A Scalable Procedure for Light-Induced Benzylic Brominations in Continuous Flow

Light on the sustainable preparation of aryl-cored dibromides

Both aryl and benzyl polybromides have gained significant importance as reactive building blocks in polymer and materials chemistry. Their preparation primarily relies on established synthetic methods using molecular bromine or N-bromosuccinimide, known for their reliability and effectiveness. However, from a sustainability perspective, these methods suffer from the generation of stoichiometric amounts of byproducts and often encounter selectivity troubles. To mitigate these issues, we extended the greener peroxide-bromide halogenation method, initially developed for monobromides, to afford aryl-cored polybromides in high yields. The same method can be employed in two variants modulated by light irradiation. This external switch can be used to selectively trigger side-chain or core halogenation.

Chemoselective Synthesis of 3-Bromomethyloxindoles via Visible-Light-Induced Radical Cascade Bromocyclization of Alkenes

A novel visible-light-induced radical cascade bromocyclization of N-arylacrylamides has been accomplished. This reaction overcomes the overbromination at the benzene rings suffered in traditional electrophilic reactions, thus enabling the first highly chemoselective synthesis of valuable 3-bromomethyloxindoles. The combination of pyridine and anhydrous medium is identified as the key factor for the high chemoselectivity in the current photoreaction system, which might work by suppressing the in situ generation of low-concentration Br2 from N-bromosuccinimide. Moreover, the mild reaction conditions ensure the generation of a wide range of the new desired products with excellent functional group tolerance.

Parallel Paired Photoelectrochemical Bromination of Alkylarenes with Electrochemical Pinacol Coupling

A paired electrochemical method for paralleling benzylic bromination of alkylarenes under irradiation with reductive pinacol coupling in a divided cell has been developed. A variety of benzyl bromides at the anode and pinacols at the cathode were obtained simultaneously in moderate-to-high faradaic efficiency. This parallel paired electrochemical protocol showed a broad substrate scope and high chemoselectivity as well as high synthetic and faradaic efficiencies.

A Convenient Synthesis of Novel Isoxazolidine and Isoxazole Isoquinolinones Fused Hybrids

Isoxazolidine, isoxazole, and isoquinolinone rings are present in the structure of several natural products and/or pharmaceutically interesting compounds. In this work, facile and efficient pathways have been developed for the preparation of fused frameworks bearing those heterocycles. The successful approaches for both isoxazolidine/isoquinolinone and isoxazole/isoquinolinone hybrid syntheses relied initially on 1,3-dipolar cycloadditions of nitrones and nitrile oxides to indenone and 2-propargylbenzamide, respectively. The construction of the isoquinolinone lactam system followed by performing a selective Schmidt reaction for isoxazolidine derivatives (two steps overall), whereas the isoxazole lactams were reached via an Ullmann-type cyclisation (three steps overall). Key observations were made regarding the stereo- and regioselectivities of the reactions employed, and small libraries of the targeted hybrids were prepared, demonstrating the general applicability of these strategies.

Visible-Light-Driven Organocatalytic Alkoxylation of Benzylic C-H Bonds

A variety of strategies for direct alkoxylation of the benzyl C-H bond have been developed toward the construction of benzyl ethers. The light-induced benzyl C-H bond alkoxylation provides an alternative strategy for the synthesis of these important intermediates. The photocatalyzed alkoxylation of the benzyl C-H bond has dominated by metal-catalyzed methods. Herein, we reported a light-driven organocatalytic approach for alkoxylation of the benzyl C-H bond by the use of 9,10-dibromoanthracene as a photocatalyst and employing N-fluorobenzenesulfonimide as an oxidant. This reaction proceeds at room temperature and is capable of converting a variety of alkyl biphenyl and coupling partners, including a variety of alcohol and carboxylic acid, as well as peroxide, to the desired products under 400 nm light irradiation.

Azolation of Benzylic C-H Bonds via Photoredox-Catalyzed Carbocation Generation

A visible-light photoredox-catalyzed method is reported that enables the coupling between benzylic C-H substrates and N-H azoles. Classically, medicinally relevant N-benzyl azoles are produced via harsh substitution conditions between the azole and a benzyl electrophile in the presence of strong bases at high temperatures. Use of C-H bonds as the alkylating partner streamlines the preparation of these important motifs. In this work, we report the use of N-alkoxypyridinium salts as a critically enabling reagent for the development of a general C(sp3)-H azolation. The platform enables the alkylation of electron-deficient, -neutral, and -rich azoles with a range of C-H bonds, most notably secondary and tertiary partners. Moreover, the protocol is mild enough to tolerate benzyl electrophiles, thus offering an orthogonal approach to existing SN2 and cross-coupling methods.

