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Visible-light Photoredox Catalysis

Introduction to Photoredox Catalysis

Visible light photoredox catalysis has emerged as a powerful tool in organic synthesis, building upon the foundation set by early pioneers in the areas of radical chemistry and photochemistry. Photoredox chemistry allows one to forge new bonds via open shell pathways and facilitates the rapid assembly of complex products en route to new areas of chemical space. Many transition-metal complexes and organocatalysts capable of initiating radical formation in the presence of visible light have been shown to facilitate a wide array of synthetic transformations including but not limited to cross-coupling, C-H functionalization, alkene and arene functionalization, and trifluoromethylation. A key aspect to visible light photoredox catalysis is the choice of light source.

Find the Photoreactor for your Experiment

About the SynLED Parallel Photoreactor 2.0

A long-standing challenge in the field of photoredox catalysis is reproducibility between reaction setups as well as across individual reactions performed with the same setup. Our SynLED Parallel Photoreactor 2.0 (Z744080 & Z744081) seeks to alleviate these issues by enabling small-scale photocatalysis reaction screening as well as rapid library generation while ensuring high levels of consistency across reactions and between runs with additional upgrades upon version 1. The photoreactor houses a 4 x 4 reaction block lit from the bottom with either 365 or 450 nm wavelength LEDs allowing for 16 simultaneous reactions in 2 dram or smaller scintillation or microwave vials. The newly designed reactor adds an improved heat sink technology and a performance quartz LED cover to protect the light source from solvents while still enabling excellent light penetration. The setup was designed to fit on a conventional stir plate, including a round cutout to fit firmly on IKA brand stir plates.

Figure 1.SynLED Parallel Photoreactor

SynLed Parallel Photoreactor 2.0 Features and Specs:

Bottom-lit LEDs (365 or 450 nm) across a 4x4 reaction block array provides consistent light intensity (320 lm) and angle (60° and 45°, respectively)
Built-in heat sink technology provides consistent temperature to each parallel reaction
Uses 1-2 dram scintillation vials or microwave vials (O.D. of 1.7 cm or less)
Power supply is wall plug power supply 700mA 12 W. Includes universal plug adapter
Compatible with IKA magnetic stirrer with round plate
Compatible with liquid scintillation vials (8 mL Wheaton, Z188719)

SynLED Parallel Photoreactor in the Literature

About the Penn PhD Photoreactor M2

The Penn PhD Photoreactor M2 (Z744035) is a benchtop instrument designed to facilitate consistency and reproducibility in photoredox catalysis across a wide range of reaction scales. The Photoreactor M2 combines LED illumination, mechanical stirring and cooling into one device while accepting reaction vials from 1 mL up to 40 mL. The touch screen user interface allows the chemist to precisely control temperature, intensity, stir rate and time, creating a valuable tool for repeatability, traceability, efficiency and consistency of results. The Photoreactor M2 addresses the potential to streamline synthetic sequences and create valuable strategies for addressing some of the challenges of molecule construction in drug discovery.

Figure 2.Penn PhD Photoreactor M2

Penn PhD Photoreactor M2 Features and Specs:

Modular design allows for use with a variety of wavelengths from 365nm to 450nm
360 degree reflective environment maximizes surface area photon capture.
Light shield interlock prevents user exposure to harmful light rays.
Interactive touch screen controls reaction parameters
User defined parameters including temperature, light intensity, fan speed and stirring.
Auto stop, pause and reset options
Supports vial sizes gc, 4, 8, 20, 40 ml
Temp feedback using a k-type thermocouple

Penn PhD Photoreactor Accessories

These additional light sources and vial holders can be used with the Penn PhD Photoreactor M2 for chemists and researchers to accelerate chemical reactions using photoredox catalysis.

Penn PhD Photoreactor M2 in the Literature

About the LightOx PhotoReact 365

The LightOx PhotoReact 365 (Z744061) is a benchtop instrument that provides even, directed and reproducible UV illumination at 365nm suitable for photochemical and photobiological reactions. The touch screen interface allows the user to control light intensity, energy delivery rate, and time. LED health over time is monitored using an in-built UV sensor enabling the user to produce rapid, reproducible results. This instrument is suitable for high-throughput screening in multiwell plates, providing researchers with a tool to screen high numbers of compounds and reactions essential for drug discovery.

