Editing Cyclopentadiene (section)

==Production and reactions==
[[File:AW Cyclopentadiene.jpg|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&nbsp;g/[[tonne|t]]) and by steam [[Cracking (chemistry)|cracking]] of [[Petroleum naphtha|naphtha]] (about 14&nbsp;kg/t).<ref name=Ullmann/> To obtain cyclopentadiene monomer, commercial dicyclopentadiene is cracked by heating to around 180&nbsp;°C. The monomer is collected by distillation and used soon thereafter.<ref>{{OrgSynth | title = Cyclopentadiene and 3-Chlorocyclopentene | prep = cv4p0238 | collvol = 4 | collvolpages = 238 | first= Robert Bruce |last=Moffett | year = 1962}}</ref> 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&nbsp;°C to allow alkylated derivatives to be manipulated selectively.<ref>{{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}}</ref> 
[[File:Prostaglandin Diels-Alder Corey (cropped2).png|400 px|thumb|center|Diene-selective Diels–Alder reaction in Corey's total synthesis of prostaglandin F2α]]

Even more [[fluxional molecule|fluxional]] are the derivatives C<sub>5</sub>H<sub>5</sub>E(CH<sub>3</sub>)<sub>3</sub> (E = [[silicon|Si]], [[germanium|Ge]], [[tin|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.<ref>{{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}}</ref> Famously, cyclopentadiene dimerizes. The conversion occurs in hours at room temperature, but the monomer can be stored for days at −20&nbsp;°C.<ref name=Ullmann>{{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}}</ref>

===Deprotonation===
{{main|Cyclopentadienyl anion}}
The compound is unusually [[acid]]ic (p''K''<sub>a</sub>&nbsp;= 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 complex]]es.

===Metallocene derivatives===
{{main|Metallocene}}
Metallocenes and related [[Cyclopentadienyl complex|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 metal]]s:<ref>{{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}}</ref> As typical example, [[nickelocene]] forms upon treating [[nickel(II) chloride]] with sodium cyclopentadienide in [[tetrahydrofuran|THF]].<ref>{{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}}</ref>

: NiCl<sub>2</sub> + 2&nbsp;NaC<sub>5</sub>H<sub>5</sub> → Ni(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> + 2&nbsp;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 solvent]]s.<ref>{{cite journal |title = Permethylmetallocene: 5. Reactions of Decamethylruthenium Cations |year = 1985 |last1 = Kolle |first1 = U. |last2 = Grub |first2 = J. |journal = [[Journal of Organometallic Chemistry|J. Organomet. Chem.]] |volume = 289 |issue = 1 |pages = 133–139 |doi =10.1016/0022-328X(85)88034-7 }}</ref>

===Organic synthesis===
It was the starting material in [[Leo Paquette]]'s 1982 synthesis of [[dodecahedrane]].<ref>{{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. }}</ref> The first step involved [[redox|reductive]] dimerization of the molecule to give [[Fulvalene|dihydrofulvalene]], not simple addition to give dicyclopentadiene.

[[File:DodecahedranePrecursorSynthesis.png|thumb|center|400px|The start of Paquette's 1982 dodecahedrane synthesis. Note the dimerisation of cyclopentadiene in step&nbsp;1 to dihydrofulvalene.]]
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