Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

We describe a protocol to photogenerate N-heterocyclic carbenes (NHCs) by UV irradiation of a 2-isopropylthioxanthone/imidazolium tetraphenylborate salt system. Methods to characterize the photoreleased NHC and elucidate the photochemical mechanism are proposed. The protocols for ring-opening metathesis photopolymerization in solution and miniemulsion illustrate the potential of this 2-component NHC photogenerating system.

Transcript

N-heterocyclic carbenes, or NHC, are key compounds that can be used as organocatalyst, as ligands, or as reactants. However, they are highly moisture-sensitive, which requires them to manipulated in a glove box. This procedure provides straightforward access to photolatent NHC in the form of photoactive imidazolium salts.

When irradiated, these salts release bare NHC, allowing NHC to be generated on demand using photochemistry. Once NHC carbenes have been photo generated, the next step is to determine the quantity released. For this purpose, we present an optional protocol based on a spectrophotometric titration using phenol red as titrant.

We demonstrate the potential of photolatent energy by opening them in photo-induced ring-opening metathesis polymerization of norbornene in miniemulsion where they interact with the non photoactive ruthenium precatalyst, to form the active catalyst in situ. To begin, place one gram of 1, 3-dimesitylimidazolium chloride in a 100 milliliter round bottom flask equipped with a stir bar. Add 30 milliliters of absolute ethanol, and stir until the solid has completely dissolved.

Place 1.35 grams of sodium tetraphenylborate in another 50 milliliter round bottom flask equipped with a stir bar, and dissolve it in 30 milliliters of absolute ethanol. Then add that tetraphenylborate solution to the stirring imidazolium solution drop wise. Once addition is complete, continue stirring the reaction mixture for 10 minutes at room temperature to obtain 1, 3-dimesitylimidazolium tetraphenylborate as a white precipitant.

Then remove the stir bar and rinse it with absolute ethanol. Collect the white product on a glass frit by vacuum filtration. And rinse the flask with ethanol.

Wash the product with 30 milliliters of ethanol, followed by 30 milliliters of ultra pure water. Transfer the product to a vial, and dry it at 60 degrees Celsius for 15 hours before characterizing it with proton and carbon NMR in deuterated dimethyl sulfoxide. To begin, combine 39 milligrams of 1, 3-dimesitylimidazolium tetraphenylborate and 7.8 milligrams of ITX, with 0.5 milliliters of anhydrous deuterated tetrahydrofuran and transfer it to a standard NMR tube.

Then place the capped NMR tube in a photo chemical reactor equipped with a circular array of tubes that will emit monochromatic radiation at 365 nanometers. Irradiate the mixture for 10 minutes to generate IMes. Next, add 0.02 milliliters of carbon disulfide to the mixture in the NMR tube, and allow the mixture to react for 12 hours to obtain the IMes carbon disulfide adduct as a red precipitant.

Collect the adduct by vacuum filtration and rinse the NMR tube with nondeuterated THF. Transfer the adduct to a small vial and let it dry in air at room temperature for 12 hours. Lastly, characterize the IMes carbon disulfide adduct by proton and carbon NMR in 0.5 milliliters of d6 DMSO.

No more than one day before the measurement, prepare at least 10 milliliters of a 0.2 millimolar solution of phenol red in anhydrous acetonitrile. To begin the measurement procedure, dissolve 1.85 milligrams of 1, 3-dimesitylimidazolium tetraphenylborate and 0.25 milligrams of ITX in 10 milliliters of anhydrous acetonitrile. Place 2 milliliters of this solution in a spectroscopic cuvette and cap it with a rubber septum.

Place the cuvette in the instrument. Then purge the imidazolium solution and the phenol red solution with nitrogen gas for two minutes. After that, irradiate the reaction mixture with a 365 nanometer, 65 watt LED light for two minutes.

Then introduce 0.1 milliliters of the phenol red solution into the cuvette containing the irradiated reaction mixture, and record a UV vis spectrum of the mixture. Repeat this process until 1.5 milliliters of the phenol red solution have been added to the cuvette. The absorption bend at 580 nanometers is increasing after addition of phenol red, then decreasing after the equivalence.