Figure 4.LightOX Photoreact 365

LightOx PhotoReact 365 Features and Specs:

Designed for the acceleration of photochemical and photobiological reactions
For use with a standard size culture multi-well plate (85.5 mm x 128 mm) or single 95 mm round dish
User selected light output from 0 mW/cm² to maximum (approx. 13.0 mW/cm²)
Variance less than 1mW/cm² (at 100% power) across the well plate surface
User control of total power output
Light intensity and temperature monitors included that records measurements every second during operation
Data export as .csv files
Device has locking lid that reduces user exposure to harmful light rays

Representative Catalysts and Applications

Late-Stage Incorporation of Small Alkyl Groups into Small Molecules of Biological Interest

Late-Stage Functionalization (LSF) of small, pharmaceutically active molecules can provide modified potency, toxicity and pharmacokinetic profiles with altogether less effort than the ground-up syntheses of analogs. Though LSF with fluorine (and fluorinated alkyl groups) is now readily achieved through use of, amongst other technologies, the zinc sulfinates developed by the Baran Group at Scripps, the incorporation of small, non-fluorous alkyl groups like methyl and ethyl groups has remained an unmet challenge. This is especially relevant as the incorporation of small alkyl groups is likely to yield molecules with more advantageous physiochemical and safety profiles vis à vis their fluorinated brethren.

Ultimately, the late-stage incorporation of methyl, ethyl and cyclopropyl groups to pharmaceutically active small molecules was achieved through a visible light photoredox strategy. Indeed, in the presence of blue light and peroxide alkyl radical precursors, newly available [Ir{dF(CF₃)ppy}₂(dtbpy)]PF₆ catalyst (747793) mediates the formation of the required alkyl radicals (and thus, alkyl group incorporation) typically achieved by methods not amenable to LSF¹.

Figure 3.Late-Stage Incorporation of Small Alkyl Groups into Small Molecules of Biological Interest

Strategy for Room Temperature Lignin Degradation

Lignin represents over 30% of non-fossil fuel based organic carbon, and as such this biomass could be made, with the advent of chemoselective degradation processes, into a primary source of carbon-based feedstock material. The Stephenson Group has recently combined two of our innovative products to achieve the selective degradation of lignin-type model compounds in one pot. Indeed, Bobbitt’s salt was used to selectively oxidize benzyl alcohols, followed by photocatalytic reductive C-O bond cleavage to the ketone and alcohol degradation products. This last transformation was achieved without degassing and in the presence of visible light².

Figure 3.Strategy for Room Temperature Lignin Degradation

Crossed Intermolecular [2+2] Cycloaddition of Styrenes

The crossed [2 + 2] cycloaddition of styrenes was achieved in high chemoselectivity through use of an appropriately tuned Ru-based visible light photocatalyst. As demonstrated by Prof. Tehshik Yoon, unsymmetrically substituted butanes can thus be prepared on gram scale with Ru(bpm)₃.

Figure 4.Crossed Intermolecular [2+2] Cycloaddition of Styrenes

Diversity of Application

Our visible light photocatalysts have been used in applications that truly span organic chemistry (and beyond). Other reactivities that have been explored include:

Reduction of unactivated alkyl, alkenyl and aryl iodides
Co-mediated water oxidation for H₂ gas production
Atom-transfer radical addition of haloalkanes to alkenes
Formal hydroaminoalkylation of alkenes

References

DiRocco DA, Dykstra K, Krska S, Vachal P, Conway DV, Tudge M. 2014. Late-Stage Functionalization of Biologically Active Heterocycles Through Photoredox Catalysis. Angew. Chem. Int. Ed.. 53(19):4802-4806. https://doi.org/10.1002/anie.201402023

Nguyen JD, Matsuura BS, Stephenson CRJ. 2014. A Photochemical Strategy for Lignin Degradation at Room Temperature. J. Am. Chem. Soc.. 136(4):1218-1221. https://doi.org/10.1021/ja4113462

Ischay MA, Ament MS, Yoon TP. 2012. Crossed intermolecular [2 + 2] cycloaddition of styrenes by visible light photocatalysis. Chem. Sci.. 3(9):2807. https://doi.org/10.1039/c2sc20658g