Lucent360â„¢ Case Study: Multiple LED Intensity / Catalyst Screening in a Single Experiment

The Lucent360â„¢ LED modules and vial holders/reactors are completely interchangeable, giving users the freedom to tailor their experiments precisely to their requirements. With adjustable wavelength, light intensity, vial arrangements, and temperature control, the possibilities for customization are virtually limitless. This device allows researchers to swiftly explore the reaction parameters needed to upscale a reaction at a small scale while using the same equipment for production.

A great example of the adaptability provided by the Lucent360â„¢ is showcased through the following reaction light intensity and catalyst screening for the iridium-catalyzed C-C bond formation (refer to Figure 1).

Figure 1:

Using the Lucent 360™ multi-light screener (HCK1021-01-010), which can screen 16 reactions simultaneously across four different wavelengths and light intensities, the following reaction screen was conducted at 450 nm. Four iridium catalyst concentrations (1 mol %, 0.5 mol %, 0.1 mol %, and 0.01 mol %) were tested with four varying light intensities on the Lucent360™. Based on actinometry experiments performed in 2 ml reactions, each setting corresponds to running the reactions at 0.25 W, 0.54 W, 1.05 W, and 1.2 W effective power. These effective powers equate to photon fluxes ranging from 9.5×10⁻⁷ to 4.5×10⁻⁶ Einstein/s (moles of photons per second).

 

The multi-light intensity/catalyst screen was carried out following this protocol (Table 1):

To each 4 ml vial equipped with a Teflon septum and stir bar, a solution containing 1.1 mg Ni-glyme (5 mol %, 5 µmol) and 1.3 mg dtbbpy (5 mol %, 5 µmol) in methanol was added, and the solvent was removed under reduced pressure (16 vials). To the resulting vial, 48.9 mg Cs₂CO₃ (1.5 equiv., 150 µmol) was weighed under an inert atmosphere and capped. To this vial, a 1 ml sparged solution in DMSO containing 19.9 mg bromoacetophenone (100 µmol), 35.6 mg Boc-Val-OH (1.5 equiv., 150 µmol), and 7.7 mg biphenyl (0.5 equiv., 50 µmol) as an internal standard was added. Ir(dF-CF₃-ppy)₂(dtbbpy) was added as a solution in DMSO to provide 0.01, 0.1, 0.5, or 1 mol % catalyst. Additional DMSO was added to each reaction to bring the total reaction volume to 2.0 mL DMSO. The reaction was performed at 30 °C in the Lucent360â„¢ with 450 nm LED. Analytical samples were taken from each vial at 5, 15, 30, and 60 minutes for analysis by LC-MS (10 µL diluted to 1 mL in DMSO).

Table 1: Experimental Details (for standard conditions)

Monitoring the reaction screen over 1 hour revealed clear trends in product formation in relation to catalyst concentration and light intensity (Table 2). Each reaction with 1 mol % Ir catalyst achieved similar conversions (62-65%) at 60 minutes, irrespective of the power setting. With 0.5 mol % catalyst, 0.54 W or higher was sufficient to reach comparable conversions. Lower catalyst loadings proved detrimental to conversion, regardless of the power setting.

Table 2: Conversion at 60 minutes

While the final conversion is similar for the seven conditions highlighted in green in Table 2, significant differences were observed in the initial rates and time to reaction completion (Figure 3). Both reactions at 1.0 W and 1.2 W are completed by 30 minutes with minimal additional product formation, whereas the two lower power settings require the full 60 minutes for completion.

Figure 3: Time course plot for 1 mol % at 0.25W, 0.54W, 1.05W, and 1.21W

Based on these results, the reaction can be performed with 0.5 mol % photocatalyst and 0.54 W to achieve similar conversions to those with higher catalyst loading and light intensity (Figure 4), although at a slower pace.

Figure 4: Time course plot for 0.54 W setting with 4 catalyst concentrations

By monitoring the initial rate of the reaction at 5 minutes, it becomes evident how significantly light intensity impacts the reaction. Accelerating the initial rate of the reaction is crucial in determining the appropriate conditions for scaling up a photochemical reaction. What does this mean for this C-C bond formation reaction? We're not entirely sure yet, but we do know that without the straightforward setup of the Lucent360â„¢ and the ability to simultaneously monitor multiple light intensities, obtaining all this data would have taken much longer. For our next experiment using this reaction, perhaps we could examine four different concentrations of the nickel co-catalyst at each light intensity compared to the 0.5 mol % iridium catalyst, or screen additional wavelengths (365 nm, 380 nm, 405 nm) or vary substrate concentrations. No matter what we decide, we know it will be easy to carry out with the Lucent360â„¢.

Figure 5:

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