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This colorless liquid has a strong andunpleasant odor. At room temperature, this cyclicdiene dimerizes over the course of hours to givedicyclopentadiene via aDiels–Alder reaction. This dimer can berestored by heating to give the monomer.

The compound is mainly used for the production ofcyclopentene and its derivatives. It is popularly used as a precursor to thecyclopentadienyl anion (Cp⁻), an importantligand incyclopentadienyl complexes inorganometallic chemistry.^[7]

Production and reactions

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Cyclopentadiene production is usually not distinguished fromdicyclopentadiene since they interconvert. They are obtained from coal tar (about 10–20 g/t) and bysteam cracking ofnaphtha (about 14 kg/t).^[8] To obtain cyclopentadiene monomer, commercial dicyclopentadiene is cracked by heating to around 180 °C. The monomer is collected by distillation and used soon thereafter.^[9] It advisable to use some form offractionating column when doing this, to remove refluxing uncracked dimer.

Sigmatropic rearrangement

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The hydrogen atoms in cyclopentadiene undergo rapid[1,5]-sigmatropic shifts. The hydride shift is, however, sufficiently slow at 0 °C to allow alkylated derivatives to be manipulated selectively.^[10]

Themethoxy group ends up only on the methylene bridge, becauseDiels-Alder addition at −55 °C occurs much faster than the sigmatropic shift (excerpted fromCorey's total synthesis ofprostaglandin F_2α)^[10]

Even morefluxional are the derivatives C₅H₅E(CH₃)₃ (E =Si,Ge,Sn), wherein the heavier element migrates from carbon to carbon with a low activation barrier.

Diels–Alder reactions

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Cyclopentadiene is a highly reactivediene in theDiels–Alder reaction because minimal distortion of the diene is required to achieve the envelope geometry of the transition state compared to other dienes.^[11] Famously, cyclopentadiene dimerizes. The conversion occurs in hours at room temperature, but the monomer can be stored for days at −20 °C.^[8]

Deprotonation

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Main article:Cyclopentadienyl anion

The compound is unusuallyacidic (pK_a = 16) for ahydrocarbon, a fact explained by the high stability of thearomatic cyclopentadienyl anion,C
₅H⁻
₅.Deprotonation can be achieved with a variety of bases, typicallysodium hydride, sodium metal, andbutyl lithium. Salts of this anion are commercially available, includingsodium cyclopentadienide andlithium cyclopentadienide. They are used to preparecyclopentadienyl complexes.

Metallocene derivatives

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Main article:Metallocene

Metallocenes and relatedcyclopentadienyl derivatives have been heavily investigated and represent a cornerstone oforganometallic 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 thetransition metals:^[12] As typical example,nickelocene forms upon treatingnickel(II) chloride with sodium cyclopentadienide inTHF.^[13]

NiCl₂ + 2 NaC₅H₅ → Ni(C₅H₅)₂ + 2 NaCl

Organometallic complexes that include both the cyclopentadienyl anion and cyclopentadiene itself are known, one example of which is therhodocene derivative produced from the rhodocene monomer inprotic solvents.^[14]

Organic synthesis

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It was the starting material inLeo Paquette's 1982 synthesis ofdodecahedrane.^[15] The first step involvedreductive dimerization of the molecule to givedihydrofulvalene, not simple addition to give dicyclopentadiene.

The start of Paquette's 1982 dodecahedrane synthesis. Note the dimerisation of cyclopentadiene in step 1 to dihydrofulvalene.

Uses

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Aside from serving as a precursor to cyclopentadienyl-based catalysts, the main commercial application of cyclopentadiene is as a precursor tocomonomers. Semi-hydrogenation givescyclopentene. Diels–Alder reaction withbutadiene givesethylidene norbornene, a comonomer in the production ofEPDM rubbers.

Derivatives

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Structure oft-Bu₃C₅H₃, a prototypicalbulky cyclopentadiene^[16]

Cyclopentadiene can substitute one or more hydrogens, forming derivatives having covalent bonds:

Most of these substituted cyclopentadienes can also formanions and joincyclopentadienyl complexes.

