Highlights of Photochemical Reactions in Microflow Reactors

Photochemical transformations accelerated in continuous-flow reactors: basic concepts and applications

Continuous-flow photochemistry is used increasingly by researchers in academia and industry to facilitate photochemical processes and their subsequent scale-up. However, without detailed knowledge concerning the engineering aspects of photochemistry, it can be quite challenging to develop a suitable photochemical microreactor for a given reaction. In this review, we provide an up-to-date overview of both technological and chemical aspects associated with photochemical processes in microreactors. Important design considerations, such as light sources, material selection, and solvent constraints are discussed. In addition, a detailed description of photon and mass-transfer phenomena in microreactors is made and fundamental principles are deduced for making a judicious choice for a suitable photomicroreactor. The advantages of microreactor technology for photochemistry are described for UV and visible-light driven photochemical processes and are compared with their batch counterparts. In addition, different scale-up strategies and limitations of continuous-flow microreactors are discussed.

Rapid trifluoromethylation and perfluoroalkylation of five-membered heterocycles by photoredox catalysis in continuous flow

Trifluoromethylated and perfluoroalkylated heterocycles are important building blocks for the synthesis of numerous pharmaceutical products, agrochemicals and are widely applied in material sciences. To date, trifluoromethylated and perfluoroalkylated hetero-aromatic systems can be prepared utilizing visible light photoredox catalysis methodologies in batch. While several limitations are associated with these batch protocols, the application of microflow technology could greatly enhance and intensify these reactions. A simple and straightforward photocatalytic trifluoromethylation and perfluoroalkylation method has been developed in continuous microflow, using commercially available photocatalysts and microflow components. A selection of five-membered hetero-aromatics were successfully trifluoromethylated (12 examples) and perfluoroalkylated (5 examples) within several minutes (8-20 min).

Multidimensional reaction screening for photochemical transformations as a tool for discovering new chemotypes

We have developed an automated photochemical microfluidics platform that integrates a 1 kW high-pressure Hg vapor lamp and allows for analytical pulse flow or preparative continuous flow reactions. Herein, we will discuss the use of this platform toward the discovery of new chemotypes through multidimensional reaction screening. We will highlight the ability to discretely control wavelengths with optical filters, allowing for control of reaction outcomes.

Three in the spotlight: Photoinduced stereoselective synthesis of (Z)-acyloxyacrylamides through a multicomponent approach

We report a straightforward approach to synthesize 2-acyloxyacrylamides, which are useful synthons in organic synthesis. This involves a photoactivated multicomponent reaction, performed both in batch and under continuous flow conditions. This process affords the desired compounds in a stereoselective fashion from readily available starting materials in one step, without the aid of metal catalysis. This paper illustrates the preliminary work, the extensive experiments carried out to understand the limitations of the approach, and the optimization of the conditions for the synthesis of these particular captodative olefins.

Microfluidic reactors for photocatalytic water purification

Photocatalytic water purification utilizes light to degrade the contaminants in water and may enjoy many merits of microfluidics technology such as fine flow control, large surface-area-to-volume ratio and self-refreshing of reaction surface. Although a number of microfluidic reactors have been reported for photocatalysis, there is still a lack of a comprehensive review. This article aims to identify the physical mechanisms that underpin the synergy of microfluidics and photocatalysis, and, based on which, to review the reported microfluidic photocatalytic reactors. These microreactors help overcome different problems in bulk reactors such as photon transfer limitation, mass transfer limitation, oxygen deficiency, and lack of reaction pathway control. They may be scaled up for large-throughput industrial applications of water processing and may also find niche applications in rapid screening and standardized tests of photocatalysts.

OLEDs as prospective light sources for microstructured photoreactors

The use of OLEDs to initiate photochemical reactions is demonstrated for the first time by conducting photooxygenations in a modular microstructured photoreactor.

OLEDs as prospective light sources for microstructured photoreactors

The use of OLEDs to initiate photochemical reactions is demonstrated for the first time by conducting photooxygenations in a modular microstructured photoreactor.

