Pentamethylcyclopentadiene: Difference between revisions

|Verifiedfields = changed

|Watchedfields = changed

|verifiedrevid = 418038654

|Name = Pentamethylcyclopentadiene

|ImageFile = Pentamethylcyclopentadien.svg

|ImageSize = 135

|ImageAlt = Skeletal formula of pentamethylcyclopentadiene

|ImageFile1 = Pentamethylcyclopentadiene molecule ball.png

|ImageSize1 = 160

|ImageAlt1 = Ball-and-stick model of the pentamethylcyclopentadiene molecule

|PIN = 1,2,3,4,5-Pentamethylcyclopenta-1,3-diene

|Section1={{Chembox Identifiers

|CASNo_Ref = {{cascite|correct|??}}

|CASNo = 4045-44-7

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = DSE3MRZ77C

|SMILES = CC1=C(C)C(C)C(C)=C1C

|PubChem = 77667

|ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}}

|ChemSpiderID = 70069

|InChI = 1/C10H16/c1-6-7(2)9(4)10(5)8(6)3/h6H,1-5H3

|InChIKey = WQIQNKQYEUMPBM-UHFFFAOYAI

|StdInChI_Ref = {{stdinchicite|changed|chemspider}}

|StdInChI = 1S/C10H16/c1-6-7(2)9(4)10(5)8(6)3/h6H,1-5H3

|StdInChIKey_Ref = {{stdinchicite|changed|chemspider}}

|StdInChIKey = WQIQNKQYEUMPBM-UHFFFAOYSA-N

}}

|Section2={{Chembox Properties

|C=10|H=16

|Appearance = Colorless liquid<ref name=pubchem>https://pubchem.ncbi.nlm.nih.gov/compound/1_2_3_4_5-Pentamethylcyclopentadiene</ref>

|Odor = Mild<ref name=pubchem></ref>

|Density = 0.87 g/cm³<ref name=sigmaaldrich>https://www.sigmaaldrich.com/GB/en/sds/aldrich/214027</ref>

|Solubility = Sparingly soluble

|BoilingPtC = 55 to 60

|BoilingPt_notes = at {{convert|13|mmHg|kPa|abbr=on}}

}}

|Section7={{Chembox Hazards

|MainHazards = [[Flammable]]

|GHSSignalWord = Warning

|GHSPictograms = {{GHS02}}

|HPhrases = {{H-phrases|H226}}

|FlashPtC = 114

}}

'''1,2,3,4,5-Pentamethylcyclopentadiene''' is a [[cyclic compound|cyclic]] [[diene]] with the formula {{chem2|C5(CH3)5H}}, often written {{chem2|C5Me5H}}, where Me is [[Methyl group|{{chem2|CH3}}]].<ref>{{cite book|last1=Elschenbroich |first1=C. |last2=Salzer |first2=A. |title=Organometallics: A Concise Introduction |date=1989 |page=47 |isbn=9783527278183| publisher=[[Wiley-VCH|VCH]]}}</ref> It is a colorless liquid.<ref name=pubchem></ref>

1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the [[ligand]] ''1,2,3,4,5-pentamethylcyclopentadienyl'', which is often denoted '''Cp*''' ({{chem2|C5Me5}}) and read as "C P star", the "star" signifying the five methyl groups radiating from the core of the ligand. Thus, the 1,2,3,4,5-pentamethylcyclopentadiene's formula is also written Cp*H. In contrast to less-substituted [[cyclopentadiene]] derivatives, Cp*H is not prone to dimerization.

Pentamethylcyclopentadiene is commercially available. It was first prepared from [[tiglaldehyde]] and 2-butenyllithium, via 2,3,4,5-tetramethylcyclopent-2-enone, with a [[Nazarov cyclization reaction]] as a key step.<ref>{{cite journal | first= L. |last=De Vries | title= Preparation of 1,2,3,4,5-Pentamethyl-cyclopentadiene, 1,2,3,4,5,5-Hexamethyl-cyclopentadiene, and 1,2,3,4,5-Pentamethyl-cyclopentadienylcarbinol | journal = [[J. Org. Chem.]] | volume = 25 | year = 1960 | pages = 1838 | doi = 10.1021/jo01080a623 | issue = 10}}</ref>

:[[File:Cp*H_Synthesis_1.svg|525px|Synthesis of pentamethylcyclopentadiene from tiglaldehyde]]