Plot the absorbance at 580 nanometers as a function of titrant volume to determine the titration end point. Evaluate two more two milliliter portions of the starting imidazolium ITX solution in clean cuvettes in the same way using other irradiation times. Calculate the NHC yields, and plot the yield as a function of irradiation time.

To begin preparing the miniemulsion, dissolve 15 grams of hecta oxyethylene sterile ether in 150 milliliters of ultra pure water. Transfer this solution to an annular LED photoreactor, add a stir bar, and seal it with a rubber septum. Insert a sonication probe with an airtight seal into the photoreactor, and sparge the solution with nitrogen gas for one hour.

During that time, combine in a 50 milliliter round bottom flask 4.94 grams of norbornene, 2.85 milliliters of hexadecane, and 6 milliliters of 1, 2-dichloroethane. Seal the flask with a high vacuum stopper. Degas the mixture with three freeze pump thaw cycles, with 30 seconds of vacuum per cycle.

Place 6 milliliters of dichloroethane in another 50 milliliter round bottom flask, sealed with a high vacuum stopper, and degas it in the same way. In the glove box, add to the degassed dichloroethane 162 milligrams of 1, 3-dimesitylimidazolium tetraphenylborate, 33 milligrams of ITX, and 30 milligrams of the ruthenium precatalyst. Then combine the norbornene and precatalyst solutions under nitrogen.

Introduce 15 milliliters of the combined monomer and precatalyst solution into the aqueous emulsifier solution in the photoreactor, while stirring at about 500 RPM. Continue stirring the mixture for one hour to form a rough macroemulsion. Then place the photoreactor in an ice bath.

Sonicate the mixture for 10 minutes in five second pulses to form the miniemulsion. Next, replace the sonication probe with an LED lamp equipped with a water cooling system and protected by a cladding tube under a nitrogren flux. Place the sealed photoreactor in the photo cabinet to shield the user from UV radiation, and start the water cooling system.

Irradiate the monomer miniemulsion at 365 nanometers for 100 minutes to obtain the polymer latex. During the irradiation, periodically turn off the LED lamp and take a 4 milliliter aliquot of the miniemulsion to monitor the reaction progress. To check the particle size, in a glass cuvette dilute 20 microliters of the aliquot with 5 milliliters of ultra pure water, and perform dynamic light scattering.

To evaluate the norbornene conversation, perform gas chromatography using hexadecane as an internal standard. Precipitate the remainder of the aliquot with 20 milliliters of acetone. Collect the polymer by vacuum filtration, dry it under vacuum, and determine the molecular weight using size exclusion chromatography.

The photolatent NHC 1, 3-dimesitylimidazolium tetraphenylborate was obtained in high yield from anion metathesis. Both proton and carbon NMR showed excellent product purity. UV irradiation of a mixture of the imidazolium tetraphenylborate and ITX resulted in deprotonation of the carbon between the nitrogen atoms to form IMes at about 50%yield.

The IMes formation was confirmed by generation of an IMes carbon disulfide adduct from the asirradiated mixture. Photobleaching experiments showed photobleaching of ITX only in the presence of tetraphenylborate. No photobleaching was observed in a mixture of ITX and the imidazolium chloride salt, indicating that ITX does not directly abstract a hydrogen from imidazolium.

These results suggested that the IMes photogeneration mechanism involved electron transfer from tetraphenylborate to excited ITX, followed by a second step of proton transfer from imidazolium cation to ITX radical anion. This was consistent with spectrophotometric titration data, which showed a progressive release of IMes during irradiation. The maximum yield was achieved at five minutes of irradiation.

Photo ROMP of norbornene using a ruthenium precatalyst was successfully performed in both solution and miniemulsion. 70-80%conversion of norbornene was achieved after 100 minutes of irradiation in miniemulsion. The polynorbornene particles were only slightly larger than the original norbornene miniemulsion droplets, and they were almost perfectly spherical when viewed by transmission electron microscopy.

Before using photolatent energies in a reaction, it is important to determine the relationship between the quantity of energy released by UV radiation and the conditions of irradiation. The protocol for determining the yield of photogenerated energy is based on a simple spectrophotometric titration. Which is unusual, as it takes place in nonaqueous conditions.

We believe that this procedure for generating energies on demand is of a great use for chemists who desire to generate energies at a specific moment during your reaction.

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Journal (Chemistry)

Journal (Chemistry)

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

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