Name: accessing valuable ligand supports for transition metals: a modified, intermediate scale preparation of 1,2,3,4,5-pentamethylcyclopentadiene
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Description: Watch this Scientific Journal Video about Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene at JoVE.com

We are grateful to the National Science Foundation (CHE-1300508) and Mount St. Mary's University (Startup and Summer Faculty Development) for generous support of this work. Ben Rupert (University of Delaware, Mass Spectrometry Facility) is acknowledged for LIFDI mass spectral analyses.

1. Synthesis of an Isomeric Mixture of 3,4,5-Trimethyl-2,5-heptadien-4-ols
<ol>
	<li>Fill an oven dried, 500 mL beaker with 200 mL of hexanes and cover with an oven dried watch glass.
	<ol>
		<li>In an empty hood, use clean scissors to cut half inch pieces of lithium wire. Wipe each lithium piece on a paper towel to remove excess mineral oil, until all oil appears to be removed from the metal&#39;s surface, and place in the beaker containing hexanes.</li>
		<li>Fill an oven dried, 250 mL beaker with 10...

<ol>
	<li>Kealy, T. J., Pauson, P. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+type+of+organo-iron+compound.">A new type of organo-iron compound.</a> Nature. 168, 1039-1040 (1951).</li><li>Wilkinson, G., Rosenblum, M., Whiting, M. C., Woodward, R. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+structure+of+iron+bis-cyclopentadienyl.">The structure of iron bis-cyclopentadienyl.</a> J. Am. Chem. Soc. 74, 2125-2126 (1952).</li><li>Fischer, E. O., Pfab, W. Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cyclopentadien-Metallkomplexe,+ein+neuer+Typ+metallorganischer+Verbindungen.">Cyclopentadien-Metallkomplexe, ein neuer Typ metallorganischer Verbindungen.</a> Z. Naturforsch. 76, 377-379 (1952).</li><li>Pauson, P. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ferrocene-how+it+all+began.">Ferrocene-how it all began.</a> J. Organomet. Chem. 637-639, 3-6 (2001).</li><li>Lauher, J. W., Hoffmann, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Structure+and+chemistry+of+bis(cyclopentadienyl)-MLn+complexes.">Structure and chemistry of bis(cyclopentadienyl)-MLn complexes.</a> J. Am. Chem. Soc. 98, 1729-1742 (1976).</li><li>Resa, I., Carmona, E., Gutierrez-Puebla, E., Monge, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Decamethyldizincocene,+a+stable+compound+of+Zn(I)+with+a+Zn-Zn+bond.">Decamethyldizincocene, a stable compound of Zn(I) with a Zn-Zn bond.</a> Science. 305, 1136-1138 (2004).</li><li>Brintzinger, H., Bercaw, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nature+of+so-called+titanocene,+(C10H10Ti)2.">Nature of so-called titanocene, (C10H10Ti)2.</a> J. Am. Chem. Soc. 92, 6182-6185 (1970).</li><li>King, R. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Some+applications+of+metal+carbonyl+anions+in+the+synthesis+of+unusual+organometallic+compounds.">Some applications of metal carbonyl anions in the synthesis of unusual organometallic compounds.</a> Acc. Chem. Res. 3, 417-427 (1970).</li><li>Chirik, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Group+4+transition+metal+sandwich+complexes:+still+fresh+after+almost+60+years.">Group 4 transition metal sandwich complexes: still fresh after almost 60 years.</a> Organometallics. 29, 1500-1517 (2010).</li><li>Bengali, A. A., Schultz, R. H., Moore, C. B., Bergman, R. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Activation+of+the+C-H+bonds+in+neopentane+and+neopentane-d12+by+(&#951;5-C5(CH3)5)Rh(CO)2:+Spectroscopic+and+temporal+resolution+of+rhodium-krypton+and+rhodium-alkane+complex+intermediates.">Activation of the C-H bonds in neopentane and neopentane-d12 by (&#951;5-C5(CH3)5)Rh(CO)2: Spectroscopic and temporal resolution of rhodium-krypton and rhodium-alkane complex intermediates.