The Development of Visible-Light Photoredox Catalysis in Flow

Advancements in hydrochlorination of alkenes

The hydrochlorination of alkenes has been extensively studied in research and is commonly featured in organic chemistry textbooks as an exemplification of the Markovnikov rule. However, the application of this reaction is typically limited to specific alkenes, such as highly substituted ones, styrenes, or strained systems. Conversely, monosubstituted or 1,2-disubstituted alkenes do not readily react with HCl gas or solutions of HCl gas at practical rates. The challenges associated with hydrochlorination reactions for these "non-activated" alkenes have spurred considerable research efforts over the past 30 years, which constitute the primary focus of this review. The discussion begins with classical polar hydrochlorinations, followed by metal-promoted radical hydrochlorinations, and concludes with a brief overview of recent anti-Markovnikov hydrochlorinations.

Fluoroalkyl Sulfoximines for Versatile Photocatalytic Radical Fluoroalkylations

Fluoroalkyl sulfoximines, which serve as electron-accepting fluoroalkyl radical sources, are easy-to-handle, solid, and bench-stable chemicals. Fluoroalkyl radicals can be generated from sulfoximine reagents using strong one-electron injectors, such as a highly reducing photoredox catalyst in the excited state. Our group has developed photocatalytic radical di- and mono-fluoromethylation and α-monofluoroalkylation of olefins with the corresponding fluoroalkyl sulfoximines. In this personal account, appropriate combinations of fluoroalkyl sulfoximines and photoredox catalysts, leading to successful radical fluoroalkylation, have been discussed.

Photochemical diazidation of alkenes enabled by ligand-to-metal charge transfer and radical ligand transfer

Vicinal diamines are privileged synthetic motifs in chemistry due to their prevalence and powerful applications in bioactive molecules, pharmaceuticals, and ligand design for transition metals. With organic diazides being regarded as modular precursors to vicinal diamines, enormous efforts have been devoted to developing efficient strategies to access organic diazide generated from olefins, themselves common feedstock chemicals. However, state-of-the-art methods for alkene diazidation rely on the usage of corrosive and expensive oxidants or complicated electrochemical setups, significantly limiting the substrate tolerance and practicality of these methods on large scale. Toward overcoming these limitations, here we show a photochemical diazidation of alkenes via iron-mediated ligand-to-metal charge transfer (LMCT) and radical ligand transfer (RLT). Leveraging the merger of these two reaction manifolds, we utilize a stable, earth abundant, and inexpensive iron salt to function as both radical initiator and terminator. Mild conditions, broad alkene scope and amenability to continuous-flow chemistry rendering the transformation photocatalytic were demonstrated. Preliminary mechanistic studies support the radical nature of the cooperative process in the photochemical diazidation, revealing this approach to be a powerful means of olefin difunctionalization.

A One-Pot Photochemical Method for the Generation of Functionalized Aminocyclopentanes

Detailed herein is the development of a photochemical intermolecular formal [3+2] cycloaddition between cyclopropylimines and substituted alkenes to generate aminocyclopentane derivatives. The Schiff base of the cyclopropylimine was designed to enable a masked N-centered radical approach in which the requisite open-shell character was achieved upon excitation with visible light. The cycloaddition products were directly converted to N-functionalized aminocyclopentanes via solvolysis and N-acylation. The photochemical component of this reaction sequence was demonstrated to operate in continuous flow.

Stereoselective, Ruthenium-Photocatalyzed Synthesis of 1,2-Diaminotruxinic Bis-amino Acids from 4-Arylidene-5(4H)-oxazolones

The irradiation of (Z)-2-phenyl-4-aryliden-5(4H)-oxazolones 1 in deoxygenated CH₂Cl₂ at 25 °C with blue light (465 nm) in the presence of [Ru(bpy)₃](BF₄)₂ (5% mole ratio) as a triplet photocatalyst promotes the [2+2] photocycloaddition of the C=C bonds of the 4-arylidene moiety, thus allowing the completely regio- and stereoselective formation of cyclobutane-bis(oxazolone)s 2 as single stereoisomers. Cyclobutanes 2 have been unambiguously characterized as the μ-isomers and contain two E-oxazolones coupled in an anti-head-to-head form. The use of continuous-flow techniques in microreactors allows the synthesis of cyclobutanes 2 in only 60 min, compared with the 24–48 h required in batch mode. Ring opening of the oxazolone heterocycle in 2 with a base affords the corresponding 1,2-diaminotruxinic bis-amino esters 3, which are also obtained selectively as μ-isomers. The ruthenium complex behaves as a triplet photocatalyst, generating the reactive excited state of the oxazolone via an energy-transfer process. This reactive excited state has been characterized as a triplet diradical ³(E/Z)-1* by laser flash photolysis (transient absorption spectroscopy). This technique also shows that this excited state is the same when starting from either (Z)- or (E)-oxazolones. Density functional theory calculations show that the first step of the [2+2] cycloaddition between ³(E/Z)-1* and (Z)-1 is formation of the C(H)–C(H) bond and that (Z) to (E) isomerization takes place at the 1,4-diradical thus formed.

