Utilization of microflow reactors to carry out synthetically useful organic photochemical reactions

Heterogeneous photochemical reaction enabled by an ultrasonic microreactor

The presence of solids as starting reagents/reactants or products in flow photochemical reactions can lead to reactor clogging and yield reduction from side reactions. We address this limitation with a new ultrasonic microreactor for continuous solid-laden photochemical reactions. The ultrasonic photochemical microreactor is characterized by the liquid and solid residence time distribution (RTD) and the absorbed photon flux in the reactor via chemical actinometry. The solid-handling capability of the ultrasonic photochemical microreactor is demonstrated with a silyl radical-mediated metallaphotoredox cross-electrophile coupling with a solid base as a reagent.

Process intensification in continuous flow organic synthesis with enabling and hybrid technologies

Industrial organic synthesis is time and energy consuming, and generates substantial waste. Traditional conductive heating and mixing in batch reactors is no longer competitive with continuous-flow synthetic methods and enabling technologies that can strongly promote reaction kinetics. These advances lead to faster and simplified downstream processes with easier workup, purification and process scale-up. In the current Industry 4.0 revolution, new advances that are based on cyber-physical systems and artificial intelligence will be able to optimize and invigorate synthetic processes by connecting cascade reactors with continuous in-line monitoring and even predict solutions in case of unforeseen events. Alternative energy sources, such as dielectric and ohmic heating, ultrasound, hydrodynamic cavitation, reactive extruders and plasma have revolutionized standard procedures. So-called hybrid or hyphenated techniques, where the combination of two different energy sources often generates synergistic effects, are also worthy of mention. Herein, we report our consolidated experience of all of these alternative techniques.

Review on Additive Manufacturing of Catalysts and Sorbents and the Potential for Process Intensification

Additive manufacturing of catalyst and sorbent materials promises to unlock large design freedom in the structuring of these materials, and could be used to locally tune porosity, shape and resulting parameters throughout the reactor along both the axial and transverse coordinates. This contrasts catalyst structuring by conventional methods, which yields either very dense randomly packed beds or very open cellular structures. Different 3D-printing processes for catalytic and sorbent materials exist, and the selection of an appropriate process, taking into account compatible materials, porosity and resolution, may indeed enable unbounded options for geometries. In this review, recent efforts in the field of 3D-printing of catalyst and sorbent materials are discussed. It will be argued that these efforts, whilst promising, do not yet exploit the full potential of the technology, since most studies considered small structures that are very similar to structures that can be produced through conventional methods. In addition, these studies are mostly motivated by chemical and material considerations within the printing process, without explicitly striving for process intensification. To enable value-added application of 3D-printing in the chemical process industries, three crucial requirements for increased process intensification potential will be set out: i) the production of mechanically stable structures without binders; ii) the introduction of local variations throughout the structure; and iii) the use of multiple materials within one printed structure.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

Revisiting aromatic diazotization and aryl diazonium salts in continuous flow: highlighted research during 2001–2021

Aryl diazonium salts play an important role in chemical transformations; however their explosive nature limits their applications in batch.

Enantioselective synthesis of heterocyclic compounds using photochemical reactions

Different methods for the direct enantioselective photochemical synthesis of heterocycles are presented. Currently, asymmetric catalysis with templates involving hydrogen bonds or metal complexes is intensively investigated. Enzyme catalysis can be simplified under photochemical conditions. For example, in multi enzyme systems, one or more enzyme catalytic steps can be replaced by simple photochemical reactions. Chiral induction in photochemical reactions performed with homochiral crystals is highly efficient. Such reactions can also be carried out with crystalline inclusion complexes. Inclusion of a photochemical substrate and an enantiopure compound in zeolites also leads to enantioselective compounds. In all these methods, the conformational mobility of the photochemical substrates is reduced or controlled. Memory of chirality is a particular case in which a chiral information is temporally lost but the rigid conformations stabilize the molecular structure which leads to the formation of enantiopure compounds. Such studies allows a profound understanding on how particular conformations determine the configuration of the final products.Graphical abstract.