</a> J. Am. Chem. Soc. 116, 9585-9589 (1994).</li><li>Shima, T., Hu, S., Luo, G., Kang, X., Luo, Y., Hou, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dinitrogen+cleavage+and+hydrogenation+by+a+trinuclear+titanium+polyhydride+complex.">Dinitrogen cleavage and hydrogenation by a trinuclear titanium polyhydride complex.</a> Science. 340, 1549-1552 (2013).</li><li>Negishi, E. -. I., Takahashi, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Alkene+and+alkyne+complexes+of+zirconocene.+Their+preparation,+structure,+and+novel+transformations.">Alkene and alkyne complexes of zirconocene. Their preparation, structure, and novel transformations.</a> Bull. Chem. Soc. Jpn. 71, 755-769 (1998).</li><li>Rosenthal, U., Burlakov, V. V., Arndt, P., Baumann, W., Spannenberg, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+titancocene+complex+of+bis(trimethylsilyl)acetylene:+Synthesis,+structure,+and+chemistry.">The titancocene complex of bis(trimethylsilyl)acetylene: Synthesis, structure, and chemistry.</a> Organometallics. 22, 884-900 (2003).</li><li>Jordan, R. F., Bradley, P. K., LaPointe, R. E., Taylor, D. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cationic+zirconium+catalysts+for+carbon-carbon+bond+forming+chemistry.">Cationic zirconium catalysts for carbon-carbon bond forming chemistry.</a> New J. Chem. 14, 505-511 (1990).</li><li>Ewen, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Symmetry+rules+and+reaction+mechanisms+of+Ziegler-Natta+catalysts.">Symmetry rules and reaction mechanisms of Ziegler-Natta catalysts.</a> J. Mol. Catal. 128, 103-109 (1998).</li><li>Manriquez, J. M., Bercaw, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+of+a+dinitrogen+complex+of+bis(pentamethylcyclopentadienyl)zirconium(II).+Isolation+and+protonation+leading+to+the+stoichiometric+reduction+of+dinitrogen+to+hydrazine.">Preparation of a dinitrogen complex of bis(pentamethylcyclopentadienyl)zirconium(II). Isolation and protonation leading to the stoichiometric reduction of dinitrogen to hydrazine.</a> J. Am. Chem. Soc. 96, 6229-6230 (1974).</li><li>Brintzinger, H. H., Bercaw, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bis(pentamethylcyclopentadienyl)titanium(II).+Isolation+and+reactions+with+hydrogen,+nitrogen,+and+carbon+monoxide.">Bis(pentamethylcyclopentadienyl)titanium(II). Isolation and reactions with hydrogen, nitrogen, and carbon monoxide.</a> J. Am. Chem. Soc. 93, 2045-2046 (1971).</li><li>Lehman, M. C., Gary, J. B., Boyle, P. D., Sanford, M. S., Ison, E. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+solvent+and+ancillary+ligands+on+the+catalytic+H/D+exchange+reactivity+of+Cp*IrIII(L)+complexes.">Effect of solvent and ancillary ligands on the catalytic H/D exchange reactivity of Cp*IrIII(L) complexes.</a> ACS Catal. 3, 2304-2310 (2013).</li><li>Pitman, C. L., Finster, O. N. L., Miller, A. J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cyclopentadiene-mediated+hydride+transfer+from+rhodium+complexes.">Cyclopentadiene-mediated hydride transfer from rhodium complexes.</a> Chem. Commun. 52, 9105-9108 (2016).</li><li>Tarantino, K. T., Miller, D. C., Callon, T. A., Knowles, R. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bond-weakening+catalysis:+Conjugate+aminations+enabled+by+the+soft+homolysis+of+strong+N-H+bonds.">Bond-weakening catalysis: Conjugate aminations enabled by the soft homolysis of strong N-H bonds.