Continuous flow synthesis of ketones from carbon dioxide and organolithium or Grignard reagents

Regioselective synthesis of N-containing polycyclic compounds via radical annulation cyclization of 1,7-dienes with aldehydes

A convenient method for oxidant-promoted radical cascade acylation or decarbonylative alkylation of 1,7-dienes with aldehydes has been established. This method allows for the rapid construction of N-containing polycyclic skeletons in a highly regio- and stereoselective manner. This transformation provides a simple and efficient method for the preparation of a range of tetrahydro-6H-indeno[2,1-c]quinolinone derivatives by sequential formation of three new carbon-carbon bonds. Additionally, this radical cascade cyclization can selectively convert aldehydes into aroyl/primary aliphatic acyl radicals and secondary or tertiary alkyl radicals.

Nickel Catalyzed Carbonylation/Carboxylation Sequence via Double CO2 Incorporation

A carbonylation-carboxylation synthetic sequence, via double CO2 fixation, is described. The productive merger of a Ni-catalyzed cross-electrophile coupling manifold, with the use of AlCl3, triggered a cascade reaction with the formation of three consecutive C-C bonds in a single operation. This strategy traces an unprecedented synthetic route to ketones under Lewis acid assisted carbon dioxide valorization. Computational insights revealed a unique double function of AlCl3, and labeling (13CO2) experiments validate the genuine incorporation of CO2 in both functional groups.

Sustainable and practical formation of carbon-carbon and carbon-heteroatom bonds employing organo-alkali metal reagents

Metal-catalysed cross-coupling reactions are amongst the most widely used methods to directly construct new bonds. In this connection, sustainable and practical protocols, especially transition metal-catalysed cross-coupling reactions, have become the focus in many aspects of synthetic chemistry due to their high efficiency and atom economy. This review summarises recent advances from 2012 to 2022 in the formation of carbon-carbon bonds and carbon-heteroatom bonds by employing organo-alkali metal reagents.

Synthesis of Unsymmetrical Ketones Using Chelation-Controlled Sequential Substitution of N-Triazinylamide/Weinreb Amide by Organometallic Reagents

N-(2,4-Dimethoxy-1,3,5-triazinyl)amide was found to exhibit similar behavior to N-methoxy-N-methylamide (Weinreb amide) but higher reactivity for nucleophilic substitution by organometallic reagents. Triazinylamide suppresses overaddition, leading to the formation of a tertiary alcohol by the chelating ability of the triazinyl and carbonyl groups. Ureas possessing both triazinylamino and methoxy(methyl)amino groups underwent sequential nucleophilic substitution with different organometallic reagents, which furnished unsymmetrical ketones without any detectable tertiary alcohols.

Continuous‐Flow Divergent Lithiation of 2,3‐Dihalopyridines: Deprotolithiation versus Halogen Dance

We describe herein the first halogen dance (HD) in continuous flow on 2-chloro-3-bromopyridine by selectively trapping a (pyridin-4-yl)lithium species that is known to undergo the halogen-dance process. In addition, this lithiated intermediate was trapped at lower temperature before the HD occurs. The HD process was extended to fluoro-iodopyridines by using various electrophiles to afford 28 examples with yields ranging from 42 to 97 % with very short residence times. Finally, scale up of the reaction was demonstrated, affording a promising space-time yield (STY) of 4.2 kg.h^-1 .L^-1 .

Mechanochemistry: New Tools to Navigate the Uncharted Territory of "Impossible" Reactions

Mechanochemical transformations have made chemists enter unknown territories, forcing a different chemistry perspective. While questioning or revisiting familiar concepts belonging to solution chemistry, mechanochemistry has broken new ground, especially in the panorama of organic synthesis. Not only does it foster new "thinking outside the box", but it also has opened new reaction paths, allowing to overcome the weaknesses of traditional chemistry exactly where the use of well-established solution-based methodologies rules out progress. In this Review, the reader is introduced to an intriguing research subject not yet fully explored and waiting for improved understanding. Indeed, the study is mainly focused on organic transformations that, although impossible in solution, become possible under mechanochemical processing conditions, simultaneously entailing innovation and expanding the chemical space.

