Pentamethylcyclopentadiene

Structural formula
General
Surname	Pentamethylcyclopentadiene
Molecular formula	C 10 H 16
Brief description	colorless to yellowish liquid
External identifiers / databases
CAS number 4045-44-7 EC number 223-743-4 ECHA InfoCard 100,021,586 PubChem 77667 Wikidata Q1927394
properties
Molar mass	136.24 g mol −1
Physical state	liquid
density	0.84 g cm −3 (20 ° C)
boiling point	58 ° C (17 hPa)
Refractive index	1.474 (20 ° C, 589 nm)
safety instructions
GHS labeling of hazardous substances Caution H and P phrases H: 226 P: 210
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic alkene or a five-fold methylated derivative of cyclopentadiene (Cp). 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor of the ligand 1,2,3,4,5-pentamethylcyclopentadienyl (C ₅ Me ₅ ^- ), which is often abbreviated as Cp *; the star stands for the five methyl groups arranged radially. Cp * is an important reagent in organometallic chemistry. In contrast to less substituted cyclopentadiene derivatives, pentamethylcyclopentadiene does not tend to dimerize .

synthesis

Pentamethylcyclopentadiene is commercially available. For the first time it was prepared from trans -2-methyl-2-butenal via 2,3,4,5-tetramethylcyclopent-2-enone.

Alternatively, 2-butenyllithium can be reacted with ethyl acetate , followed by an acid-catalyzed dehydrocyclization:

2 MeCH = C (Li) Me + MeC (O) OEt → (MeCH = C (Me)) ₂ C (OLi) Me + LiOEt

(MeCH = C (Me)) ₂ C (OLi) Me + H ⁺ → Cp * H + H ₂ O + Li ⁺

Organometallic derivatives

Pentamethylcyclopentadiene is an important precursor for organometallic compounds. The anionic form pentamethylcyclopentadienyl is suitable as a ligand because the five carbon atoms in the ring are bonded to the metal atom.

Synthesis of Cp * complexes

Representative Cp * metal complexes
Cp * 2 Fe	yellow
Cp * TiCl 3	red
[Cp * Fe (CO) 2 ] 2	red-purple
[Cp * RhCl 2 ] 2	red
[Cp * IrCl 2 ] 2	orange
Cp * Re (CO) 3	colorless
Cp * Mo (CO) 2 CH 3	orange

Some exemplary reactions for the synthesis of common Cp * metal complexes (see table) are:

${\displaystyle {\ce {Cp^{\ast }H{+}C4H9Li->Cp^{\ast }Li{+}C4H10}}}$

${\displaystyle {\ce {Cp^{\ast }Li{+}TiCl4->Cp^{\ast }TiCl3{+}LiCl}}}$

${\displaystyle {\ce {2Cp^{\ast }H{+}2Fe(CO)5->{[Cp^{\ast }Fe(CO)2]2}{+}H2}}}$

An obsolete method used in teaching for the synthesis of Cp * complexes uses hexamethyl- Dewar-benzene . This method has traditionally been used to prepare chlorine-bridged [Cp * IrCl ₂ ] ₂ and [Cp * RhCl ₂ ] ₂ dimers. The synthesis is based on a hydrohalic acid -induced rearrangement of hexamethyl-Dewar-benzene to a substituted pentamethylcyclopentadiene and the subsequent reaction with iridium (III) chloride hydrate or rhodium (III) chloride hydrate.

Comparison between Cp * and Cp

Pentamethylcyclopentadienyl complexes differ in several respects from the more common cyclopentadienyl derivatives. As a more electron-rich ligand, Cp * is a stronger donor and less easy to remove from the central metal atom. This results in a higher thermal stability of Cp * complexes. Its steric demand enables the isolation of complexes with fragile ligands, reduces intermolecular interactions and reduces the tendency towards polymeric structures. As a rule, Cp * complexes are also very soluble in non-polar solvents.

Individual evidence

1. ↑ Entry on 1,2,3,4,5-pentamethylcyclopentadiene at ChemicalBook , accessed on January 20, 2014.
2. ↑ ^{a b c d e} data sheet 1,2,3,4,5-pentamethylcyclopentadiene (PDF) from Merck , accessed on December 29, 2012.
3. ↑ Overview of Cp * Compounds: Elschenbroich, C. and Salzer, A. Organometallics: a Concise Introduction (1989) p. 47
4. ↑ L. de Vries: 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 . In: J. Org. Chem. . 25, No. 10, 1960, p. 1838. doi : 10.1021 / jo01080a623 .
5. ↑ S. Threlkel, JE Bercaw, PF Seidler, JM Stryker, RG Bergman: 1,2,3,4,5-pentamethyl cyclopentadienes In: Organic Synthesis . 65, 1987, p. 42, doi : 10.15227 / orgsyn.065.0042 ; Coll. Vol. 8, 1993, p. 505 ( PDF ).
6. ↑ Fendrick, CM; Schertz, LD; Mintz, EA; Marks, TJ: Large-Scale Synthesis of 1,2,3,4,5-Pentamethylcyclopentadienes . In: Inorganic Syntheses . 29, 1992, pp. 193-198. doi : 10.1002 / 9780470132609.ch47 .
7. ↑ Yamamoto, A. Organotransition Metal Chemistry: Fundamental Concepts and Applications. (1986) p. 105
8. ↑ RB King, MB Bisnette: Organometallic chemistry of the transition metals XXI. Some π-pentamethylcyclopentadienyl derivatives of various transition metals . In: Journal of Organometallic Chemistry . 8, No. 2, 1967, pp. 287-297. doi : 10.1016 / S0022-328X (00) 91042-8 .
9. ↑ LA Paquette, GR Krow: Electrophilic Additions to Hexamethyldewarbenzene . In: Tetrahedron Lett. . 9, No. 17, 1968, pp. 2139-2142. doi : 10.1016 / S0040-4039 (00) 89761-0 .
10. ↑ R. Criegee, H. Gruner: Acid-catalyzed Rearrangements of Hexamethyl-prismane and Hexamethyl-Dewar-benzene . In: Angew. Chem. Int. Ed. Engl . 7, No. 6, 1968, pp. 467-468. doi : 10.1002 / anie.196804672 .
11. ↑ JW Kang, K. Mosley, PM Maitlis: Mechanisms of Reactions of Dewar Hexamethylbenzene with Rhodium and Iridium Chlorides . In: Chem. Commun. . No. 21, 1968, pp. 1304-1305. doi : 10.1039 / C19680001304 .
12. ↑ JW Kang, PM Maitlis: Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium (III) Chloride . In: J. Am. Chem. Soc. . 90, No. 12, 1968, pp. 3259-3261. doi : 10.1021 / ja01014a063 .
13. ^ BD Gupta, AJ Elias: Basic Organometallic Chemistry . Universities Press, Hyderabad 2010, ISBN 978-1-4398-4968-2 , pp. 101-102 .

See also

• Cyclopentadiene (Cp)
• List of ligand abbreviations