Method Article

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

DOI:

10.3791/55366

⸱

March 20th, 2017

 ,  , 
1Department of Science, Mount St. Mary's University

In This Article

Summary

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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.

Abstract

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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.

Introduction

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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 °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.

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Protocol

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1. Synthesis of an Isomeric Mixture of 3,4,5-Trimethyl-2,5-heptadien-4-ols

  1. Fill an oven dried, 500 mL beaker with 200 mL of hexanes and cover with an oven dried watch glass.
    1. 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's surface, and place in the beaker containing hexanes.
    2. Fill an oven dried, 250 mL beaker with 100 mL of hexanes and cover with an oven dried watch glass. Tare this setup on a balance.
    3. Using oven dried tongs, remove the lithium wire from the first beaker, wipe quickly on a clean paper towel to remove any residual oil or hexanes, and transfer to the tared beaker. Repeat this process until 14.0 g (2.03 mol) of lithium wire have been added to the beaker.
  2. Attach an oven-dried, 1 L three-neck flask equipped with a stir bar to a Schlenk line under a flow of argon in a fume hood. Heat gun the flask for a few minutes and allow it to cool under argon.
    Caution: It is important to use argon to prevent a potential reaction between lithium and dinitrogen to form lithium nitride.
    1. Once cooled, use tongs to quickly wipe the lithium pieces on a paper towel and then transfer the lithium into the flask under an argon counter flow.
    2. Submerge any excess lithium strips in mineral oil for storage. Rinse the tongs and scissors with copious amounts of water in an empty sink. Dry each with a paper towel and submerge the towels in water in the sink.
      NOTE: No solvent bottles or paper towels should be near the sink.
      1. Immerse any paper towels or gloves that have been used to cut lithium in a half-filled bowl of water and place in an empty sink. In an empty hood, slowly pour the hexanes from the beakers in 1.1-1.1.3 into a bowl containing water. Add water carefully into the empty beakers to quench any residual lithium.
      2. Rinse and wipe down the hood where the lithium was cut with water before continuing with the rest of the procedure. Submerge these paper towels in water in the sink before continuing.
    3. Add approximately 500 mL of diethyl ether, collected directly from a solvent purification system, to the reaction flask via cannula transfer.
    4. Attach an oven dried reflux condenser, an addition funnel equipped with an inert gas sidearm, and a glass stopper to the three neck flask. Place a plastic bowl underneath the reaction setup in case of flask rupture.
    5. Add 2-bromo-2-butene (29.0 g, 0.218 mol), which has been dried over 4 Å molecular sieves overnight, to the addition funnel via syringe.
    6. Add approximately 3-4 mL of the 2-bromo-2-butene dropwise over the course of 5 minutes to initiate the reaction, whereupon reflux occurs and the solution becomes cloudy. If gentle reflux does not occur on its own after 10-15 minutes of stirring, then warm the flask gently with a heat gun for two-three minutes until reflux is achieved. This is usually adequate to initiate the reaction.
      Caution: Even if the reaction does not initiate immediately, controlled addition of 2-bromo-2-butene is critical to keep the reaction under control.
    7. Add the remainder of the 2-bromo-2-butene in the addition funnel dropwise over the course of 90 minutes at a rate that maintains moderate reflux throughout the time period. The reaction mixture will turn a light green color.
    8. Syringe a mixture of ethyl acetate (40.0 g, 0.454 mol), which has been dried over 4 Å molecular sieves overnight, and 2-bromo-2-butene (103.85 g, 0.782 mol) into the addition funnel. Add this mixture to the reaction at a sufficient rate to maintain a controlled but vigorous reflux over the course of four hours.
    9. After addition is complete, syringe a small amount of ethyl acetate (2.71 g, 0.0308 mol) into the addition funnel and add dropwise to the reaction over 5 minutes. The mixture will be a cloudy orange/yellow color by the end of these additions. Allow the reaction to stir overnight at ambient temperature.
  3. The next day, replace the addition funnel with a rubber septum.
    1. Under an argon counter flow, remove the septum and use dry tongs to grab a piece of the unreacted lithium. To recover as much product as possible, rinse the lithium wire quickly with diethyl ether from a wash bottle and allow the solution to drip back into the reaction vessel. Then add the lithium piece to a 500 mL beaker filled with 250 mL of water.
      Caution: The lithium pieces should be added individually to the beaker and the strip should be allowed to fully react before another is added. This minimizes the risk of fire given the significant amount of H2 generated during the quenching process.
      NOTE: Other methods to quench the excess lithium, such as careful addition to isopropanol, also work but require significantly longer times to fully consume the excess lithium.
    2. Repeat 1.3.1 until all large pieces of lithium are quenched, which should require 60-90 minutes on the given scale.
    3. While still under argon, add a saturated ammonium chloride solution (250 mL) slowly dropwise to the stirring reaction mixture over the course of an hour, quenching the reaction mixture and any small pieces of unreacted lithium. The reaction may reflux gently during the process and become more difficult to stir based on solid formation. Continue addition until the layer of sediment completely dissolves at the bottom of the flask, forming a defined aqueous layer.
    4. Allow the reaction to cool for 30 min after addition of the solution is complete.
    5. Pour the contents of the flask into a 1 L separatory funnel and isolate the diethyl ether layer.
    6. Extract the aqueous layers with three 150 mL portions of diethyl ether.
    7. Combine all four diethyl ether layers and rotavap to a volume of approximately 75-100 mL.