References

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^^a ^b ^c ^d ^e ^f ^gNIOSH Pocket Guide to Chemical Hazards."#0170".National Institute for Occupational Safety and Health (NIOSH).
^William M. Haynes (2016).CRC Handbook of Chemistry and Physics [Physical Constants of Organic Compounds]. Vol. 97. CRC Press/Taylor and Francis. p. 276 (3-138).ISBN 978-1498754286.
^^a ^b ^cWilliam M. Haynes; David R. Lide; Thomas J. Bruno, eds. (2016).CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data (2016-2017, 97th ed.). Boca Raton, Florida: CRC Press.ISBN 978-1-4987-5428-6.OCLC 930681942.
^Faustov, Valery I.; Egorov, Mikhail P.; Nefedov, Oleg M.; Molin, Yuri N. (2000). "Ab initio G2 and DFT calculations on electron affinity of cyclopentadiene, silole, germole and their 2,3,4,5-tetraphenyl substituted analogs: structure, stability and EPR parameters of the radical anions".Phys. Chem. Chem. Phys.2 (19):4293–4297.Bibcode:2000PCCP....2.4293F.doi:10.1039/b005247g.
^"Cyclopentadiene".Immediately Dangerous to Life or Health Concentrations.National Institute for Occupational Safety and Health.
^Scharpen, LeRoy H.; Laurie, Victor W. (1965-10-15)."Structure of Cyclopentadiene".The Journal of Chemical Physics.43 (8):2765–2766.Bibcode:1965JChPh..43.2765S.doi:10.1063/1.1697207.ISSN 0021-9606.
^Hartwig, J. F. (2010).Organotransition Metal Chemistry: From Bonding to Catalysis. New York, NY: University Science Books.ISBN 978-1-891389-53-5.
^^a ^bHönicke, Dieter; Födisch, Ringo; Claus, Peter; Olson, Michael. "Cyclopentadiene and Cyclopentene".Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH.doi:10.1002/14356007.a08_227.ISBN 978-3-527-30673-2.
^Moffett, Robert Bruce (1962)."Cyclopentadiene and 3-Chlorocyclopentene".Organic Syntheses;Collected Volumes, vol. 4, p. 238.
^^a ^bCorey, E. J.; Weinshenker, N. M.; Schaaf, T. K.; Huber, W. (1969). "Stereo-controlled synthesis of prostaglandins F-2a and E-2 (dl)".Journal of the American Chemical Society.91 (20):5675–5677.doi:10.1021/ja01048a062.PMID 5808505.
^Levandowski, Brian; Houk, Ken (2015). "Theoretical Analysis of Reactivity Patterns in Diels–Alder Reactions of Cyclopentadiene, Cyclohexadiene, and Cycloheptadiene with Symmetrical and Unsymmetrical Dienophiles".J. Org. Chem.80 (7):3530–3537.doi:10.1021/acs.joc.5b00174.PMID 25741891.
^Girolami, G. S.; Rauchfuss, T. B.;Angelici, R. J. (1999).Synthesis and Technique in Inorganic Chemistry. Mill Valley, CA: University Science Books.ISBN 0-935702-48-2.
^Jolly, W. L. (1970).The Synthesis and Characterization of Inorganic Compounds. Englewood Cliffs, NJ: Prentice-Hall.ISBN 0-13-879932-6.
^Kolle, U.; Grub, J. (1985). "Permethylmetallocene: 5. Reactions of Decamethylruthenium Cations".J. Organomet. Chem.289 (1):133–139.doi:10.1016/0022-328X(85)88034-7.
^Paquette, L. A.; Wyvratt, M. J. (1974). "Domino Diels–Alder reactions. I. Applications to the rapid construction of polyfused cyclopentanoid systems".J. Am. Chem. Soc.96 (14):4671–4673.Bibcode:1974JAChS..96.4671P.doi:10.1021/ja00821a052.
^Reiners, Matthis; Ehrlich, Nico; Walter, Marc D. (2018). "Synthesis of Selected Transition Metal and Main Group Compounds with Synthetic Applications".Inorganic Syntheses. Vol. 37. p. 199.doi:10.1002/9781119477822.ch8.ISBN 978-1-119-47782-2.S2CID 105376454.

External links

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v t e Cycloalkenes
Alkenes	Cyclopropene Cyclobutene Cyclopentene Cyclohexene Cycloheptene Cyclooctene Cyclononene Cyclodecene
Dienes	Cyclobutadiene Cyclopentadiene Cyclohexadiene 1,3-Cyclohexadiene 1,4-Cyclohexadiene Cycloheptadiene 1,3-Cycloheptadiene 1,4-Cycloheptadiene Cyclooctadiene 1,5-Cyclooctadiene
Trienes	Benzene Cycloheptatriene
Tetraenes	Cyclooctatetraene Cyclononatetraene

v t e Annulenes
Even–numbered	Cyclobutadiene Benzene Cyclooctatetraene Cyclodecapentaene Cyclododecahexaene Cyclotetradecaheptaene Cyclohexadecaoctaene Cyclooctadecanonaene
Odd–numbered	Cyclopropene Cyclopentadiene Cycloheptatriene Cyclononatetraene
Compounds initalics arearomatic