</a> J. Am. Chem. Soc. 137, 6440-6443 (2015).</li><li>Bolig, A. D., Brookhart, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Activation+of+sp3+C-H+bonds+with+cobalt(I):+Catalytic+synthesis+of+enamines.">Activation of sp3 C-H bonds with cobalt(I): Catalytic synthesis of enamines.</a> J. Am. Chem. Soc. 129, 14544-14545 (2007).</li><li>Hung-Low, F., Tye, J. W., Cheng, S., Bradley, C. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=sp2+C-H+activation+of+dimethyl+fumarate+by+a+[(Cp*Co)2-&#181;-(&#951;4:&#951;4-toluene)]+complex.">sp2 C-H activation of dimethyl fumarate by a [(Cp*Co)2-&#181;-(&#951;4:&#951;4-toluene)] complex.</a> Dalton Trans. 41 (26), 8190-8197 (2012).</li><li>Hung-Low, F., Krogman, J. P., Tye, J. W., Bradley, C. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+more+labile+low+electron+count+Co(I)+sources:+mild,+catalytic+functionalization+of+activated+alkanes+using+a+[(Cp*Co)2-&#181;-(&#951;4:&#951;4-arene)]+complex.">Development of more labile low electron count Co(I) sources: mild, catalytic functionalization of activated alkanes using a [(Cp*Co)2-&#181;-(&#951;4:&#951;4-arene)] complex.</a> Chem. Commun. 48 (3), 368-370 (2012).</li><li>Andjaba, J. M., Tye, J. W., Yu, P., Pappas, I., Bradley, C. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cp*Co(IPr):+synthesis+and+reactivity+of+an+unsaturated+Co(I)+complex.">Cp*Co(IPr): synthesis and reactivity of an unsaturated Co(I) complex.</a> Chem. Commun. 52, 2469-2472 (2016).</li><li>Fendrick, C. M., Schertz, L. D., Mintz, E. A., Marks, T. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Large-scale+synthesis+of+1,2,3,4,5-pentamethylcyclpentadiene.">Large-scale synthesis of 1,2,3,4,5-pentamethylcyclpentadiene.</a> Inorg. Synth. 29, 193-198 (1992).</li><li>Threlkel, R. S., Bercaw, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Convenient+synthesis+of+alkyltetramethylcyclopentadienes+and+phenyltetramethylcyclopentadiene.">A Convenient synthesis of alkyltetramethylcyclopentadienes and phenyltetramethylcyclopentadiene.</a> J. Organomet. Chem. 136, 1-5 (1977).</li><li>Manriquez, J. M., Fagan, P. J., Schertz, L. D., Marks, T. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=1,2,3,4,5-pentamethylcyclopentadiene.">1,2,3,4,5-pentamethylcyclopentadiene.</a> Inorg Synth. 28, 317-320 (1990).</li><li>Threlkel, R. S., Bercaw, J. E., Seidler, P. F., Stryker, J. M., Bergman, R. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=1,2,3,4,5-pentamethylcyclopentadiene.">1,2,3,4,5-pentamethylcyclopentadiene.</a> Org. Synth. 65, (1987).</li><li>Koelle, U., Kossakowski, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Di-&#181;-chloro-bis[(&#951;5-pentachlororuthenium(III)],+[Cp*RuCl2]2+and+Di-&#181;-methoxo-bis(&#951;5-pentamethylcyclomethylcyclopentadienyl)+diruthenium(II),+[Cp*RuOMe]2.">Di-&#181;-chloro-bis[(&#951;5-pentachlororuthenium(III)], [Cp*RuCl2]2 and Di-&#181;-methoxo-bis(&#951;5-pentamethylcyclomethylcyclopentadienyl) diruthenium(II), [Cp*RuOMe]2.</a> Inorg. Synth. 29, 225-228 (1992).</li><li>White, C., Yates, A., Maitlis, P. 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During preparation of the heptadienol mixture, it is important to clean the lithium prior to initiating the reaction with the 2-bromo-2-butene. This is accomplished by wiping off residual mineral oil used for storage on paper towels, to the point that the oil appears fully removed from the surface, and by dissolving any remaining oil in the beaker of hexanes. The hexanes were used as received and not further dried before use in the procedure. Because of both the large scale of the reaction and an excess of lithium used, ...