2. Synthesis of 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*H)

  1. Attach an oven dried 500 mL three neck round bottom flask, equipped with a reflux condenser and stir bar, to a Schlenk line under a flow of inert gas.
    1. Charge the flask with p-toluene sulfonic acid monohydrate (8.70 g, 0.0457 mol).
    2. Cannula approximately 50 mL of diethyl ether onto the solid.
    3. Under inert gas counter flow, attach an oven dried addition funnel to the reaction setup.
    4. Charge the funnel with the 3,4,5-trimethyl-2,5-heptadien-4-ols/diethyl ether concentrate from Step 1.
    5. Add the concentrate dropwise to the stirring slurry, maintaining a gentle reflux, over the course of 1 h. Allow the reaction to stir for an additional 1 h after the addition is complete.
    6. Pour the reaction mixture into 300 mL of a saturated sodium bicarbonate solution containing 4.59 g (0.0433 mol) of sodium carbonate.
    7. Transfer the contents of the flask into a 1 L separatory funnel and isolate the diethyl ether layer. Extract the aqueous layer with three, 100 mL portions of diethyl ether.
    8. Combine the four diethyl ether layers and dry over magnesium sulfate for two hours.
    9. Filter the mixture and rotavap down the liquid until ~75-100 mL of material remains. At this stage, the concentrate can be stored in a refrigerator overnight prior to purification the following day.
  2. Transfer the concentrate to an oven dried, round bottom flask equipped with a stir bar. Connect this flask to an oven dried transfer manifold equipped with another 100 mL round bottom flask.
    1. Cool the receiving flask in ice while submerging the concentrate in an ambient temperature water bath.
    2. Apply dynamic vacuum to the setup, while stirring the concentrate, for 30-60 min to remove any residual diethyl ether.
    3. Seal the transfer setup and switch the bath for the receiving flask to a dry ice/acetone slurry while submerging the concentrate in a warm water bath. Allow the trap-to-trap distillation to continue while stirring over the course of 2-3 h, being careful to refresh the vacuum periodically (every 15-20 min) while the baths are also monitored over the same time period.
      NOTE: After transfer, 39.0 g (0.286 moles, 58% based on 2-bromo-2-butene) of Cp*H can be isolated as a pale yellow oil.

