Cyclopentadiene production is usually not distinguished from dicyclopentadiene since they interconvert. They are obtained from coal tar (about 10–20 g/tonne) and by steam cracking of naphtha (about 14 kg/tonne).[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 of fractionating column when doing this, to remove refluxing uncracked dimer.
Sigmatropic rearrangement
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]
Diene-selective Diels-Alder reaction in Corey's total synthesis of prostaglandin F2α.
Even more fluxional 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
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.[11] Famously, cyclopentadiene dimerizes. The conversion occurs in hours at room temperature, but the monomer can be stored for days at −20 °C.[8]
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:[12] As typical example, nickelocene forms upon treating nickel(II) chloride with sodium cyclopentadienide in THF.[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 the rhodocene derivative produced from the rhodocene monomer in protic solvents.[14]
Organic synthesis
It was the starting material in Leo Paquette's 1982 synthesis of dodecahedrane.[15] The first step involved reductive dimerization of the molecule to give dihydrofulvalene, 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
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.
^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). ISBN978-1498754286.
^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.
^LeRoy H. Scharpen and Victor W. Laurie (1965): "Structure of cyclopentadiene". The Journal of Chemical Physics, volume 43, issue 8, pages 2765-2766. doi:10.1063/1.1697207
^Hartwig, J. F. (2010). Organotransition Metal Chemistry: From Bonding to Catalysis. New York, NY: University Science Books. ISBN978-1-891389-53-5.
^Corey, 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): 5675–7. doi:10.1021/ja01048a062. PMID5808505.
^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. PMID25741891.
^Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. (1999). Synthesis and Technique in Inorganic Chemistry. Mill Valley, CA: University Science Books. ISBN0-935702-48-2.
^Reiners, Matthis; Ehrlich, Nico; Walter, Marc D. (2018). Synthesis of 1,3,5-Tri-tert-Butylcyclopenta-1,3-diene and Its Metal Complexes Na{1,2,4-(Me3C)3C5H2} and Mg{η5-1,2,4-(Me3C)3C5H2)2. Inorganic Syntheses. Vol. 37. p. 199. doi:10.1002/9781119477822.ch8. S2CID105376454.
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