Since the discovery and structural elucidation of ferrocene in the 1950s,1,2,3,4 cyclopentadienyl (Cp) substituted ligands have played a vital role in the development of organometallic chemistry. These ligands have served as versatile ancillary supports for a range of metals, leading to studies of unusual structure and bonding,5,6,7 the activation and functionalization of small molecules,8,9,10,11,12,13 and catalysis, including olefin polymerization.14,15
The 1,2,3,4,5-pentamethylcyclopentadienyl (Cp*) anion has proven to be a particularly valuable ligand in transition and main group metal chemistry, as the methyl groups impart greater steric protection, increased electron donation by the anionic ligand, and block potential activation of the cyclopentadienyl ring.16,17 The Cp* ligand remains relevant even today, as the anion has recently been utilized to support H/D exchange by Ir(III),18 hydride transfer by Rh,19 and conjugate aminations mediated by Ti(III).20
Our interest in the Cp* ligand stems from the desire to access reactive sources of cobalt(I) for use in small molecule activation.21 These studies have resulted in the generation of both Cp*CoI and Cp*CoIL (L = N-heterocyclic carbene) equivalents for use in sp3 and sp2 C-H bond oxidative addition.22,23,24 As access to our Cp*Co(II) starting materials necessitate significant quantities of 1,2,3,4,5-pentamethylcyclopentadiene, we desired a multigram synthesis of Cp*H, given the substantial commercial cost of the ligand.
Two major methods currently exist for the large scale preparation of Cp*H, each of which presents inherent technical challenges. A procedure developed by Marks and coworkers involves a two-step synthesis of 2,3,4,5-tetramethylcyclopent-2-enone followed by installation of the final methyl group using methyl lithium.25 The synthesis is described on a massive scale, using a 12 L reaction vessel and mechanical stirring, while also requiring sustained low temperature cooling at 0 &#176;C for four days.
An alternative procedure originally developed by Bercaw and coworkers,26 and later adapted by Marks,27 utilizes in situ generation of an alkenyl lithium for nucleophilic attack of ethyl acetate to produce an isomeric mixture of 3,4,5-trimethyl-2,5-heptadien-4-ols followed by acid mediated cyclization to provide Cp*H. The initial reports of this method were performed on a large (3-5 L) scale and required mechanical stirring. In addition, a significant excess of lithium metal was used, complicating quenching and subsequent workup of the intermediate heptadienols. A subsequent revision of the procedure scales down the reaction and the amount of lithium,28 but safe quenching of the reaction mixture remains an issue. Reproducibility in the initiation of the alkenyl lithium, due to differences in lithium source and purity or dryness of the 2-bromo-2-butene reactant are further noted concerns. Given these issues with the commonly used procedures for preparing Cp*H, we looked to develop better access to the ligand on an intermediate scale (30-40 g) which would circumvent use of specialty laboratory glassware and equipment, improve reaction reproducibility and safety, and simplify workup and ligand purification.
Here we report that synthesis of 1,2,3,4,5-pentamethylcyclopentadiene, based on modifications of the existing procedure developed by Bercaw and coworkers. The revised synthesis and purification of the ligand accomplishes the major goals outlined above, while permitting access to substantial amounts (39 g) of Cp*H in good yield (58%). The procedure offers other additional benefits, including a more controlled quench of excess lithium during the production of the intermediate heptadienols and a simplified isolation of Cp*H of adequate purity for subsequent metallation with transition metals. To demonstrate the utility of the prepared ligand, it was used to synthesize two [Cp*MCl2]2 (M = Ir, Ru) complexes. The revised protocol outlined below complements existing procedures and provides a simpler and more accessible entry point into the chemistry of a ubiquitous ancillary ligand support in organometallic chemistry.

