Utilization of Stop-flow Micro-tubing Reactors for the Development of Organic Transformations

Podsumowanie

A protocol for organic reaction screening using stop-flow micro-tubing (SFMT) reactors employing gaseous reactants and/or visible-light mediated reactions is presented.

Streszczenie

A new reaction screening technology for organic synthesis was recently demonstrated by combining elements from both continuous micro-flow and conventional batch reactors, coined stop-flow micro-tubing (SFMT) reactors. In SFMT, chemical reactions that require high pressure can be screened in parallel through a safer and convenient way. Cross-contamination, which is a common problem in reaction screening for continuous flow reactors, is avoided in SFMT. Moreover, the commercially available light-permeable micro-tubing can be incorporated into SFMT, serving as an excellent choice for light-mediated reactions due to a more effective uniform light exposure, compared to batch reactors. Overall, the SFMT reactor system is similar to continuous flow reactors and more superior than batch reactors for reactions that incorporate gas reagents and/or require light-illumination, which enables a simple but highly efficient reaction screening system. Furthermore, any successfully developed reaction in the SFMT reactor system can be conveniently translated to continuous-flow synthesis for large scale production.

Wprowadzenie

Flow chemistry is well poised towards the movement of green and sustainable processes^1,2. In contrast to batch reactors, continuous flow reactors possess significant advantages, such as improved thermal management, enhanced mixing control, and internal pressure regulation. These advantages greatly reduce the formation of by-products in the continuous flow system. Furthermore, continuous-flow enhances the biphasic gas-liquid reactions within the micro-tubing due to the excellent interfacial surface area of the reagents in different states. Continuous flow reactors also provide a good platform for photosynthesis due to the enhanced and uniform light illumination across the micro-tubing³.

Despite success in continuous-flow technology, there are still limitations in reaction screening for parameters that involve catalysts, solvents and reagents². Changes made to the pressure in the flow system will drastically affect the flow equilibrium. Moreover, a classic continuous flow system generally is limited to one reaction screening at a time, making it time consuming for efficient parallel reaction screening. The reaction time in continuous flow synthesis is also limited by its micro-tubing reactor size. Furthermore, continuous flow screening is prone to cross-contamination at higher temperature, even though carrier medium is employed between different reactions⁴.

Hence, to address the difficulty of screening discrete parameters in continuous-flow systems, we developed a stop-flow micro-tubing (SFMT) reactor system for reaction screening that involves gaseous reagents and/or photo-mediated reactions². SFMT reactors comprise elements of both batch reactors and continuous flow reactors. The introduction of shut-off valves entraps the reagents within the micro-tubing, a concept that is similar to a batch reactor, and when the system is pressurized, the SFMT behaves as a miniature high-pressure reactor. The SFMT can then be submerged into a water or oil bath, introducing heat to the reactor system. Visible lights can also be shone on the micro-tubing during the reaction period to facilitate photo-mediated reactions.

In SFMT, flammable or toxic gases, such as ethylene, acetylene, and carbon monoxide, can be utilized to generate valuable chemicals in a safer way compared to batch reactors^1,2,4. It is an asset to use such reactive gases as they are inexpensive chemical feedstocks and can be readily removed after reactions are completed, providing a cleaner procedure². On the contrary, most reaction development carried out in batch reactors tends to exclude the use of reactive gases due to its inconvenience and risk of explosion at elevated pressure and temperature. If gaseous reagents are employed, they are usually introduced into batch reactors via bubbling or balloons. This generally gave lower reproducibility or reactivity due to the low mixing efficiency at the interface. Although high-pressure vessels are commonly applied to enhance reactivity and solubility of gases, they are laborious with a risk of explosion, especially with flammable gases. In addition, the opaque surface of those commonly used high-pressure reactors made it unsuitable for photo-mediated reactions. Hence, reactions that consist of gaseous reagents and photo-mediated reactions are generally left unexplored. In this context, SFMT reactors provide an ideal platform because the gaseous reagents can be utilized within the micro-tubing with the assistance of a back pressure regulator (BPR) to regulate the internal pressure in a safe and convenient manner². Apart from reactions that involve gaseous reagents, visible-light promoted synthesis also displays great promises for organic synthesis^5,6. However, one of the greatest downfall of visible-light mediated reactions is the scalability in conventional batch reactors due to the attenuation effect of photon transport in large vessels⁷. If high-power light sources are used, over-irradiation may result in by-product formation. Moreover, gaseous reagents have seldom been applied in photo-chemical reactions mainly due to the complex apparatus system when using gas-phase reactants at high pressure². Through the introduction of a narrow channel, like SFMT, a high-pressure gas environment can be easily achieved under light irradiation.