3. Synthesis of [Cp*RuCl2]229

  1. Charge 1.00 g (3.95 mmol) of ruthenium(III) chloride trihydrate in an oven dried 100 mL Schlenk flask equipped with a stir bar under a flow of inert gas on a Schlenk line.
    1. Cannula transfer 50 mL of methanol, dried over Mg and distilled prior to use, onto the solid and stir for 30 min.
    2. Filter the solution under inert gas through an oven dried air free filter funnel, filled with an inch of oven dried Celite, into another 100 mL Schlenk flask equipped with a stir bar.
    3. Remove the air-free filter funnel and replace with a septum.
    4. Add Cp*H (1.20 g, 8.81 mmol) by syringe, attach an oven dried reflux condenser equipped with an inert gas sidearm adapter under a flow of inert gas.
    5. Seal the sidearm on the Schlenk flask and reflux for 4 h under a flow of inert gas.
  2. Allow the reaction to cool and replace the reflux condenser with a glass stopper under a flow of inert gas.
    1. Remove the solvent in vacuo and dry on a Schlenk line for three hours. Transfer the flask to a glovebox.
    2. Inside the glovebox, rinse the solid with pentane and scrape the solid from the sides of the flask. Filter the black solid onto a medium porosity frit, rinse with 50 mL of pentane, and dry for an additional hour in vacuo.
      Note: [Cp*RuCl2]2 (1.53 g, 63% based on Ru) is isolated as a dark solid. Characterization matches literature reports.29

4. Synthesis of [Cp*IrCl2]230

  1. Charge 1.00 g (2.60 mmol) of iridium(III) chloride hydrate in an oven dried 100 mL Schlenk flask equipped with a stir bar under a flow of inert gas on a Schlenk line.
    1. Cannula transfer 50 mL of methanol onto the solid.
    2. Add Cp*H (0.50 g, 3.67 mmol) by syringe, attach an oven dried reflux condenser equipped with an inert gas sidearm adapter under a flow of inert gas.
    3. Seal the sidearm on the Schlenk flask and reflux for 48 h under inert gas flow.
  2. Allow the reaction to cool and replace the reflux condenser with a septum under a flow of inert gas.
    1. Cool the flask in an ice bath for 30 min.
    2. Filter the reaction in air through a 15 mL medium porosity frit.
    3. Rinse with 50 mL of diethyl ether and dry the solid in vacuo for two hours.
      NOTE: [Cp*IrCl2]2 (1.23 g, 59% based on Ir) is isolated as an orange/red solid. Characterization matches literature reports.30

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Results

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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...

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Discussion

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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, ...

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Disclosures

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The authors have nothing to disclose.

Acknowledgements

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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.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Materials
Lithium wire (in mineral oil)Aldrich278327-100G>98%
2-bromo-2-butene (mixture of cis/trans isomers)Acros200016-364  98%, dried over molecular sieves from an oven overnight before use
HexanesMilliporeHX0299-3GR ACS, used as received
Ethyl actetateMilliporeEX0240-3GR ACS, dried over molecular sieves from an oven overnight before use
Ammonium chlorideAldrich213330-2.5kgACS Reagent
Diethyl etherMilliporeEX0190-5GR ACS, collected from a solvent purification system before use
Magnesium sulfateAldrich793612-500gAnhydrous, reagent grade
p-toluene sulfonic acid monohydrateFisherA320-500ACS Certified
Sodium bicarbonateFisher5233-500ACS Certified
Sodium carbonateAmresco0585-500g
Ruthenium(III) chloride trihydratePressure Chemical475040% Metal
Iridium(III) chloride hydratePressure Chemical573053% Metal
MethanolAvantor3016-22AR ACS, distilled from Mg before use
PentaneJ. T. BakerT007-09>98%, dried with a solvent purification system before use
Chloroform-dAldrich151823-150G99.8 atom % D
Molecular sieves 4 ÅAldrich208590-1KGdried in an oven at 140 °C before use
Celite 545AcrosAC34967-0025dried in an oven at 140 °C before use
NameCompanyCatalog NumberComments
Equipment
Schlenk line, with vacuum and inert gas manifoldsCustomNAUsed in Preps 1-4
Solvent transfer manifoldChemglassAF-0558-01Used in 2.2
Airfree filter funnelChemglassAF-0542-22Used in 3.1.3
GloveboxVacuum AtmospheresOMNIUsed in 3.2.2

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Pentamethylcyclopentadiene SynthesisLigand PreparationOrganometallic ChemistryLithium QuenchingSchlenk Line TechniqueCannula TransferVacuum DistillationTransition Metal ComplexesNMR Spectroscopy

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