<table border="0" cellpadding="0" cellspacing="0" width="573">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Materials</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Lithium wire (in mineral oil)</td>
			<td style="width:71px;">Aldrich</td>
			<td>278327-100G</td>
			<td style="width:167px;">&#62;98%</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">2-bromo-2-butene (mixture of cis/trans isomers)</td>
			<td style="width:71px;">Acros</td>
			<td>200016-364&#160;&#160;</td>
			<td style="width:167px;">98%, dried over molecular sieves from an oven overnight before use</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Hexanes</td>
			<td style="width:71px;">Millipore</td>
			<td style="width:119px;">HX0299-3</td>
			<td style="width:167px;">GR ACS, used as received</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">Ethyl actetate</td>
			<td style="width:71px;">Millipore</td>
			<td style="width:119px;">EX0240-3</td>
			<td style="width:167px;">GR ACS, dried over molecular sieves from an oven overnight before use</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Ammonium chloride</td>
			<td style="width:71px;">Aldrich</td>
			<td style="width:119px;">213330-2.5kg</td>
			<td style="width:167px;">ACS Reagent</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Diethyl ether</td>
			<td style="width:71px;">Millipore</td>
			<td style="width:119px;">EX0190-5</td>
			<td style="width:167px;">GR ACS, collected from a solvent purification system before use</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Magnesium sulfate</td>
			<td style="width:71px;">Aldrich</td>
			<td style="width:119px;">793612-500g</td>
			<td style="width:167px;">Anhydrous, reagent grade</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">p-toluene sulfonic acid monohydrate</td>
			<td style="width:71px;">Fisher</td>
			<td style="width:119px;">A320-500</td>
			<td style="width:167px;">ACS Certified</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sodium bicarbonate</td>
			<td style="width:71px;">Fisher</td>
			<td style="width:119px;">5233-500</td>
			<td style="width:167px;">ACS Certified</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sodium carbonate</td>
			<td style="width:71px;">Amresco</td>
			<td style="width:119px;">0585-500g</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Ruthenium (III) chloride trihydrate</td>
			<td style="width:71px;">Pressure Chemical</td>
			<td style="width:119px;">4750</td>
			<td style="width:167px;">40% Metal</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Iridium (III) chloride hydrate</td>
			<td style="width:71px;">Pressure Chemical</td>
			<td style="width:119px;">5730</td>
			<td style="width:167px;">53% Metal</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Methanol</td>
			<td style="width:71px;">Avantor</td>
			<td style="width:119px;">3016-22</td>
			<td style="width:167px;">AR ACS, distilled from Mg before use</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Pentane</td>
			<td style="width:71px;">J. T. Baker</td>
			<td style="width:119px;">T007-09</td>
			<td style="width:167px;">&#62;98%, dried with a solvent purification system before use</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Chloroform-d</td>
			<td style="width:71px;">Aldrich</td>
			<td style="width:119px;">151823-150G</td>
			<td style="width:167px;">99.8 atom % D</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Molecular sieves 4&#197;&#160;</td>
			<td style="width:71px;">Aldrich</td>
			<td style="width:119px;">208590-1KG</td>
			<td style="width:167px;">dried in an oven at 140 &#176;C before use&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Celite 545</td>
			<td style="width:71px;">Acros</td>
			<td style="width:119px;">AC34967-0025</td>
			<td style="width:167px;">dried in an oven at 140 &#176;C before use&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Name</td>
			<td style="width:71px;">Company</td>
			<td style="width:119px;">Catalog Number</td>
			<td style="width:167px;">Comments</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Equipment</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Schlenk line, with vacuum and inert gas manifolds</td>
			<td style="width:71px;">Custom</td>
			<td style="width:119px;">NA</td>
			<td>Used in Preps 1-4</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Solvent transfer manifold</td>
			<td style="width:71px;">Chemglass</td>
			<td style="width:119px;">AF-0558-01</td>
			<td style="width:167px;">Used in 2.2</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Airfree filter funnel</td>
			<td style="width:71px;">Chemglass</td>
			<td style="width:119px;">AF-0542-22</td>
			<td style="width:167px;">Used in 3.1.3</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;">Glovebox</td>
			<td style="width:71px;">Vacuum Atmospheres</td>
			<td style="width:119px;">OMNI</td>
			<td style="width:167px;">Used in 3.2.2</td>
		</tr>
	</tbody>
</table>