Photocatalytic Oxidative Bromination of 2,6-Dichlorotoluene to 2,6-Dichlorobenzyl Bromide in a Microchannel Reactor

Photocatalytic oxidative benzylic bromination with hydrobromic acid (HBr) and hydrogen peroxide (H₂O₂) is a green process for the synthesis of benzyl bromides, but suffers from the risk of explosion when performing it in a batch reactor. This disadvantage could be overcome by running the reaction in a microchannel reactor. In this work, a green and safe process for the synthesis of 2,6-dichlorobenzyl bromide (DCBB) was developed by conducting selective benzylic bromination of 2,6-dichlorotoluene (DCT) with H₂O₂ as an oxidant and HBr as a bromine source in a microchannel reactor under light irradiation. The reaction parameters were optimized, and the conversion of DCT reached up to 98.1% with a DCBB yield of 91.4% under the optimal reaction conditions.

Photoinduced Organocatalyzed Atom Transfer Radical Polymerization (O-ATRP): Precision Polymer Synthesis Using Organic Photoredox Catalysis

The development of photoinduced organocatalyzed atom transfer radical polymerization (O-ATRP) has received considerable attention since its introduction in 2014. Expanding on many of the advantages of traditional ATRP, O-ATRP allows well-defined polymers to be produced under mild reaction conditions using organic photoredox catalysts. As a result, O-ATRP has opened access to a range of sensitive applications where the use of a metal catalyst could be of concern, such as electronics, certain biological applications, and the polymerization of coordinating monomers. However, key limitations of this method remain and necessitate further investigation to continue the development of this field. As such, this review details the achievements made to-date as well as future research directions that will continue to expand the capabilities and application landscape of O-ATRP.

Generation of Oxyphosphonium Ions by Photoredox/Cobaloxime Catalysis for Scalable Amide and Peptide Synthesis in Batch and Continuous-Flow

Phosphine-mediated deoxygenative nucleophilic substitutions, such as the Mitsunobu reaction, are of great importance in organic synthesis. However, the conventional protocols require stoichiometric oxidants to trigger the formation of the oxyphosphonium intermediates for the subsequent nucleophilic additions. Through dual catalysis of photoredox and cobaloxime, we realized a radical strategy for the catalytic formation of acyloxyphosphonium ions that enables direct amidation. The deoxygenative protocol exhibits a broad scope and has been used in the late-stage amidation of drug molecules. In addition to batch reactions, a continuous-flow reactor was developed, enabling rapid peptide synthesis on gram scale. The successful assembly of a tetrapeptide on the solid support further demonstrated the versatility of this photocatalytic system. Moreover, experimental and computational studies are consistent with the hypothesis of acyloxyphosphonium ions being formed as the key intermediates.

Photocatalysis in the Life Science Industry

In the pursuit of new pharmaceuticals and agrochemicals, chemists in the life science industry require access to mild and robust synthetic methodologies to systematically modify chemical structures, explore novel chemical space, and enable efficient synthesis. In this context, photocatalysis has emerged as a powerful technology for the synthesis of complex and often highly functionalized molecules. This Review aims to summarize the published contributions to the field from the life science industry, including research from industrial-academic partnerships. An overview of the synthetic methodologies developed and strategic applications in chemical synthesis, including peptide functionalization, isotope labeling, and both DNA-encoded and traditional library synthesis, is provided, along with a summary of the state-of-the-art in photoreactor technology and the effective upscaling of photocatalytic reactions.