Imposed dynamic irradiation to intensify photocatalytic reactions

Dynamic irradiation is a potent option to influence the interaction between photochemical reactions and mass transport to design high performant and efficient photochemical processes. To systematically investigate the impact of this parameter, the photocatalytic reduction of nitrobenzene was conducted as a test reaction. Dynamic irradiation was realized through provoked secondary flow patterns, multiple spatially distributed light emitting diodes (LEDs) and electrical pulsation of LEDs. A combined experimental and theoretical approach revealed significant potential to enhance photochemical processes. The reaction rate was accelerated by more than 70% and even more important the photonic efficiency was increased by more than a factor of 4. This renders imposed dynamic irradiation an innovative and powerful tool to intensify photoreactions on the avenue to large scale sustainable photochemical processes.

Toward the Scale-Up of a Bicyclic Homopiperazine via Schmidt Rearrangement and Photochemical Oxaziridine Rearrangement in Continuous-Flow

The scale-up of a chiral bicyclic homopiperazine of pharmaceutical interest was investigated. The outcome and safety profile of a key batch ring-expansion step via Schmidt rearrangement was improved using continuous-flow chemistry. The selectivity of nitrogen insertion for the ring expansion was improved via an alternative photochemical oxaziridine rearrangement under mild conditions, which when converted to continuous-flow in a simple and efficient flow reactor allowed the first photochemical scale-up of a homopiperazine.

Laser synthesis: a solvent-free approach for the preparation of phenylthiazolo[5,4-b]pyridine derivatives

We describe here a rapid and straightforward solvent-free method to access phenylthiazolo[5,4-b]pyridines using a Nd-YAG laser NANO-NY81-10 (λ = 355 nm, 10 Hz pulse frequency; 8 ns pulse duration).

Biphasic nucleophilic aromatic substitution using a microreactor under droplet formation conditions

Biphasic nucleophilic aromatic substitution of 4-fluoronitrobenzene proceeded efficiently using a packed bed reactor.

Radiometric measurement techniques for in-depth characterization of photoreactors – part 1: 2 dimensional radiometry

A spatially resolving radiometric scanning method for the evaluation of the radiation field of photoreactors is presented, allowing for a knowledge driven development of highly efficient photoreactors and scale-up.

Flow Photochemistry as a Tool in Organic Synthesis

Photochemical transformations of molecular building blocks have become an important and widely recognized research field in the past decade. Detailed and deep understanding of novel photochemical catalysts and reaction concepts with visible light as the energy source has enabled a broad application portfolio for synthetic organic chemistry. In parallel, continuous-flow chemistry and microreaction technology have become the basis for thinking and doing chemistry in a novel fashion with clear focus on improved process control for higher conversion and selectivity. As can be seen by the large number of scientific publications on flow photochemistry in the recent past, both research topics have found each other as exceptionally well-suited counterparts with high synergy by combining chemistry and technology. This review will give an overview on selected reaction classes, which represent important photochemical transformations in synthetic organic chemistry, and which benefit from mild and defined process conditions by the transfer from batch to continuous-flow mode.

Visible-Light Flow Reactor Packed with Porous Carbon Nitride for Aerobic Substrate Oxidations

A triphasic photocatalytic reactor employing a mesoporous carbon nitride photocatalyst and aerobic O₂ was assembled to operate under continuous flow conditions. This reactor design allows for facile downstream processing and reusability in multiple flow cycles. The selective aerobic oxidation of alcohols and amines was chosen to demonstrate the applicability and performance advantage of this flow approach compared to that of conventional batch photochemistry. This precious-metal-free photocatalytic flow system operates under benign reaction conditions (visible light, low pressure, and mild temperature) and will stimulate the exploration of other oxidative reactions in a sustainable, scalable, and affordable manner.