accessing valuable ligand supports for transition metals: a modified, intermediate scale preparation of 1,2,3,4,5-pentamethylcyclopentadiene

A reliable, intermediate scale preparation of 1,2,3,4,5-pentamethylcyclopentadiene (Cp*H) is presented, based on modifications of existing protocols that derive from initial 2-bromo-2-butene lithiation followed by acid mediated dienol cyclization. The revised synthesis and purification of the ligand avoids the use of mechanical stirring while still permitting access to significant quantities (39 g) of Cp*H in good yield (58%). The procedure offers other additional benefits, including a more controlled quench of excess lithium during the production of the intermediate heptadienols and a simplified isolation of Cp*H of sufficient purity for metallation with transition metals. The ligand was subsequently used to synthesize [Cp*MCl2]2 complexes of both iridium and ruthenium to demonstrate the utility of the Cp*H prepared and purified by our method. The procedure outlined herein affords substantial quantities of a ubiquitous ancillary ligand support used in organometallic chemistry while minimizing the need for specialized laboratory equipment, thus providing a simpler and more accessible entry point into the chemistry of 1,2,3,4,5-pentamethylcyclopentadiene.

The protocol described above for Cp*H synthesis relies on modification of the three step procedure developed by Bercaw and coworkers and modified by Marks (Figure 1). The air sensitive alkenyl lithium is prepared in situ from a mixture of cis and trans-2-butene via a lithium/halogen exchange reaction and is subsequently quenched with ethyl acetate to prepare an isomeric mixture of heptadienols. The mixture can be used without further purificatio...

Watch this Scientific Journal Video about Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene at JoVE.com

Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene

Reliable, intermediate scale preparation of 1,2,3,4,5-pentamethylcyclopentadiene (Cp*H) is presented. The revised protocol for the synthesis and purification of the ligand minimizes the need for specialized laboratory equipment while simplifying reaction workups and product purification. Use of Cp*H in the synthesis of [Cp*MCl2]2 complexes (M = Ru, Ir) is also described.

A reliable, intermediate scale preparation of 1,2,3,4,5-pentamethylcyclopentadiene (Cp*H) is presented, based on modifications of existing protocols that ...