Hence, this detailed video aims to help more scientists understand the advantages and the procedure of using SFMT for condition screening of gas-involved transformations and light-mediated reactions.

Protokół

Refer to all relevant material safety data sheets (MSDS) before handling any possible toxic and carcinogenic chemicals. Conduct appropriate risk assessments before starting any reactions, including the use of engineering controls, such as fume hoods and gas cylinders, as well as wearing sufficient personal protective equipment. Proper training should be conducted before using any highly flammable gas to avoid any accidents caused by mishandling of the gas cylinders.

1. Gas-involved Reaction²

1. Preparation of acetylene tank
   Set gas regulator of acetylene tank to 20 psi (137895 Pa), above the desired back-pressure of 5 psi (34474 Pa) used in the system.
   Note: Refer to Figure 1 on more details of the gas regulator set up.
   Note: Back-pressure regulator (BPR) is set at the end of the tubing, refer to Figure 2 and 3 for more details on the SFMT set-up.
2. Preparation of 4-iodoanisole solution
   1. Add a 10 mm magnetic stir bar into a 10 mL round-bottom flask.
   2. Measure 58.5 mg 4-iodoanisole with a weighing balance and transfer to the round-bottom flask.
      Caution: Aryl halides are irritants and can be harmful. Consult the relevant MSDSs before proceeding.
   3. Add 8.5 mg Pd(PPh₃)₂Cl₂, 1.0mg copper(I) iodide, 21.0 mg 1, 3, 5-trimethoxybenzene (internal standard) and 80 µL N,N-Diisopropylethylamine (DIPEA) into the same round-bottom flask. Add approximately 2.5 mL of dimethyl sulfoxide (DMSO) to the round-bottom flask.
      Caution: Pd(PPh₃)₂Cl₂, copper(I) iodide, DIPEA are irritants and can be harmful. Consult the relevant MSDSs before proceeding.
      Caution: 1, 3, 5-trimethoxybenzene are flammable and volatile. Keep away from ignition sources.
      Caution: DMSO is a toxic chemical. Consult the relevant MSDSs before proceeding.
   4. Seal the round bottom flask with a rubber septum and mixture was stir on a heat plate at room temperature and pressure until all the solid have dissolved.
      Note: Further sonication can be done to ensure a homogenous solution.
   5. Degas the reaction mixture with argon-filled balloon for approximately 15 min while maintaining a constant stirring on heat plate. Remove both needles after 15 min to ensure an inert environment within the round-bottom flask.
      Note: Refer to Figure 4 for details on degas procedure.
3. Mixing of liquid-gas layer in SFMT reactor
   1. Extract all the reaction mixture from the round bottom flask with a 8 mL stainless steel syringe connected to a long needle via a needle connector through the rubber septum. Remove the needle and attach the stainless steel syringe to the syringe pump. Connect the syringe to High purity Perfluoroalkoxy alkanes (HPFA) tubing (O.D. 1/16", I.D. 0.03", 300 cm, volume = 1.37 mL) via a T-connector.
      Note: Use a needle connector to connect both the stainless steel and long needle, refer to Figure 5 for more details on using the needle connector.
      Note: All air bubbles should be remove from the stainless steel syringe before attaching to the syringe pump.
      Note: Ensure that all the tubing are tighten before connecting the reaction mixture to the set-up to reduce exposure of air, refer to Figure 2 and 3 on the connections for the tubing.
   2. Set flow rate of the syringe pump to 300 µL/min for the reaction mixture to be pumped into the HPFA tubing. Adjust the flow rate of acetylene with the needle valve to about 1:1 liquid:gas ratio along the plugs. Equilibrated ratio was maintained until the HPFA tubing is filled with gas/liquid slug reagents.
      Caution: Acetylene is highly flammable. Keep away from ignition sources.
      Note: BPR is placed in acetone vial before purging the tubing with acetylene gas.
      Note: Purge the tubing with acetylene gas first until bubble is observed in the acetone vial for the BPR to ensure that the pressure is built up within the SFMT reactor before pumping the reaction mixture into the SFMT reactor. Refer to Figure 6 for better illustration of the liquid:gas ratio.
   3. Close the valve at the end when all the liquid had been injected into the HPFA tubing or when liquid start to leak from the BPR. Pump in more acetylene until the liquid stops moving in the tubing in order to maintain the pressure within the tubing. Close the valve at the start point and close the needle valve once complete. Transfer the whole set-up into the oil bath and incubate for 2 hours.
      Note: The valves are kept above the oil bath to prevent contamination from the silicon oil.
      Note: Pre-heat the oil bath to the desired temperature before transferring the SFMT reactor to it.
   4. After 1 hour, pump the reaction mixture into a 10 mL vial using a 8 mL stainless steel syringe. Fill an 8 mL stainless steel syringe with diethyl ether (approximately 4.0 mL) to wash out any residue in the tubing.
      Caution: Diethyl ether is highly flammable. Keep away from all ignition sources.
      Note: Hexane could be used to wash away the silicon oil before proceeding to avoid contamination for subsequent steps.
   5. Saturated NH₄Cl aqueous solution (4.0 mL) was added to the combined organic layer, followed by a liquid-liquid extraction with 1.5 mL diethyl ether, with the aid of a separatory funnel.
      Caution: NH₄Cl may be harmful. Consult the relevant MSDSs before proceeding.
   6. Conduct a gas chromatography mass spectrum (GC-MS) analysis with the organic layer to determine the yield.
      Note: 1, 3, 5-trimethoxybenzene was added in step 1.2.3 as an internal standard.
      Note: An internal standard calibration curve was plot with different mass of the product to derive a linear regression curve. The yield of the product is interpolated from the linear regression curve. Refer to Ref. 2 for more details on the calibration curve.