Rapid Synthesis of α-Chiral Piperidines via a Highly Diastereoselective Continuous Flow Protocol

A practical continuous flow protocol has been developed using readily accessible N-(tert-butylsulfinyl)-bromoimine and Grignard reagents, providing various functionalized piperidines (34 examples) in superior results (typically >80% yield and with >90:10 dr) within minutes. The high-performance scale-up is smoothly carried out, and efficient synthesis of the drug precursor further showcases its utility. This flow process offers rapid and scalable access to enantioenriched α-substituted piperidines.

Electrochemical Dearomative Dicarboxylation of Heterocycles with Highly Negative Reduction Potentials

The dearomative dicarboxylation of stable heteroaromatics using CO₂ is highly challenging but represents a very powerful method for producing synthetically useful dicarboxylic acids, which can potentially be employed as intermediates of biologically active molecules such as natural products and drug leads. However, these types of transformations are still underdeveloped, and concise methodologies with high efficiency (e.g., high yield and high selectivity for dicarboxylations) have not been reported. We herein describe a new electrochemical protocol using the CO₂ radical anion (E_1/2 of CO₂ = -2.2 V in DMF and -2.3 V in CH₃CN vs SCE) that produces unprecedented trans-oriented 2,3-dicarboxylic acids from N-Ac-, Boc-, and Ph-protected indoles that exhibit highly negative reduction potentials (-2.50 to -2.94 V). On the basis of the calculated reduction potentials, N-protected indoles with reduction potentials up to -3 V smoothly undergo the desired dicarboxylation. Other heteroaromatics, including benzofuran, benzothiophene, electron-deficient furans, thiophenes, 1,3-diphenylisobenzofuran, and N-Boc-pyrazole, also exhibit reduction potentials more positive than -3 V and served as effective substrates for such dicarboxylations. The dicarboxylated products thus obtained can be derivatized into useful synthetic intermediates for biologically active compounds in few steps. We also show how the dearomative monocarboxylation can be achieved selectively by choice of the electrolyte, solvent, and protic additive; this strategy was then applied to the synthesis of an octahydroindole-2-carboxylic acid (Oic) derivative, which is a useful proline analogue.

D. dos Santos AC, Kohlhepp SV, Mendoza A. Direct Addition of Grignard Reagents to Aliphatic Carboxylic Acids Enabled by Bulky turbo ‐Organomagnesium Anilides

The synthesis of ketones through addition of organometallic reagents to aliphatic carboxylic acids is a straightforward strategy that is limited to organolithium reagents. More desirable Grignard reagents can be activated and controlled with a bulky aniline‐derived turbo‐Hauser base. This operationally simple procedure allows the straightforward preparation of a variety of aliphatic and perfluoroalkyl ketones alike from functionalized alkyl, aryl and heteroaryl Grignard reagents.

Construction of an α-chiral pyrrolidine library with a rapid and scalable continuous flow protocol

The cost-efficient development of an α-chiral pyrrolidine library is achieved via a rapid continuous flow protocol under mild conditions.

Mechanochemical Grignard Reactions with Gaseous CO2 and Sodium Methyl Carbonate

A one-pot three-step protocol for the preparation of Grignard reagents from organobromides in a ball mill and their subsequent reactions with gaseous carbon dioxide (CO 2 ) or sodium methyl carbonate providing aryl and alkyl carboxylic acids in up to 82% yield is reported. Noteworthy are the short reaction times and the significantly reduced solvent amounts [2.0 equiv. for liquid assisted grinding (LAG) conditions]. Unexpectedly, aryl bromides with methoxy substituents lead to symmetric ketones as major products.