The design and synthesis of benzylpiperazine-based edaravone derivatives and their neuroprotective activities

New edaravone derivatives containing a benzylpiperazine moiety are designed and synthesized. The structures are characterized by 1H NMR, 13C NMR, and high-resolution mass spectrometry. The potential neuroprotective activities of the target compounds are evaluated in differentiated rat pheochromocytoma cells (PC12 cells) and in mice subjected to acute cerebral ischemia. Most of the target compounds showed neuroprotective activities both in vivo and in vitro, especially 1-(4-(4-fluorobenzyl) piperazin-1-yl)-2-(4-(5-hydroxy-3-methyl-1 H-pyrazol-1-yl)phenoxy)ethanone and 1-(4-(4-nitrobenzyl)piperazin-1-yl)-2-(4-(5-hydroxy-3-methyl-1 H-pyrazol-1-yl)phenoxy)ethanone, which displayed significant protective effects on cell viability against damage caused by H2O2, and remarkably prolonged the survival time of mice subjected to acute cerebral ischemia and decreased the mortality rate at all doses. These compounds represent lead compounds for the further discovery of neuroprotective agents for treating cerebral ischemic stroke. Molecular docking studies and basic structure–activity relationships are also presented.

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.

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.

Developing flow photo-thiol–ene functionalizations of cinchona alkaloids with an autonomous self-optimizing flow reactor

Continuous flow photo-thiol–ene reactions on cinchona alkaloids with a variety of organic thiols have been developed using enabling technologies such as a self-optimizing flow photochemical reactor.

Forgotten and forbidden chemical reactions revitalised through continuous flow technology

Continuous flow technology has played an undeniable role in enabling modern chemical synthesis, whereby a myriad of reactions can now be performed with greater efficiency, safety and control. As flow chemistry furthermore delivers more sustainable and readily scalable routes to important target structures a growing number of industrial applications are being reported. In this review we highlight the impact of flow chemistry on revitalising important chemical reactions that were either forgotten soon after their initial report as necessary improvements were not realised due to a lack of available technology, or forbidden due to unacceptable safety concerns relating to the experimental procedure. In both cases flow processing in combination with further reaction optimisation has rendered a powerful set of tools that make such transformations not only highly efficient but moreover very desirable due to a more streamlined construction of desired scaffolds. This short review highlights important contributions from academic and industrial laboratories predominantly from the last 5 years allowing the reader to gain an appreciation of the impact of flow chemistry.

Deconjugative α-Alkylation of Cyclohexenecarboxaldehydes: An Access to Diverse Terpenoids

A general and efficient method for the deconjugative α-alkylation of α,β-unsaturated aldehydes promoted by a synergistic effect between ^tBuOK and NaH, which considerably increases the reaction rate under mild conditions, is reported. The β,γ-unsaturated aldehyde, resulting from the α-alkylation, is transformed in high yield into the corresponding allyl acetate via a lead(IV) acetate-mediated oxidative fragmentation. This strategy could be used for the construction of the carbon skeleton of a wide variety of alkyl or arylterpenoids.

Reactivity of substrates with multiple competitive reactive sites toward NBS under neat reaction conditions promoted by visible light

Regioselectivity of visible-light-induced transformations of a range of (hetero)aryl alkyl-substituted ketones bearing several competitive reactive sites (α-carbonyl, benzyl and aromatic ring) with N-bromosuccinimide (NBS) was studied under solvent-free reaction conditions (SFRC) and in the absence of inert-gas atmosphere, radical initiators and catalysts. An 8-W energy-saving household lamp was used for irradiation. Heterogeneous reaction conditions were dealt with throughout the study. All substrates were mono- or dibrominated at the α-carbonyl position, and additionally, some benzylic or aromatic bromination was observed in substrates with benzylic carbon atoms or electron-donating methoxy groups, respectively. Surprisingly, ipso-substitution of the acyl group with a bromine atom took place with (4-methoxynaphthyl) alkyl ketones. While the addition of the radical scavenger TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxy) decreased the extent of α- and ring bromination, it completely suppressed the benzylic bromination and α,α-dibromination with NBS under SFRC.

Evaluation of a Continuous-Flow Photo-Bromination Using N-Bromosuccinimide for Use in Chemical Manufacture

A continuous-flow photo-bromination reaction on benzyl and phenyl groups was conducted using N-bromosuccinimide as the bromine source inside a preparatory-scale glass plate reactor. This flow reactor system was capable of independently controlling light intensity, wavelength, and reaction temperature, hence exerting an exceptional level of control over the reaction. A short optimisation study for the synthesis of 2-bromomethyl-4-trifluoromethoxyphenylboronic acid pinacol ester resulted in best conditions of 20°C and 10min residence time using an LED (light-emitting diode) array at 405nm and acetonitrile as the solvent. The present study evaluates the potential for this easy-to-handle bromination system to be scaled up for chemical manufacture inside a continuous-flow glass plate reactor. The combination with an in-line continuous flow liquid–liquid extraction and separation system, using a membrane separator, demonstrates the potential for continuous flow reaction with purification in an integrated multi-stage operation with minimal manual handling in between.

