Cyclopentadiene

File:AW Cyclopentadiene.jpg

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). To obtain cyclopentadiene monomer, commercial dicyclopentadiene is cracked by heating to around 180 °C. The monomer is collected by distillation and used soon thereafter.{{OrgSynth | title = Cyclopentadiene and 3-Chlorocyclopentene | prep = cv4p0238 | collvol = 4 | collvolpages = 238 | first= Robert Bruce |last=Moffett | year = 1962}} It advisable to use some form of fractionating column when doing this, to remove refluxing uncracked dimer.

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.{{cite journal |last1=Corey |first1=E. J. |last2=Weinshenker |first2=N. M. |last3=Schaaf |first3=T. K. |last4=Huber |first4=W. |year=1969 |title=Stereo-controlled synthesis of prostaglandins F-2a and E-2 (dl)|journal=Journal of the American Chemical Society |volume=91 |issue=20 |pages=5675–5677 |doi=10.1021/ja01048a062 |pmid=5808505}}

File:Prostaglandin Diels-Alder Corey (cropped2).png

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.

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.{{cite journal |first1=Brian |last1=Levandowski |first2=Ken |last2=Houk |date=2015 |title=Theoretical Analysis of Reactivity Patterns in Diels–Alder Reactions of Cyclopentadiene, Cyclohexadiene, and Cycloheptadiene with Symmetrical and Unsymmetrical Dienophiles |doi=10.1021/acs.joc.5b00174 |pmid=25741891 |journal=J. Org. Chem. |volume=80 |issue=7 |pages=3530–3537}} Famously, cyclopentadiene dimerizes. The conversion occurs in hours at room temperature, but the monomer can be stored for days at −20 °C.{{Ullmann|first1=Dieter |last1=Hönicke |first2=Ringo |last2=Födisch |first3=Peter |last3=Claus |first4=Michael |last4=Olson |title=Cyclopentadiene and Cyclopentene |DOI=10.1002/14356007.a08_227}}

{{main|Cyclopentadienyl anion}}

The compound is unusually acidic (pK_a = 16) for a hydrocarbon, a fact explained by the high stability of the aromatic cyclopentadienyl anion, {{chem|C|5|H|5|−}}. 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.

{{main|Metallocene}}

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:{{cite book |author1-link=Gregory S. Girolami |author3-link=Robert Angelici |last1=Girolami |first1=G. S. |last2=Rauchfuss |first2=T. B. |last3=Angelici |first3=R. J. |title=Synthesis and Technique in Inorganic Chemistry |year=1999 |publisher=University Science Books |location=Mill Valley, CA |isbn=0-935702-48-2}} As typical example, nickelocene forms upon treating nickel(II) chloride with sodium cyclopentadienide in THF.{{cite book |last1=Jolly |first1=W. L. |title=The Synthesis and Characterization of Inorganic Compounds |url=https://archive.org/details/synthesischaract0000joll |url-access=registration |year=1970 |publisher=Prentice-Hall |location=Englewood Cliffs, NJ |isbn=0-13-879932-6}}

: 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.{{cite journal |title = Permethylmetallocene: 5. Reactions of Decamethylruthenium Cations |year = 1985 |last1 = Kolle |first1 = U. |last2 = Grub |first2 = J. |journal = J. Organomet. Chem. |volume = 289 |issue = 1 |pages = 133–139 |doi =10.1016/0022-328X(85)88034-7 }}

It was the starting material in Leo Paquette's 1982 synthesis of dodecahedrane.{{cite journal |title= Domino Diels–Alder reactions. I. Applications to the rapid construction of polyfused cyclopentanoid systems |journal= J. Am. Chem. Soc. |year= 1974 |volume= 96 |issue= 14 |pages= 4671–4673 |doi= 10.1021/ja00821a052 |author1-link=Leo Paquette |last1=Paquette |first1= L. A. |last2= Wyvratt |first2= M. J. }} The first step involved reductive dimerization of the molecule to give dihydrofulvalene, not simple addition to give dicyclopentadiene.

File:DodecahedranePrecursorSynthesis.png

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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.

File:(t-Bu)3C5H3.png{{cite book |doi=10.1002/9781119477822.ch8 |title=Inorganic Syntheses |year=2018 |last1=Reiners |first1=Matthis |last2=Ehrlich |first2=Nico |last3=Walter |first3=Marc D. |chapter=Synthesis of Selected Transition Metal and Main Group Compounds with Synthetic Applications |volume=37 |page=199 |isbn=978-1-119-47782-2 |s2cid=105376454}}]]

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.

• Aromaticity

{{Reflist}}

• [http://www.inchem.org/documents/icsc/icsc/eics0857.htm International Chemical Safety Card 0857]
• [https://www.cdc.gov/niosh/npg/npgd0170.html NIOSH Pocket Guide to Chemical Hazards]

{{Cycloalkenes}}

{{Annulenes}}

{{Cyclopentadiene complexes}}

Category:Annulenes

Category:Five-membered rings