Regioselective Magnesiations and Zincations of Aromatics and Heterocycles Triggered by Lewis Acids

Mixed TMP‐bases (TMP=2,2,6,6‐tetramethylpiperidyl), such as TMPMgCl ⋅ LiCl, TMP₂Mg ⋅ 2LiCl, TMPZnCl ⋅ LiCl and TMP₂Zn ⋅ 2LiCl, are outstanding reagents for the metalation of functionalized aromatics and heterocycles. In the presence of Lewis acids, such as BF₃ ⋅ OEt₂ or MgCl₂, the metalation scope of such bases was dramatically increased, and regioselectivity switches were achieved in the presence or absence of these Lewis acids. Furthermore, highly reactive lithium bases, such as TMPLi or Cy₂NLi, are also compatible with various Lewis acids, such as MgCl₂ ⋅ 2LiCl, ZnCl₂ ⋅ 2LiCl or CuCN ⋅ 2LiCl. Performing such metalations in continuous flow using commercial setups permitted practical and convenient reaction conditions.

Continuous Flow Acylation of (Hetero)aryllithiums with Polyfunctional N,N-Dimethylamides and Tetramethylurea in Toluene

The continuous flow reaction of various aryl or heteroaryl bromides in toluene in the presence of THF (1.0 equiv) with sec -BuLi (1.1 equiv) provided at 25 °C within 40 sec the corresponding aryllithiums which were acylated with various functionalized N,N-dimethylamides including easily enolizable amides at -20 °C within 27 sec, producing highly functionalized ketones in 48-90% yield (36 examples). This method was well suited for the preparation of α-chiral ketones such as naproxene and ibuprofen derived ketones with 99% ee . A one-pot stepwise bis-addition of two different lithium organometallics to 1,1,3,3-tetramethyurea (TMU) provided unsymmetrical ketones in 69-79% yield (9 examples).

Efficient carboxylation of styrene and carbon dioxide by single-atomic copper electrocatalyst

Electrocarboxylation of olefins with carbon dioxide (CO₂) is a potential approach to produce carboxylates as synthetic intermediates of polymer and pharmaceuticals. Nonetheless, due to the intrinsic inertness of CO₂ at ambient conditions, the electrocarboxylation efficiency has been quite limited, typically with high applied potentials and low current densities. In this work, we demonstrate that nitrogen-coordinated single-atomic copper sites on carbon framework (Cu/NC) served as an excellent electrocatalyst for electrocarboxylation of styrene with CO₂. The Cu/NC catalyst allowed to efficiently activate CO₂, followed by nucleophilic attack to carboxylate styrene to produce phenylsuccinic acid, thus leading the reaction toward the CO₂ activation pathway. The enhanced CO₂ activation capability enabled increased selectivity and activity for electrocarboxylation of styrene. The Faradaic efficiency of electrocarboxylation was 92%, suggesting most of the activated CO₂ proceeded to react with styrene rather than direct reduction to CO or CH₄. The electrocarboxylation exhibited almost 100% product selectivity toward phenylsuccinic acid, with a high partial current density of 58 mA·cm^-2 at -2.2 V (vs. Ag/AgI), corresponding to an outstanding production rate of 216 mg·cm^-2·h^-1, substantially exceeding previously reported works. Our work suggests an exciting perspective in electrocarboxylation of olefins by rational design of CO₂ activation electrocatalysts.

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

Due to their intrinsic physical properties, which includes being able to perform as volatile liquids at room and biological temperatures, fragrance ingredients/intermediates make ideal candidates for continuous-flow manufacturing. This review highlights the potential crossover between a multibillion dollar industry and the flourishing sub-field of flow chemistry evolving within the discipline of organic synthesis. This is illustrated through selected examples of industrially important transformations specific to the fragrances and flavours industry and by highlighting the advantages of conducting these transformations by using a flow approach. This review is designed to be a compendium of techniques and apparatus already published in the chemical and engineering literature which would constitute a known solution or inspiration for commonly encountered procedures in the manufacture of fragrance and flavour chemicals.