1,1,2-Tribromoethyl arenes: novel and highly efficient precursors for the synthesis of 1-bromoalkynes and α-bromoketones

A novel tribromination method to prepare versatile intermediate 1,1,2-tribromoethyl arenes, which can not only be transformed to synthetically valuable 1-bromoalkynes via elimination but also be hydrolyzed to a variety of α-bromoketones, was developed.

Achieving selectivity in porphyrin bromination through a DoE-driven optimization under continuous flow conditions

The post-functionalization of porphyrins through the bromination in β position of the pyrrolic rings is a relevant transformation because the resulting bromoderivatives are useful synthons to covalently link a variety of chemical architectures to a porphyrin ring. However, single bromination of porphyrins is a challenging reaction for the abundancy of reactive β-pyrrolic positions in the aromatic macrocycle. We herein report a synthetic procedure for the efficient preparation of 2-bromo-5,10,15,20-tetraphenylporphyrin (1) under continuous flow conditions. The use of flow technology allows to reach an accurate control over critical reaction parameters such as temperature and reaction time. Furthermore, by performing the optimization process through a statistical DoE (Design of Experiment) approach, these parameters could be properly adjusted with a limited number of experiments. This process led us to a better understanding of the relevant factors that govern porphyrins monobromination and to obtain compound 1 with an unprecedent 80% yield.

Flow Chemistry in Contemporary Chemical Sciences: A Real Variety of Its Applications

Flow chemistry is an area of contemporary chemistry exploiting the hydrodynamic conditions of flowing liquids to provide particular environments for chemical reactions. These particular conditions of enhanced and strictly regulated transport of reagents, improved interface contacts, intensification of heat transfer, and safe operation with hazardous chemicals can be utilized in chemical synthesis, both for mechanization and automation of analytical procedures, and for the investigation of the kinetics of ultrafast reactions. Such methods are developed for more than half a century. In the field of chemical synthesis, they are used mostly in pharmaceutical chemistry for efficient syntheses of small amounts of active substances. In analytical chemistry, flow measuring systems are designed for environmental applications and industrial monitoring, as well as medical and pharmaceutical analysis, providing essential enhancement of the yield of analyses and precision of analytical determinations. The main concept of this review is to show the overlapping of development trends in the design of instrumentation and various ways of the utilization of specificity of chemical operations under flow conditions, especially for synthetic and analytical purposes, with a simultaneous presentation of the still rather limited correspondence between these two main areas of flow chemistry.

Pushing the boundaries of C–H bond functionalization chemistry using flow technology

C–H functionalization chemistry is one of the most vibrant research areas within synthetic organic chemistry. While most researchers focus on the development of small-scale batch-type transformations, more recently such transformations have been carried out in flow reactors to explore new chemical space, to boost reactivity or to enable scalability of this important reaction class. Herein, an up-to-date overview of C–H bond functionalization reactions carried out in continuous-flow microreactors is presented. A comprehensive overview of reactions which establish the formal conversion of a C–H bond into carbon–carbon or carbon–heteroatom bonds is provided; this includes metal-assisted C–H bond cleavages, hydrogen atom transfer reactions and C–H bond functionalizations which involve an SE-type process to aromatic or olefinic systems. Particular focus is devoted to showcase the advantages of flow processing to enhance C–H bond functionalization chemistry. Consequently, it is our hope that this review will serve as a guide to inspire researchers to push the boundaries of C–H functionalization chemistry using flow technology.

Synthesis of Alkyl Halides from Aldehydes via Deformylative Halogenation

An unprecedented deformylative halogenation of aldehydes to alkyl halides is presented. Under oxidative conditions, 1,4-dihydropyridine (DHP), derived from an aldehyde, generated a C(sp3)- radical that coupled with a halogen radical that was generated from inexpensive and atom-economical halogen sources (NaBr, NaI, or HCl), to yield an alkyl halide. Because of the mild conditions, a wide range of functional groups were tolerated, and excellent site selectivity was achieved.

Photochemical benzylic bromination in continuous flow using BrCCl3 and its application to telescoped p-methoxybenzyl protection

Photochemical benzylic bromination in flow using BrCCl₃, which is compatible with electron-rich aromatics, allowing in situ p-methoxybenzyl bromide formation and PMB-protection.