Visible-Light-Induced Deaminative Alkylation/Cyclization of Alkyl Amines with N-Methacryloyl-2-phenylbenzoimidazoles in Continuous-Flow Organo-Photocatalysis

Herein, we present a metal-free visible-light-induced eosin-y-catalyzed deaminative strategy for the sequential alkylation/cyclization of N-methacryloyl-2-phenylbenzoimidazoles with alkyl amine-derived Katritzky salts, which provides an efficient avenue for the construction of various benzo[4,5]imidazo[2,1-a]isoquinolin-6(5H)-one derivatives in moderate to excellent yields under mild reaction conditions. The key enabling feature of this novel reaction includes utilization of redox-active pyridinium salts from abundant and inexpensive primary amine feedstocks that were converted into alkyl radicals via C-N bond scission and subsequent alkylation/cyclization with N-methacryloyl-2-phenylbenzoimidazoles by the formation of two new C-C bonds. In addition, we implemented this protocol for a variety of amino acids, affording the products in moderate yields. Moreover, the novel, environmentally benign batch protocol was further carried out in a continuous-flow regime by utilizing a perfluoroalkoxy alkane tubing microreactor under optimized reaction conditions with a blue light-emitting diode light source, enabling excellent yields and a shorter reaction time (19 min) versus the long reaction time (16 h) of the batch reaction. The reaction displays excellent functional group tolerance, easy operation, scalability, mild reaction conditions, and broad synthetic utility.

Visible-Light-Mediated Z-Stereoselective Monoalkylation of β,β-Dichlorostyrenes by Photoredox/Nickel Dual Catalysis

Metal-catalyzed alkylation of 1,1-dihalovinyl moiety commonly suffers from both a lack of stereoselectivity and the overreaction leading to the dialkylation product. The methodology described herein features a new pathway to alkylate stereoselectively β,β-dichlorostyryl substrates to provide the Z-trisubstituted olefin only with fair to good yields. This cross-coupling reaction bears on the smooth and photoinduced formation of a C-centered radical that engages in a nickel-catalyzed organometallic cycle to form the key Csp2–Csp3 bond.

Rapid Optimization of Photoredox Reactions for Continuous-Flow Systems Using Microscale Batch Technology

Photoredox catalysis has emerged as a powerful and versatile platform for the synthesis of complex molecules. While photocatalysis is already broadly used in small-scale batch chemistry across the pharmaceutical sector, recent efforts have focused on performing these transformations in process chemistry due to the inherent challenges of batch photocatalysis on scale. However, translating optimized batch conditions to flow setups is challenging, and a general approach that is rapid, convenient, and inexpensive remains largely elusive. Herein, we report the development of a new approach that uses a microscale high-throughput experimentation (HTE) platform to identify optimal reaction conditions that can be directly translated to flow systems. A key design point is to simulate the flow-vessel pathway within a microscale reaction plate, which enables the rapid identification of optimal flow reaction conditions using only a small number of simultaneous experiments. This approach has been validated against a range of widely used photoredox reactions and, importantly, was found to translate accurately to several commercial flow reactors. We expect that the generality and operational efficiency of this new HTE approach to photocatalysis will allow rapid identification of numerous flow protocols for scale.

Mechanistic Investigation and Optimization of Photoredox Anti-Markovnikov Hydroamination

The reaction mechanism and the origin of the selectivity for the photocatalytic intermolecular anti-Markovnikov hydroamination of unactivated alkenes with primary amines to furnish secondary amines have been revealed by time-resolved laser kinetics measurements of the key reaction intermediates. We show that back-electron transfer (BET) between the photogenerated aminium radical cation (ARC) and reduced photocatalyst complex (Ir(II)) is nearly absent due to rapid deprotonation of the ARC on the sub-100 ns time scale. The selectivity for primary amine alkylation is derived from the faster addition of the primary ARCs (as compared to secondary ARCs) to alkenes. The turnover of the photocatalyst occurs via the reaction between Ir(II) and a thiyl radical; the in situ formation of an off-cycle disulfide from thiyl radicals suppresses this turnover, diminishing the efficiency of the reaction. With these detailed mechanistic insights, the turnover of the photocatalyst has been optimized, resulting in a >10-fold improvement in the quantum yield. These improvements enabled the development of a scalable flow protocol, demonstrating a potential strategy for practical applications with improved energy efficiency and cost-effectiveness.

Solar and Visible Light Assisted Peptide Coupling

Amino acid and peptide couplings are widely used in fields related to pharma and materials. Still, current peptide synthesis continues to rely on the use of expensive, water sensitive, and waste-generating coupling reagents, which are often prepared in multi-step sequences and used in excess. Herein is described a peptide coupling reaction design that relies mechanistically on sun-light activation of a 4-dimethylamino-pyridine-alkyl halide charge-transfer complex to generate a novel coupling reagent in situ. The resulting coupling method is rapid, does not require dry solvents or inert atmosphere, and is compatible with all the most common amino acids and protecting groups. Peptide couplings can be run on gram-scale, without the use of special equipment. This method has a significantly reduced environmental and financial footprint compared to standard peptide coupling reactions. Experimental and computational studies support the proposed mechanism.

