Cyclopentadiene is an organic compound with the formula C<sub>5</sub>H<sub>6</sub>. It is often abbreviated CpH because the cyclopentadienyl anion is abbreviated Cp<sup>−</sup>.
This colorless liquid has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels–Alder reaction. This dimer can be heated to induce thermal cracking and regenerate the monomer through a retro-Diels–Alder reaction
The compound is mainly used for the production of cyclopentene and its derivatives. It is popularly used as a precursor to the cyclopentadienyl anion (Cp<sup>−</sup>), an important ligand in cyclopentadienyl complexes in organometallic chemistry.
Production and reactions
thumb|left|Cyclopentadiene monomer in an ice bath
Cyclopentadiene production is usually not distinguished from dicyclopentadiene since they interconvert. They are obtained from coal tar (about 10–20 g/t) and by steam cracking of naphtha (about 14 kg/t). It advisable to use some form of fractionating column when doing this, to remove refluxing uncracked dimer.
Sigmatropic rearrangement
The hydrogen atoms in cyclopentadiene undergo rapid [[sigmatropic reaction|[1,5]-sigmatropic shifts]]. The hydride shift is, however, sufficiently slow at 0 °C to allow alkylated derivatives to be manipulated selectively.
400 px|thumb|class=skin-invert-image|center|The [[methoxy group ends up only on the methylene bridge, because Diels-Alder addition at −55 °C occurs much faster than the sigmatropic shift (excerpted from Corey's total synthesis of prostaglandin F<sub>2α</sub>) Famously, cyclopentadiene dimerizes. The conversion occurs in hours at room temperature, but the monomer can be stored for days at −20 °C.
Deprotonation
The compound is unusually acidic (pK<sub>a</sub> = 16) for a hydrocarbon due to the high stability of the aromatic cyclopentadienyl anion, . Deprotonation can be achieved with a variety of bases, typically sodium hydride, sodium metal, and butyllithium. Salts of this anion are commercially available, including sodium cyclopentadienide and lithium cyclopentadienide. They are used to prepare cyclopentadienyl complexes.
Metallocene derivatives
Metallocenes and related cyclopentadienyl derivatives have been heavily investigated and represent a cornerstone of organometallic chemistry owing to their high stability. The first metallocene characterised, ferrocene, was prepared the way many other metallocenes are prepared by combining alkali metal derivatives of the form MC<sub>5</sub>H<sub>5</sub> with dihalides of the transition metals: As typical example, nickelocene forms upon treating nickel(II) chloride with sodium cyclopentadienide in THF.
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Organometallic complexes that include both the cyclopentadienyl anion and cyclopentadiene itself are known, one example of which is the rhodocene derivative produced from the rhodocene monomer in protic solvents.
Organic synthesis
It was the starting material in Leo Paquette's 1982 synthesis of dodecahedrane. The first step involved reductive dimerization of the molecule to give dihydrofulvalene, not simple addition to give dicyclopentadiene.
[[File:DodecahedranePrecursorSynthesis.png|thumb|class=skin-invert-image|
center|400px|The start of Paquette's 1982 dodecahedrane synthesis. Note the dimerisation of cyclopentadiene in step 1 to dihydrofulvalene.]]
Uses
Aside from serving as a precursor to cyclopentadienyl-based catalysts, the main commercial application of cyclopentadiene is as a precursor to comonomers. Semi-hydrogenation gives cyclopentene. Diels–Alder reaction with butadiene gives ethylidene norbornene, a comonomer in the production of EPDM rubbers.
Derivatives
[[File:(t-Bu)3C5H3.png|thumb|class=skin-invert-image|
right|144 px|Structure of t-Bu<sub>3</sub>C<sub>5</sub>H<sub>3</sub>, a prototypical bulky cyclopentadiene]]
Cyclopentadiene can substitute one or more hydrogens, forming derivatives having covalent bonds:
- Bulky cyclopentadienes
- Calicene
- Cyclopentadienone
- Di-tert-butylcyclopentadiene
- Methylcyclopentadiene
- Pentamethylcyclopentadiene
- Pentacyanocyclopentadiene
Most of these substituted cyclopentadienes can also form anions and join cyclopentadienyl complexes.
See also
- Aromaticity
References
External links
- International Chemical Safety Card 0857
- NIOSH Pocket Guide to Chemical Hazards