Dichlorophenylacrylonitriles as AhR Ligands That Display Selective Breast Cancer Cytotoxicity in vitro

Knoevenagel condensation of 3,4-dichloro- and 2,6-dichlorophenylacetonitriles gave a library of dichlorophenylacrylonitriles. Our leads (Z)-2-(3,4-dichlorophenyl)-3-(1H-pyrrol-2-yl)acrylonitrile (5) and (Z)-2-(3,4-dichlorophenyl)-3-(4-nitrophenyl)acrylonitrile (6) displayed 0.56±0.03 and 0.127±0.04 μm growth inhibition (GI₅₀ ) and 260-fold selectivity for the MCF-7 breast cancer cell line. A 2,6-dichlorophenyl moiety saw a 10-fold decrease in potency; additional nitrogen moieties (-NO₂ ) enhanced activity (Z)-2-(2,6-dichloro-3-nitrophenyl)-3-(2-nitrophenyl)acrylonitrile (26) and (Z)-2-(2,6-dichloro-3-nitrophenyl)-3-(3-nitrophenyl)acrylonitrile (27), with the corresponding -NH₂ analogues (Z)-2-(3-amino-2,6-dichlorophenyl)-3-(2-aminophenyl)acrylonitrile (29) and (Z)-2-(3-amino-2,6-dichlorophenyl)-3-(3-aminophenyl)acrylonitrile (30) being more potent. Despite this, both 29 (2.8±0.03 μm) and 30 (2.8±0.03 μm) were found to be 10-fold less cytotoxic than 6. A bromine moiety effected a 3-fold enhancement in solubility with (Z)-3-(5-bromo-1H-pyrrol-2-yl)-2-(3,4-dichlorophenyl)acrylonitrile 18 relative to 5 at 211 μg mL^-1 . Modeling-guided synthesis saw the introduction of 4-aminophenyl substituents (Z)-3-(4-aminophenyl)-2-(3,4-dichlorophenyl)acrylonitrile (35) and (Z)-N-(4-(2-cyano-2-(3,4-dichlorophenyl)vinyl)phenyl)acetamide (38), with respective GI₅₀ values of 0.030±0.014 and 0.034±0.01 μm. Other analogues such as 35 and 36 were found to have sub-micromolar potency against our panel of cancer cell lines (HT29, colon; U87 and SJ-G2, glioblastoma; A2780, ovarian; H460, lung; A431, skin; Du145, prostate; BE2-C, neuroblastoma; MIA, pancreas; and SMA, murine glioblastoma), except compound 38 against the U87 cell line. A more extensive evaluation of 38 ((Z)-N-(4-(2-cyano-2-(3,4-dichlorophenyl)vinyl)phenyl)acetamide) in a panel of drug-resistant breast carcinoma cell lines showed 10-206 nm potency against MDAMB468, T47D, ZR-75-1, SKBR3, and BT474. Molecular Operating Environment docking scores showed a good correlation between predicted binding efficiencies and observed MCF-7 cytotoxicity. This supports the use of this model in the development of breast-cancer-specific drugs.

An FEP Microfluidic Reactor for Photochemical Reactions

Organic syntheses based on photochemical reactions play an important role in the medical, pharmaceutical, and polymeric chemistry. For years, photochemistry was performed using high-pressure mercury lamps and immersion-wells. However, due to excellent yield, control of temperature, selectivity, low consumption of reagents and safety, the microreactors made of fluorinated ethylene propylene (FEP) tubings have recently been used more frequently. Fluoropolymers are the material of choice for many types of syntheses due to their chemical compatibility and low surface energy. The use of tubing restricts the freedom in designing 2D and 3D geometries of the sections of the microreactors, mixing sections, etc., that are easily achievable in the format of a planar chip. A chip microreactor made of FEP is impracticable to develop due to its high chemical inertness and high melting temperature, both of which make it difficult (or impossible) to bond two plates of polymer. Here, we demonstrate a 'click' system, where the two plates of FEP are joined together mechanically using a tenon and a mortise. The concept was presented by us previously for a preparation polytetrafluoroethylene (PTFE) microreactor (Szymborski et al. Sensors Actuators, B Chem. 2017, doi:10.1016/j.snb.2017.09.035). Here, we use the same strategy for FEP plates, test the use of the chips in photochemistry and also describe a custom-designed non-transparent polyethylene (PE) mask-holder with a circular opening to guide and focus the ultraviolet (UV) illumination. The solutions that we describe offer tight microreactor chips, preventing any leakage either of the liquid reagents or of UV light outside the reactor. This allows for conducting photochemical synthesis without a fume hood and without special protection against UV radiation.

Halogenation of organic compounds using continuous flow and microreactor technology

Halogenation reactions involving highly reactive halogenating agents can be performed safely and with improved efficiency and selectivity under continuous flow conditions.