Metal-Free ATRP Catalyzed by Visible Light in Continuous Flow

ATRP of methyl methacrylate catalyzed by Eosin Y, an inexpensive and an environmental benign dye, was performed in a continuous flow reactor made of FEP tubing and irradiated by visible light green LEDs. The reaction under flow conditions was significantly more rapid and controlled compared to that in batch giving 90% of polymerization after only 3 h of irradiation. The formed polymers in flow have M n measured by GPC and DOSY NMR in accordance with the theoretical values and show low dispersities (Ð < 1.5). The livingness of the polymers has been confirmed by LED on and LED off experiments and by the synthesis of block copolymers. The protocol described herein serves as a "proof of concept" of using Eosin Y as a photocatalyst for controlled polymerization and of using 1D and 2D NMR for polymer characterization. The protocol could be replicated in the future for other reversible-deactivation radical polymerizations.

Late stage C-H functionalization via chalcogen and pnictogen salts

Late-stage functionalization (LSF) of heteroarenes can dramatically accelerate SAR studies by enabling the installation of functional groups that would otherwise complicate a synthetic sequence. Although heteroaryl halides and boronic esters have well-established chemistries for LSF, alternatives that enable site-selective C-H functionalization are highly attractive. Recently, three unrelated cationic groups (phosphonium, pyridinium, and thianthrenium), which can replace C-H bonds late stage, have been identified as precursors to various functional groups. This review will discuss the synthesis and application of these three salts with an emphasis on their use for LSF and application to medicinal chemistry.

Synthetic Photoelectrochemistry

Photoredox catalysis (PRC) and synthetic organic electrochemistry (SOE) are often considered competing technologies in organic synthesis. Their fusion has been largely overlooked. We review state-of-the-art synthetic organic photoelectrochemistry, grouping examples into three categories: 1) electrochemically mediated photoredox catalysis (e-PRC), 2) decoupled photoelectrochemistry (dPEC), and 3) interfacial photoelectrochemistry (iPEC). Such synergies prove beneficial not only for synthetic "greenness" and chemical selectivity, but also in the accumulation of energy for accessing super-oxidizing or -reducing single electron transfer (SET) agents. Opportunities and challenges in this emerging and exciting field are discussed.

A method to determine the correct photocatalyst concentration for photooxidation reactions conducted in continuous flow reactors

When conducting a photooxidation reaction, the key question is what is the best amount of photocatalyst to be used in the reaction? This work demonstrates a fast and simple method to calculate a reliable concentration of the photocatalyst that will ensure an efficient reaction. The determination is based on shifting the calculation away from the concentration of the compound to be oxidized to utilizing the limitations on the total light dose that can be delivered to the catalyst. These limitations are defined by the photoflow setup, specifically the channel height and the emission peak of the light source. This method was tested and shown to work well for three catalysts with different absorption properties through using LEDs with emission maxima close to the absorption maximum of each catalyst.

Recent applications of porphyrins as photocatalysts in organic synthesis: batch and continuous flow approaches

In this review we present relevant and recent applications of porphyrin derivatives as photocatalysts in organic synthesis, involving both single electron transfer (SET) and energy transfer (ET) mechanistic approaches. We demonstrate that these highly conjugated photosensitizers show increasing potential in photocatalysis since they combine both photo- and electrochemical properties which can substitute available metalloorganic photocatalysts. Batch and continuous-flow approaches are presented highlighting the relevance of enabling technologies for the renewal of porphyrin applications in photocatalysis. Finally, the reaction scale in which the methodologies were developed are highlighted since this is an important parameter in the authors' opinion.