2. Photo-mediated Reaction⁵

1. Add 30.8 mg benzylidenemalonitrile, 4.1 mg 9-mesityl-10-methylacridinium perchlorate, 67.3 mg tetramethylethylene and 2.0 mL dichloroethane into a 10 mL silicon septa vial.
   Caution: Benzylidenemalonitrile, 9-mesityl-10-methylacridinium perchlorate, tetramethylethylene and dichloroethane are highly flammable. Keep away from all ignition sources.
2. Degas for about 15 minutes with argon-filled balloon. Remove both needles after 15 min to ensure an inert environment within the vial.
   Note: Refer to Figure 4 for details on degas procedure.
3. Purge the HPFA tubing (O.D. 1/16", I.D. 0.03", 340 cm, volume = 1.5 mL) with argon gas for approximately 5 min by direct connection of the SFMT reactor to the argon gas cylinder with an Union body PEEK. Close both valves to entrap the argon gas within the HPFA tubing after reaching the indicating time of 5 min.
   Note: Refer to Figure 5 for more details on using the Union body PEEK.
4. With a 3 mL disposable syringe attached with a long needle, extract the reaction mixture from 10 mL silicon septa vial. Remove the needle and connect the disposable syringe to the HPFA tubing via a syringe connector. Open both valves to pump in the reaction mixture manually. Close both valves again once the HPFA tubing has been filled with the reaction mixture.
   Note: Refer to Figure 5 for more details on using the syringe connector.
   Note: Mix the reaction mixture well with the syringe to ensure a homogenous solution before pumping into the HPFA tubing.
   Note: There may be excess solvent that will exceed the tubing volume. Place the tubing end on a waste can to collect any overflowed reaction mixture.
5. Place the SFMT reactor in the middle of the blue LED (λ_max = 425 nm, 2 m, 20 W) stripe to ensure equal exposure of the HPFA tubing. The HPFA was exposed for irradiation for approximately 5-48 hours.
   Note: The length of the blue LED stripe is set to 2 meters to provide enough energy for the reaction to proceed.
6. Pump out the reaction mixture with 3 mL disposable syringe into a clean round-bottom flask with a syringe connector piece. Wash out any residue with excess diethyl ether using a 3 mL disposable syringe into the same round-bottom flask.
   Note: Refer to Figure 5 for more details on using the syringe connector.
7. Measure 0.06 mmol of 1, 3, 5-trimethoxybenzene (internal standard) and add to combined organic mixture. Remove excess solvent under reduced pressure with a rotavap machine.
8. Measure 0.6 mL of deuterated chloroform with 1 mL disposable syringe attached with long needle and add to the concentrated crude product. Transfer the deuterated mixture into a clean NMR tube for crude ¹H NMR analysis.
   Note: The integral (x) for internal standard at 6.10 ppm is used to calculate the conversion rate by comparing the integral (y) of product formed at 3.38 ppm.
   