The overall goal of this procedure is to provide a detailed and updated method for intermediate scale synthesis of 1, 2, 3, 4, 5-Pentamethylcyclopentadiene, or CP star H.This method improves upon prior preparations of CP star H, a valuable ligand in in organo-metallic chemistry, by eliminating the need for mechanical stirring and by providing a safer quenching of excess lithium. The main advantage of this technique is that the CP star H can be prepared in significant quantities, in a safe manor, using minimal amount of specialized glassware and equipment. Visual demonstration of this method is important as the reagents used in the procedure are air and water reactive at certain steps, and because controlled quenching of excess lithium is necessary.To begin the procedure, in a clean empty hood, place 200 milliliters of hexanes in a beaker, and cover the beaker with a watch glass. Place a piece of paper towel in the hood, then cut a half inch piece of lithium wire, stored in mineral water, with clean scissors. Wipe the lithium wire piece on the paper towel until no oil is visible on the wire surface.Place the lithium wire in the beaker of hexanes. Cut approximately 14 grams in this way, then tare a covered beaker containing 100 milliliters of hexanes. Place a fresh piece of paper towel in the hood, using tongs remove a piece of lithium wire from the larger beaker of hexanes.Quickly wipe the piece on the clean paper towel, and place the piece in the tared beaker. Transfer 14 grams of lithium wire into the tared beaker in this way. Stir any excess lithium in mineral oil.Connect a one liter, three neck flask, equipped with a stir bar to a Schlenk line. Open the flask to a constant flow of Argon gas, and use a heat gun to heat the flask for two to three minutes. And then allow the glass to cool to room temperature.Use tongs to transfer the lithium pieces to the flask under a counter flow of Argon. Wiping each piece on a paper towel before addition. Close the reaction flask to air.Thoroughly rinse the scissors and tongs with water in an empty sink. Dry the scissors and tongs with paper towels. And submerge the towels in a half filled bowl of water in the sink.Collect any paper towels and gloves used to cut or handle lithium, and immerse them in the bowl of water in the sink. In an empty hood, slowly poor the hexanes from both beakers into a fresh beaker of water. Slowly add water to the empty beakers to quench any residual lithium.Use water and paper towels to clean the lithium preparation area. Immerse the paper towels in water in the sink before proceeding. Once lithium is in the flask, and the lithium preparation area is cleaned, draw 500 milliliters of diethyl ether from a solvent purification system.Use cannula transfer, to add diethyl ether to the lithium containing reaction flask. Connect an addition funnel with an inert gas sidearm, and a reflux condenser to the reaction flask. Use a syringe to transfer 29 grams of dry 2-bromo-2-butene to the addition funnel.Add three to four milliliters of 2-bromo-2-butene drop wise over 5 minutes, while stirring, to initiate the reaction. As indicated by a gentle reflux commencing. If after 10 to 15 minutes of stirring, a gentle reflux is not observed, carefully warm the reaction flask with a heat gun for two to three minutes until reflux is achieved.Once the reaction has been initiated, resume drop wise addition of 2-bromo-2-butene over 90 minutes maintaining a moderate reflux throw. Next use a syringe to load a mixture of 40 grams of ethyl-acetate into 103.85 grams of 2-bromo-2-butene into the addition funnel. Continuously dispense the mixture into the reaction flask over the course of four hours to maintain controlled vigorous reflux.On the next day replace the addition funnel with a rubber septum. Obtain a wash bottle containing diethyl ether, and place 250 milliliters of water in a beaker. With an Argon counter flow in place, remove the septum.Use tongs to remove one piece of unreactive lithium wire from the reaction flask. Quickly rinse the wire and tongs with diethyl ether, and allow the rinse to drip into the flask. Quench the lithium piece in the water, and stop for the reaction flask.Once the lithium piece has fully reacted, repeat the process until each large piece of unreacted lithium has been removed and quenched. Then draw 60 milliliters of a saturated ammonium chloride solution into a syringe. Attach needle to the syringe.Pierce the septum and add a total of 250 milliliters of the ammonium chloride solution over a period of 60 to 90 minutes. Then allow the mixture to cool for 30 minutes. Use a separatory funnel to isolate the diethyl ether layer.Then extract the aqueous layer with 100 milliliter portions of diethyl ether three times. Combine and concentrate the diethyl ether portions to obtain the isomeric mixture of heptadienals and diethyl ether. To begin the synthesis, under a flow of inert gas add 8.70 grams of Para-Toluenesulfonic acid monohydrate, to a 500 milliliter three neck round bottom flask, equipped with a reflux condenser and a stir bar.Add 50 milliliters of diethyl ether by cannula transfer, and begin stirring the mixture. Charge an addition funnel with a mixture heptadienals and diethyl ether prepared from the previous section, and add it drop wise to the stirring reaction mixture over the course of an hour. Once the crude product is obtained, connect a transfer manifold, and a 100 milliliter receiving round bottom flask to the crude product flask.Secure the receiving flask in an ice bath, and the crude product flask in a room temperature water bath on a stir plate. Apply a dynamic vacuum while stirring for 30 to 60 minutes to remove residual diethyl ether. Then seal the transfer set up to establish static vacuum.Place the receiving flask in a dry ice and acetone bath, and the crude product flask in a warm water bath. Allow trap to trap distillation to proceed while stirring for two to three hours, refreshing the vacuum and baths as needed to obtain CP star H as a pale yellow oil. Using this method, CP star H was synthesized and isolated in an intermediate scale, in 50 to 60 percent yield.Excess lithium wire, with the lithium halogen exchange was removed and quenched outside the reaction vessel following synthesis of the intermediate isomeric mixture of heptodienals. The product was isolated with sufficient purity to use as a starting material in direct metallation reactions. Ruthenium chloride trihydrate was refluxed with the CP star H product in methanol, to obtain the bridging ruthenium three halide complex in moderate yield.The proton NMR specturm of the isolated paramagnetic product was consistent with literature values. A bridging iridium halide complex was prepared in moderate yield by a similar method. Proton and carbon NMR spectra of the idolated diamagnetic product were consistent with previous literature values.While attempting this procedure, it's important to remember to carefully quench the excess lithium, or materials that have come into contact with they alkali metal, and to perform the lithiation step under Argon. After watching this video you should have a good understanding of how to prepare 1, 2, 3, 4, 5-Pentamethylcyclopentadiene on a large scale, and how to use the ligand in the preparation of CP star supportive transition metal complexes.

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