Alternatively, 2-butenyllithium adds to [[ethyl acetate]] followed by acid-catalyzed dehydrocyclization:<ref>{{OrgSynth | first1= S. |last1=Threlkel |first2=J. E. |last2=Bercaw |first3=P. F. |last3=Seidler |first4=J. M. |last4=Stryker |first5=R. G. |last5=Bergman | prep = cv8p0505 | title = 1,2,3,4,5-Pentamethylcyclopentadiene | collvol = 8 | collvolpages = 505 | year = 1993}}</ref><ref>{{cite book | last1= Fendrick |first1=C. M. |last2=Schertz |first2=L. D. |last3=Mintz|first3=E. A. |last4=Marks |first4=T. J. | title = Large-Scale Synthesis of 1,2,3,4,5-Pentamethylcyclopentadiene | volume = 29 | pages = 193–198 | year = 1992 | doi = 10.1002/9780470132609.ch47 | series = Inorganic Syntheses | isbn = 978-0-470-13260-9}}</ref>

:[[File:Cp*H_Synthesis_2.svg|525px|Synthesis of pentamethylcyclopentadiene from ethyl acetate]]

:[[File:Cp*Hampoule.jpg|132px|Sample of pentamethylcyclopentadiene in ampoule.]]

{| class="toccolours floatleft" border="1" style="border-collapse: collapse;"

! {{Chemical datatable header}} | Cp*–metal complexes

| [[Decamethylferrocene|Cp*₂Fe]]

| [[Pentamethylcyclopentadienyl iridium dichloride dimer|[Cp*IrCl₂]]]₂

Cp*H is a precursor to [[organometallic]] compounds containing the {{chem|C|5|Me|5|−}} [[ligand]], commonly called Cp*⁻.<ref>{{cite book |last=Yamamoto |first=A. |title=Organotransition Metal Chemistry: Fundamental Concepts and Applications |date=1986 |page=105 |isbn=9780471891710 |publisher=[[Wiley-Interscience]] |url-access=registration |url=https://archive.org/details/organotransition00yama/page/105}}</ref>

Some representative reactions leading to such Cp*–metal complexes follow:<ref>{{cite journal | title = Organometallic chemistry of the transition metals XXI. Some π-pentamethylcyclopentadienyl derivatives of various transition metals | first1= R. B. |last1=King|first2= M. B.|last2= Bisnette | journal = [[Journal of Organometallic Chemistry|J. Organomet. Chem.]] | volume = 8 | year = 1967 | pages = 287–297 | doi = 10.1016/S0022-328X(00)91042-8 | issue = 2}}</ref>

Deprotonation with [[n-butyllithium]]:

:Cp*H + C₄H₉Li → Cp*Li + C₄H₁₀

Synthesis of (pentamethylcyclopentadienyl)titanium trichloride:

:Cp*Li + TiCl₄ → Cp*TiCl₃ + LiCl

Synthesis of (pentamethylcyclopentadienyl)iron dicarbonyl dimer from [[iron pentacarbonyl]]:

:2&nbsp;Cp*H + 2 Fe(CO)₅]] → [''η''⁵-Cp*Fe(CO)₂]₂ + H₂ + 6 CO

This method is analogous to the route to the related Cp complex, see [[cyclopentadienyliron dicarbonyl dimer]].

Some Cp* complexes are prepared using silyl transfer:

:Cp*Li + Me₃SiCl → Cp*SiMe₃ + LiCl

:Cp*SiMe₃ + TiCl₄ → Cp*TiCl₃ + Me₃SiCl

A now-obsolete route to Cp* complexes involves the use of [[hexamethyl Dewar benzene]]. This method was traditionally used for preparation of the chloro-bridged dimers [[Pentamethylcyclopentadienyl iridium dichloride dimer|[Cp*IrCl₂<nowiki>]</nowiki>₂]] and [[Pentamethylcyclopentadienyl rhodium dichloride dimer|[Cp*RhCl₂<nowiki>]</nowiki>₂]], but has been discontinued with the increased commercial availability of Cp*H. Such syntheses rely on a [[hydrohalic acid]] induced rearrangement of hexamethyl Dewar benzene<ref>{{cite journal |last1= Paquette|first1= L. A.|last2= Krow|first2= G. R.|year= 1968|title= Electrophilic Additions to Hexamethyldewarbenzene|journal= [[Tetrahedron Letters|Tetrahedron Lett.]]|volume= 9|issue= 17|pages= 2139–2142|doi= 10.1016/S0040-4039(00)89761-0}}</ref><ref>{{cite journal |last1= Criegee|first1= R.|last2= Gruner|first2= H.|year= 1968|title= Acid-catalyzed Rearrangements of Hexamethyl-prismane and Hexamethyl-Dewar-benzene|journal= [[Angewandte Chemie|Angew. Chem. Int. Ed. Engl.]]|volume= 7|issue= 6|pages= 467–468|doi= 10.1002/anie.196804672}}</ref> to a substituted pentamethylcyclopentadiene prior to reaction with the hydrate of either [[iridium(III) chloride]]<ref>{{cite journal |last1= Kang|first1= J. W.|last2= Mosley|first2= K.|last3= Maitlis|first3= P. M.|author-link3=Peter Maitlis|year= 1968|title= Mechanisms of Reactions of Dewar Hexamethylbenzene with Rhodium and Iridium Chlorides|journal= [[Chemical Communications|Chem. Commun.]]|issue= 21|pages= 1304–1305|doi= 10.1039/C19680001304}}</ref> or [[rhodium(III) chloride]].<ref>{{cite journal |last1= Kang|first1= J. W.|last2= Maitlis|first2= P. M.|author-link2=Peter Maitlis|year= 1968|title= Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium(III) Chloride|journal= [[J. Am. Chem. Soc.]]|volume= 90|issue= 12|pages= 3259–3261|doi= 10.1021/ja01014a063}}</ref>

:[[File:C5Me5Ir2Cl2viaMe6DewarBen.png|525px|Synthesis of the iridium(III) dimer [Cp*IrCl₂]₂ using hexamethyl Dewar benzene]]

===Comparison to other Cp ligands===

[[File:(t-Bu)3C5H3.png|thumb|144 px|Structure of ^''t''Bu₃C₅H₃, a prototypical bulky cyclopentadiene]]

Complexes of pentamethylcyclopentadienyl differ in several ways from the more common cyclopentadienyl (Cp) derivatives. Being more electron-rich, Cp*⁻ is a stronger donor and dissociation, like ring-slippage, is more difficult with Cp* than with Cp.<ref>{{Cite journal|last=Kuwabara|first=Takuya|last2=Tezuka|first2=Ryogen|last3=Ishikawa|first3=Mikiya|last4=Yamazaki|first4=Takuya|last5=Kodama|first5=Shintaro|last6=Ishii|first6=Youichi|date=2018-06-25|title=Ring Slippage and Dissociation of Pentamethylcyclopentadienyl Ligand in an (η 5 -Cp*)Ir Complex with a κ 3 - O , C , O Tridentate Calix[4]arene Ligand under Mild Conditions|journal=Organometallics|language=en|volume=37|issue=12|pages=1829–1832|doi=10.1021/acs.organomet.8b00257|issn=0276-7333}}</ref> The fluorinated ligand, (trifluoromethyl)tetramethylcyclopentadienyl, C₅Me₄CF₃, combines the properties of Cp and Cp*: it possesses the steric bulk of Cp* but has electronic properties similar to Cp, the electron-donation from the methyl groups being "canceled out" by the electron-accepting nature of the trifluoromethyl substituent.<ref>{{Cite journal|last=Gassman|first=Paul G.|last2=Mickelson|first2=John W.|last3=Sowa|first3=John R.|date=1992-08-01|title=1,2,3,4-Tetramethyl-5-(trifluoromethyl)cyclopentadienide: a unique ligand with the steric properties of pentamethylcyclopentadienide and the electronic properties of cyclopentadienide|journal=Journal of the American Chemical Society|volume=114|issue=17|pages=6942–6944|doi=10.1021/ja00043a065|issn=0002-7863}}</ref> Its steric bulk stabilizes complexes with fragile ligands. Its bulk also attenuates intermolecular interactions, decreasing the tendency to form polymeric structures. Its complexes also tend to be more soluble in non-polar solvents. The methyl group in Cp* complexes can undergo [[C–H activation]] leading to "[[tuck-in complex]]es". [[Bulky cyclopentadienyl ligands]] are known that are far more sterically encumbered than Cp*.

*[[Cyclopentadiene]]

[[Category:Cyclopentadienes]]

[[Category:Ligands]]