Continuous-Flow Photochemical Transformations of 1,4-Naphthoquinones and Phthalimides in a Concentrating Solar Trough Reactor

A series of photochemical transformations has been successfully conducted under continuous-flow conditions in a concentrating solar trough reactor. Photoacylations and [2+2]-photocycloadditions involving 1,4-naphthoquinones gave the corresponding photoproducts in moderate to high yields with residence times of 70min. Likewise, acetone-sensitized photodecarboxylations involving phthalimides furnished the corresponding benzylated hydroxy phthalimidines in good to excellent yields and purity with residence times of 40min. Compared with corresponding exposures to direct sunlight conducted in a solar float, flow operation generally gave superior conversions and subsequent yields.

Continuous synthesis of 2,5-hexanedione through direct C–C coupling of acetone in a Hilbert fractal photo microreactor

A Hilbert fractal photo microreactor (PMR) was developed and used in the continuous photochemical synthesis of 2,5-hexanedione (2,5-HDN) via direct C–C coupling of acetone.

Continuous Flow Photochemical and Thermal Multi-Step Synthesis of Bioactive 3-Arylmethylene-2,3-Dihydro-1H-Isoindolin-1-Ones

An effective multi-step continuous flow approach towards N-diaminoalkylated 3-arylmethylene-2,3-dihydro-1H-isoindolin-1-ones, including the local anesthetic compound AL-12, has been realized. Compared to the traditional decoupled batch processes, the combined photochemical–thermal–thermal flow setup rapidly provides the desired target compounds in superior yields and significantly shorter reaction times.

Tuning photosensitized singlet oxygen production from microgels synthesized by polymerization in aqueous dispersed media

Miniemulsion copolymerization of vinyl acetate, N-vinylcaprolactam, vinyl benzyl Rose Bengal and divinyl adipate to synthesize switchable photosensitizer-grafted polymer colloids for interfacial photooxygenation reactions.

Sensitized [2 + 2] intramolecular photocycloaddition of unsaturated enones using UV LEDs in a continuous flow reactor: kinetic and preparative aspects

A photocatalysed cycloaddition by benzophenone derivatives under flow and UVA LED irradiation is described.

Selective synthesis of azoxybenzenes from nitrobenzenes by visible light irradiation under continuous flow conditions

Herein we report a highly selective preparation method of azoxybenzenes from nitrobenzenes by visible-light irradiation using flow microreactors.

Visible-Light Photocatalytic Difluoroalkylation-Induced 1, 2-Heteroarene Migration of Allylic Alcohols in Batch and Flow

A convenient method for the preparation of sp³-rich heterocycles is reported. The method comprises a photocatalytic difluoroalkylation-induced 1,2-heteroarene migration of allylic alcohols. Here we describe for the first time the benefits of using flow to facilitate such migration reactions, including shorter reaction times, higher selectivities, and opportunities to scale the chemistry.

Electron transfer-induced reduction of organic halides with amines

Reduction of a variety of organo halides was examined by using amines as a sacrificial hydrogen source. UV light-induced reduction of vinyl and aryl halides with triethylamine proceeded smoothly to give the corresponding reduced products. High temperature heating also caused the reduction and DABCO (1,4-diazabicyclo[2.2.2]octane) also served as a good reducing reagent.

Recent Advances in Photodecarboxylations Involving Phthalimides

Owing to their favourable photophysical and electrochemical properties, phthalimides undergo a variety of highly efficient photodecarboxylation reactions. These transformations have been applied to the synthesis of macrocyclic compounds as well as bioactive addition adducts. N-Acetoxyphthalimides are versatile precursors to imidyl and alkyl radicals through photodecarboxylation and have subsequently been used for a variety of coupling reactions. The generally mild reaction conditions make these reactions attractive for green chemical applications. The process protocols were successfully transferred to novel photoreactor devices, among these falling film or continuous flow reactors.

Systematic investigations on fused π-system compounds of seven benzene rings prepared by photocyclization of diphenanthrylethenes

Molecular and crystal structures of photoproducts (n@m and n@mPP) from diphenanthrylethenes (nEm) were studied to understand the photocyclization patterns, and the photophysical properties.