3. Photo-mediated Gas-involved Reaction²

1. Preparation of acetylene tank
   Set the gas regulator of the acetylene tank to about 20 psi (137895 Pa) which is above the desired back-pressure of 5 psi (34474 Pa) in the system.
   Note: Refer to Figure 1 on more details of the gas regulator set up.
   Note: Back-pressure regulator (BPR) is set at the end of the tubing, refer to Figure 2 and 3 for more details on the SFMT set-up.
2. Preparation of bromopentafluorobenzene solution
   1. Under inert atmosphere, add 74.1 mg bromopentafluorobenzene , 2.8 mg Ir(ppy)₂(dtbbpy)PF₆ and 46.8 mg 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) into a 10 mL silicon septa. Add 3.0 mL of acetonitrile into the same 10 mL glass vial to dissolve all the reagents.
      Caution: Bromopentafluorobenzene and acetonitrile are highly flammable and volatile. Keep away from all ignition sources.
      Caution: Ir(ppy)₂(dtbbpy)PF₆ and TEMPO may be harmful. Consult the relevant MSDSs before proceeding.
   2. Degas the reaction mixture with argon-filled balloon carefully for 10 minutes in an ice bath. Remove both needles from the septa to ensure an inert atmosphere in the vial.
      Note: Refer to Figure 4 for details on degas procedure.
   3. Add 56.0 µL of DIPEA into the mixture with a 1 mL syringe and degas for another 5 minutes in an ice bath similar to step 3.2.2.
3. Mixing of liquid-gas layer in SFMT reactor
   1. With a 8 mL stainless steel syringe attached with long needle via a needle connector, extract the reaction mixture from the silicon septa vial. Remove the needle and attach the syringe to the syringe pump. Connect the outlet to T-connector.
      Note: Use a needle connector to connect both the stainless steel and long needle, refer to Figure 5 for more details on using the needle connector.
      Note: All gas should be remove from the stainless steel syringe before attaching to the syringe pump.
      Note: Ensure that all the tubing are tighten before connecting the reaction mixture to the set-up to reduce exposure of gas, refer to Figure 2 and 3 on the connections for the tubing.
   2. Set the flow apparatus' flow rate to 100 µL/min and pump the reaction mixture into the HPFA tubing (O.D. 1/16", I.D. 0.03", 300 cm, volume = 1.37 mL). Adjust the flow rate of acetylene with the needle valve until 2:1 gas/liquid ratio is observed in the plug. The ratio plugs was determine via estimation in the clear tubing.
      Note: BPR is placed in acetone vial before purging the tubing with acetylene gas.
      Note: Purge the tubing with acetylene gas first until bubble is observed in the acetone vial for the BPR to ensure that the pressure is built up within the SFMT reactor before pumping reaction mixture into the SFMT reactor.
      Note: Refer to Figure 6 for better illustration of the liquid:gas ratio but take note that the volume of the gas should be double the volume of the liquid in the plug by visual estimation.
   3. Close the valve at the end when all the liquid had been injected into the SFMT reactor (total volume 0.65 ml, 0.065 mmol) or when liquid started to leak from the BPR. Pump in more acetylene until the liquid stop moving in the tubing. Close the valve at the start point and close the needle valve once done. Transfer the whole set-up to a water bath pre-heated to 60°C, and allowed to react for 3 h under blue LED light (λ_max = 425 nm, 3 m, 30 W).
      Note: The valves are left above the water bath to prevent any contamination.
      Note: The length of the blue LED stripe is set to 3 meters to provide enough energy for the reaction to proceed.
   4. Pump the reaction mixture from the HPFA tubing with a 8 mL stainless syringe into a round-bottom flask. Wash out the residues from the tubing reactor with excess diethyl ether into the same round-bottom flask. Concentrate the mixture under reduced pressure with a rotavap machine.
      Note: Carefully reduce the pressure as the starting material and products are highly volatile.
   5. Add 0.6 mL of deuterated chloroform via a 1 mL disposable syringe into the round-bottom flask to dissolve the concentrated crude mixture. Transfer the deuterated mixture into an NMR tubing for ¹⁹F NMR analysis.
      Note: The ¹⁹F NMR spectrums of the starting material (bromopentafluorobenzene) and the 2 products (2, 3, 4, 5, 6-Pentafluorostyrene and pentafluorobenzene) were analysed to find a significant peak for each chemical. The crude ¹⁹F NMR spectrum is used to compare the integral of these 3 significant peaks in order to determine the ratio of product formed. Refer to Ref. 2 for more details on the calculation of the product conversion and product ratio.

