:Pentamethylcyclopentadiene

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.{{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}}

:File:Cp*H_Synthesis_1.svg

Alternatively, 2-butenyllithium adds to ethyl acetate followed by acid-catalyzed dehydrocyclization:{{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}}{{cite book | last1= Fendrick |first1=C. M. |last2=Schertz |first2=L. D. |last3=Mintz|first3=E. A. |last4=Marks |first4=T. J. |chapter=Large-Scale Synthesis of 1,2,3,4,5-Penta-Methylcyclopentadiene | title = Inorganic Syntheses | volume = 29 | pages = 193–198 | year = 1992 | doi = 10.1002/9780470132609.ch47 | isbn = 978-0-470-13260-9}}

:File:Cp*H_Synthesis_2.svg

:File:Cp*Hampoule.jpg

Cp*H is a precursor to organometallic compounds containing the {{chem2|C5Me5−}} ligand, commonly called {{chem2|Cp\*-}}.{{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}} Some representative reactions leading to such Cp*–metal complexes follow:{{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 = J. Organomet. Chem. | volume = 8 | year = 1967 | pages = 287–297 | doi = 10.1016/S0022-328X(00)91042-8 | issue = 2}}

Deprotonation with n-butyllithium:

:{{chem2|Cp\*H + C4H9Li -> Cp\*Li + C4H10}}

Synthesis of (pentamethylcyclopentadienyl)titanium trichloride:

:{{chem2|Cp\*Li + TiCl4 -> Cp\*TiCl3 + LiCl}}

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

:{{chem2|2 Cp\*H + 2 Fe(CO)5< -> [\h{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:

:{{chem2|Cp\*Li + Me3SiCl -> Cp\*SiMe3 + LiCl}}

:{{chem2|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 {{chem2|[Cp\*IrCl2]2|link=Pentamethylcyclopentadienyl iridium dichloride dimer}} and {{chem2|[Cp\*RhCl2]2|link=Pentamethylcyclopentadienyl rhodium dichloride dimer}}, 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{{cite journal |last1= Paquette|first1= L. A.|last2= Krow|first2= G. R.|year= 1968|title= Electrophilic Additions to Hexamethyldewarbenzene|journal= Tetrahedron Lett.|volume= 9|issue= 17|pages= 2139–2142|doi= 10.1016/S0040-4039(00)89761-0}}{{cite journal |last1= Criegee|first1= R.|last2= Gruner|first2= H.|year= 1968|title= Acid-catalyzed Rearrangements of Hexamethyl-prismane and Hexamethyl-Dewar-benzene|journal= Angew. Chem. Int. Ed. Engl.|volume= 7|issue= 6|pages= 467–468|doi= 10.1002/anie.196804672}} to a substituted pentamethylcyclopentadiene prior to reaction with the hydrate of either iridium(III) chloride{{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= Chem. Commun.|issue= 21|pages= 1304–1305|doi= 10.1039/C19680001304}} or rhodium(III) chloride.{{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|bibcode= 1968JAChS..90.3259K}}

File:C5Me5Ir2Cl2viaMe6DewarBen.png

File:(t-Bu)3C5H3.png

Complexes of pentamethylcyclopentadienyl differ in several ways from the more common cyclopentadienyl (Cp) derivatives. Being more electron-rich, {{chem2|Cp\*-}} is a stronger donor and dissociation, like ring-slippage, is more difficult with Cp* than with Cp.{{Cite journal|last1=Kuwabara|first1=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}} The fluorinated ligand, (trifluoromethyl)tetramethylcyclopentadienyl ({{chem2|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.{{Cite journal|last1=Gassman|first1=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|bibcode=1992JAChS.114.6942G |issn=0002-7863}} 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 complexes". Bulky cyclopentadienyl ligands are known that are far more sterically encumbered than Cp*.

• Cyclopentadiene
• Methylcyclopentadiene

{{reflist}}

Category:Cyclopentadienes

Category:Ligands