Vol. 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.
  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. Bibcode:1968JAChS..90.3259K. doi:10.1021/ja01014a063.
  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". 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. Bibcode:1992JAChS.114.6942G. doi:10.1021/ja00043a065. ISSN   0002-7863.
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