Cyclopentadiene


Cyclopentadiene is an organic compound with the formula C5H6. 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 restored by heating to give the monomer.
The compound is mainly used for the production of cyclopentene and its derivatives. It is popularly used as a precursor to the cyclopentadienyl ligand in cyclopentadienyl complexes in organometallic chemistry.

Production and reactions

Cyclopentadiene production is usually not distinguished from dicyclopentadiene since they are interconverted. They are obtained from coal tar and by steam cracking of naphtha. To obtain cyclopentadiene monomer, commercial dicyclopentadiene is cracked by heating to ~ 180 °C. The monomer is collected by distillation, and used soon thereafter.

Sigmatropic rearrangement

The hydrogen atoms in cyclopentadiene undergo rapid -sigmatropic shifts as indicated by 1H NMR spectra recorded at various temperatures. Even more fluxional are the derivatives C5H5E3, wherein the heavier element migrates from carbon to carbon with a low activation barrier.

Diels–Alder reactions

Cyclopentadiene is a highly reactive diene in the Diels–Alder reaction because minimal distortion of the diene is required to achieve the envelope geometry of the transition state compared to other dienes. 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 for a hydrocarbon, a fact explained by the high stability of the aromatic cyclopentadienyl anion,. Deprotonation can be achieved with a variety of bases, typically sodium hydride, sodium metal, and butyl lithium. 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 MC5H5 with dihalides of the transition metals: As typical example, nickelocene forms upon treating nickel chloride with sodium cyclopentadienide in THF.
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.

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.

Abbreviation

The commonly used abbreviation of the cyclopentadienyl anion is Cp. The abbreviation played a part in the naming of copernicium: the original proposal for the element's symbol was also Cp, but because of the abbreviation for this anion and the fact that lutetium was originally named cassiopeium and had Cp for the symbol as well, the symbol for copernicium was changed to Cn.