Visible-light-induced addition of carboxymethanide to styrene from monochloroacetic acid

Where monochloroacetic acid is widely used as a starting material for the synthesis of relevant groups of compounds, many of these synthetic procedures are based on nucleophilic substitution of the carbon chlorine bond. Oxidative or reductive activation of monochloroacetic acid results in radical intermediates, leading to reactivity different from the traditional reactivity of this compound. Here, we investigated the possibility of applying monochloroacetic acid as a substrate for photoredox catalysis with styrene to directly produce γ-phenyl-γ-butyrolactone. Instead of using nucleophilic substitution, we cleaved the carbon chlorine bond by single-electron reduction, creating a radical species. We observed that the reaction works best in nonpolar solvents. The reaction does not go to full conversion, but selectively forms γ-phenyl-γ-butyrolactone and 4-chloro-4-phenylbutanoic acid. Over time the catalyst precipitates from solution (perhaps in a decomposed form in case of fac-[Ir(ppy)₃]), which was proven by mass spectrometry and EPR spectroscopy for one of the catalysts (N,N-5,10-di(2-naphthalene)-5,10-dihydrophenazine) used in this work. The generation of HCl resulting from lactone formation could be an additional problem for organometallic photoredox catalysts used in this reaction. In an attempt to trap one of the radical intermediates with TEMPO, we observed a compound indicating the generation of a chloromethyl radical.

A Metallaphotoredox Method for the Expansion of Benzyl SAR on Electron-Deficient Amines

A metallaphotoredox reaction is described that allows for the efficient exploration of benzyl structure-activity relationships on electron-deficient amines. Typically, accessing a variety of benzyl groups on these substrates can be difficult due to the limited availability of the prerequisite building blocks, namely benzyl halides. However, the use of aryl bromides in this metallaphotoredox reaction allows for greater diversity in the benzyl piece. The reaction scope is discussed herein, including conditions for product scaleup using flow.

Electrochemistry and Photoredox Catalysis: A Comparative Evaluation in Organic Synthesis

This review provides an overview of synthetic transformations that have been performed by both electro- and photoredox catalysis. Both toolboxes are evaluated and compared in their ability to enable said transformations. Analogies and distinctions are formulated to obtain a better understanding in both research areas. This knowledge can be used to conceptualize new methodological strategies for either of both approaches starting from the other. It was attempted to extract key components that can be used as guidelines to refine, complement and innovate these two disciplines of organic synthesis.

Catalytic Aerobic Oxidation of C(sp3 )-H Bonds

Oxidation reactions are a key technology to transform hydrocarbons from petroleum feedstock into chemicals of a higher oxidation state, allowing further chemical transformations. These bulk-scale oxidation processes usually employ molecular oxygen as the terminal oxidant as at this scale it is typically the only economically viable oxidant. The produced commodity chemicals possess limited functionality and usually show a high degree of symmetry thereby avoiding selectivity issues. In sharp contrast, in the production of fine chemicals preference is still given to classical oxidants. Considering the strive for greener production processes, the use of O₂ , the most abundant and greenest oxidant, is a logical choice. Given the rich functionality and complexity of fine chemicals, achieving regio/chemoselectivity is a major challenge. This review presents an overview of the most important catalytic systems recently described for aerobic oxidation, and the current insight in their reaction mechanism.

Structure and Reactivity of Aromatic Radical Cations Generated by FeCl3

This paper describes the isolation and characterization of an aromatic radical cation generated by FeCl₃. X-ray crystallographic analysis and kinetic studies reveal the mechanism of the generation of aromatic radical cation. In the solid state, a tight ion-pair of a radical cation with FeCl₄^- is observed. Leveraging the efficient generation of the radical cation-FeCl₄^- ion pair, we explore a radical cation-induced cycloaddition of trans-anethole initiated by catalytic amount of FeCl₃. Both [4+2] cycloaddition and [2+2] cycloaddition with a broad substrate scope are also described. Moreover, a 100 g-scale reaction is demonstrated with the use of 1 mol % of FeCl₃ as a simple and a highly active initiator.

A Laser Driven Flow Chemistry Platform for Scaling Photochemical Reactions with Visible Light

Visible-light-promoted organic reactions can offer increased reactivity and selectivity via unique reaction pathways to address a multitude of practical synthetic problems, yet few practical solutions exist to employ these reactions for multikilogram production. We have developed a simple and versatile continuous stirred tank reactor (CSTR) equipped with a high-intensity laser to drive photochemical reactions at unprecedented rates in continuous flow, achieving kg/day throughput using a 100 mL reactor. Our approach to flow reactor design uses the Beer-Lambert law as a guideline to optimize catalyst concentration and reactor depth for maximum throughput. This laser CSTR platform coupled with the rationale for design can be applied to a breadth of photochemical 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.

Cross coupling of alkylsilicates with acyl chlorides via photoredox/nickel dual catalysis: a new synthesis method for ketones

Photoredox/nickel dual catalysis using silicates and acyl chlorides allows a new method of formation of ketones. Flow chemistry can be applied.