Accurate Measurement of the Photon Flux Received Inside Two Continuous Flow Microphotoreactors by Actinometry

In this study, the photon flux received in two continuous flow microphotoreactors was measured by actinometry (potassium ferrioxalate). The microphotoreactors had two different geometries and were irradiated by either a polychromatic or a monochromatic light source. A model considering the partial absorption of photons through the reactor depth and, if required, the polychromatic character of the light source and the dependence of the actinometer properties on the wavelength were formulated to describe the variation of the actinometer conversion with the irradiation time. The photon flux received in the microphotoreactors could be thus accurately calculated as a function of the emitted wavelength. The same methodology was then applied to measure the photon flux received in a batch immersion well photoreactor. The radiant power received in each photoreactor was compared to that emitted by the lamp and major differences were found, thus confirming the need for this kind of in situ measurement. Finally, some guidelines based on a knowledge of the photon flux were proposed to compare various photoreactors. They revealed in particular that the choice of the most efficient photoreactor depended on the criteria chosen to evaluate the performances (i.e. productivity, Space Time Yield).

Continuous-flow synthesis of polymer nanoparticles in a microreactor via miniemulsion photopolymerization

Water-based photopolymerization in microreactor using compact UV fluorescent lamps can create a breakthrough technology to produce polymer latexes in a safer, more environmental-friendly and energy-efficient way.

From 'Lab & Light on a Chip' to Parallel Microflow Photochemistry

Continuous-flow microreactors offer major advantages for photochemical applications. This mini-review summarizes the technological development of microflow devices in the Applied and Green Photochemistry Group at James Cook University, and its associates, from fixed microchips for microscale synthesis to flexible multicapillary systems for parallel photochemistry. Whereas the enclosed microchip offered high space–time-yields, the open capillary-type reactor showed a greater potential for further modifications. Consequently, a 10-microcapillary reactor was constructed and used successfully for process optimization, reproducibility studies, scale-up, and library synthesis. To demonstrate the superiority of microflow photochemistry over conventional batch processes, the reactors were systematically evaluated using alcohol additions to furanones as model reactions. In all cases, the microreactor systems furnished faster conversions, improved product qualities, and higher yields. UVC-induced [2+2] cycloadditions of furanone with alkenes were exemplarily examined in a capillary reactor, thus proving the broad applicability of this reactor type.

A combined continuous microflow photochemistry and asymmetric organocatalysis approach for the enantioselective synthesis of tetrahydroquinolines

A continuous-flow asymmetric organocatalytic photocyclization-transfer hydrogenation cascade reaction has been developed. The new protocol allows the synthesis of tetrahydroquinolines from readily available 2-aminochalcones using a combination of photochemistry and asymmetric Brønsted acid catalysis. The photocylization and subsequent reduction was performed with catalytic amount of chiral BINOL derived phosphoric acid diester and Hantzsch dihydropyridine as hydrogen source providing the desired products in good yields and with excellent enantioselectivities.

Microflow photochemistry: UVC-induced [2 + 2]-photoadditions to furanone in a microcapillary reactor

[2 + 2]-Cycloadditions of cyclopentene and 2,3-dimethylbut-2-ene to furanone were investigated under continuous-flow conditions. Irradiations were conducted in a FEP-microcapillary module which was placed in a Rayonet chamber photoreactor equipped with low wattage UVC-lamps. Conversion rates and isolated yields were compared to analogue batch reactions in a quartz test tube. In all cases examined, the microcapillary reactor furnished faster conversions and improved product qualities.

Continuous flow photocyclization of stilbenes - scalable synthesis of functionalized phenanthrenes and helicenes

A continuous flow oxidative photocyclization of stilbene derivatives has been developed which allows the scalable synthesis of backbone functionalized phenanthrenes and helicenes of various sizes in good yields.

Visible Light Photocatalysis: The Development of Photocatalytic Radical Ion Cycloadditions

Photochemistry has the potential to significantly impact multiple aspects of chemical synthesis, in part because photoinduced reactions can be used to construct molecular architectures that would otherwise be difficult to produce. Nevertheless, organic chemists have been slow to embrace photochemical synthesis because of technical complications associated with the use of ultraviolet light. Our laboratory has been part of an effort to design synthetically useful reactions that utilize visible light. This strategy enables the synthesis of a diverse range of organic structures by generation of a variety of reactive intermediates under exceptionally mild conditions. This Perspective article describes the reasoning that led to the conception of our first experiments in this area, the features of our reaction design that have been most powerful in the discovery of new processes, and a few of the possible future areas in which visible light photocatalysis might have a large impact.