Wyniki

In this study, SFMT is used to carry out transformations that include gaseous reagents ( Table 1), light-mediated reactions (Table 2), and reactions that involves both gaseous reagents and photo-catalysis (Table 3).

Figure 1 displays a typical set-up for the gas regulator to be connected to the gas cylinder so as to regulate the pressure of the gas being pumped into the SFMT system.

Figure 2 represents the ...

Dyskusje

The newly developed SFMT reactor is a modification of the continuous-flow system by adding shut-off valves to the micro-tubing². In this system, the flow rate of a desired volume of reagents can be halted at will, simulating a batch reactor but in micro-tubing^2,10,11. These valves aid in the trapping of desired amount of reagents in the HPFA or stainless steel tubing while...

Materiały

Name	Company	Catalog Number	Comments
Acetylene Cylinder	Chem Gas PTE LTD (Singapore)
Logato 200 series Syringe pumps	KD Scientific Inc	788200
Blue LED Strips	Inwares Pte Ltd (Singapore)	3528 FlexiGlow LED Strips
PFA Tubing High Purity 1/16" OD x .030" ID x 50ft	IDEX Health&Science	1632-L	Depending on diameter of tubings needed
KDS Stainless Steel Syringe	KD Scientific Inc	780802
Shut-Off Valve Tefzel (ETFE) with 1/16" Fittings	IDEX Health&Science	P-782
BPR Assembly 20 psi	IDEX Health&Science	P-791
Luer Adapter Female Luer - Female Union	IDEX Health&Science	P-628	Known as syringe connector in this paper
1/4-28 Female to Male Luer Assy	IDEX Health&Science	P-675	Known as needle connector in this paper
Union Body PEEK .020 thru hole, for 1/16" OD"	IDEX Health&Science	P-702-01
Super Flangeless Ferrule w/SST Ring, 1/4-28 Flat-Bottom, for 1/16" OD	IDEX Health&Science	P-250X
PEEK Low Pressure Tee Assembly 1/16" PEEK .020 thru hole	IDEX Health&Science	P-712	Known as T-connector in this paper
Super Flangeless Nut PEEK 1/4-28 Flat-Bottom, for 1/16" & 1/32" OD	IDEX Health&Science	P-255X
Micro Metering Valve Assembly, 1/4-28 Flat-Bottom, for 1/16" OD	IDEX Health&Science	P-445NF	Known as Needle valve in this paper
Shut Off Valve Assembly PEEK .020	IDEX Health&Science	P-732
Terumo Syringe without needle	Terumo medical		1 mL and 3 mL depending on the volume needed
Terumo needle	Terumo medical		22G X 1½” (0.70 X 38 mm)
Sterican needle	B | Braun Sharing Enterprise		21G X 4¾” (0.80 X 120 mm)
Bruker ACF300 (300 MHz)			For 300 MHz NMR scanning
AV-III400 (400 MHZ)			For 400 MHz NMR scanning
AMX500 (500 MHz)			For 500 MHz NMR scanning
Merck 60 (0.040-0.063 mm) mesh silica gel	Merck
4-Iodoanisole	Sigma Aldrich	I7608-100G
412740 ALDRICH Bis(triphenylphosphine) palladium(II) dichloride ≥99% trace metals basis	Sigma Aldrich	412740-5G
Copper(I) iodide purum, ≥99.5%	Sigma Aldrich	03140-100G
N,N-Diisopropylethylamine	Tokyo Chemical Industry Co., Ltd	D1599
1, 3, 5-trimethoxybenzene	Tokyo Chemical Industry Co., Ltd	P0250
2,3-Dimethyl-2-butene ≥99%	Sigma Aldrich	220159-25ML
Bromopentafluorobenzene 99%	Sigma Aldrich	B75158-10G
TEMPO Green Alternative 98%	Sigma Aldrich	214000-25G
Acetonitrile	Sigma Aldrich	271004-1L
Diethylether	Sigma Aldrich	346136-1L
Dimethyl sulfoxide	VWR chemical	23500.322- 25L
1,2-Dichloroethane	Sigma Aldrich	284505-1L
9-mesityl-10-methylacridinium perchlorate			Refer to Ref. 8 for synthesis
Ir(ppy)2(dtbbpy)PF6			Refer to Ref. 9 for synthesis