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

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.