Continuous flow as an enabling technology: a fast and versatile entry to functionalized glyoxal derivatives

We herein report two complementary strategies employing organolithium chemistry for the synthesis of glyoxal derivatives. Micro-mixer technology allows for the generation of unstable organometallic intermediates and their instantaneous in-line quenching with esters as electrophiles. Selective mono-addition was observed via putative stabilized tetrahedral intermediates. Advantages offered by flow chemistry technologies facilitate direct and efficient access to masked 1,2-dicarbonyl compounds while mitigating undesired by-product formation. These two approaches enable the production of advanced and valuable synthetic building blocks for heterocyclic chemistry with throughputs of grams per minute.

Milled Dry Ice as a C1 Source for the Carboxylation of Aryl Halides

The use of carbon dioxide as a C1 chemical feedstock remains an active field of research. Here we showcase the use of milled dry ice as a method to promote the availability of CO2 in a reaction solution, permitting practical synthesis of arylcarboxylic acids. Notably, the use of milled dry ice produces marked increases in yields relative to those obtained with gaseous CO2, as previously reported in the literature.

How to approach flow chemistry

Flow chemistry is a widely explored technology whose intrinsic features both facilitate and provide reproducible access to a broad range of chemical processes that are otherwise inefficient or problematic. At its core, a flow chemistry module is a stable set of conditions - traditionally thought of as an externally applied means of activation/control (e.g. heat or light) - through which reagents are passed. In an attempt to simplify the teaching and dissemination of this field, we envisioned that the key advantages of the technique, such as reproducibility and the correlation between reaction time and position within the reactor, allow for the redefinition of a flow module to a more synthetically relevant one based on the overall induced effect. We suggest a rethinking of the approach to flow modules, distributing them in two subclasses: transformers and generators, which can be described respectively as a set of conditions for either performing a specific transformation or for generating a reactive intermediate. The chemistry achieved by transformers and generators is (ideally) independent of the substrate introduced, meaning that they must be robust to small adjustments necessary for the adaptation to different starting materials and reagents while ensuring the same chemical outcome. These redefined modules can be used for single-step reactions or in multistep processes, where modules can be connected to each other in reconfigurable combinations to create chemical assembly systems (CAS) targeting compounds and libraries sharing structural cores. With this tutorial review, we provide a guide to the overall approach to flow chemistry, discussing the key parameters for the design of transformers and generators as well as the development of chemical assembly systems.

Continuous-Flow Reactions Mediated by Main Group Organometallics

The generation of reactive organometallic reagents in batch is often complicated by the low thermal stability of these important synthetic intermediates and can require low reaction temperatures and special reaction conditions. However, the use of continuous-flow setups and microreactors has led to a revolution in this field. In this short review, an overview is given of recent advances in this area, with a focus on the main group organometallics of Li, Na, and K.

Electroreductive Cobalt-Catalyzed Carboxylation: Cross-Electrophile Electrocoupling with Atmospheric CO2

The chemical use of CO2 as an inexpensive, nontoxic C1 synthon is of utmost topical interest in the context of carbon capture and utilization (CCU). We present the merger of cobalt catalysis and electrochemical synthesis for mild catalytic carboxylations of allylic chlorides with CO2 . Styrylacetic acid derivatives were obtained with moderate to good yields and good functional group tolerance. The thus-obtained products are useful as versatile synthons of γ-arylbutyrolactones. Cyclic voltammetry and in operando kinetic analysis were performed to provide mechanistic insights into the electrocatalytic carboxylation with CO2 .

N-Acetylation of Amines in Continuous-Flow with Acetonitrile-No Need for Hazardous and Toxic Carboxylic Acid Derivatives

A continuous-flow acetylation reaction was developed, applying cheap and safe reagent, acetonitrile as acetylation agent and alumina as catalyst. The method developed utilizes milder reagent than those used conventionally. The reaction was tested on various aromatic and aliphatic amines with good conversion. The catalyst showed excellent reusability and a scale-up was also carried out. Furthermore, a drug substance (paracetamol) was also synthesized with good conversion and yield.

i-Pr2NMgCl·LiCl Enables the Synthesis of Ketones by Direct Addition of Grignard Reagents to Carboxylate Anions

The direct preparation of ketones from carboxylate anions is greatly limited by the required use of organolithium reagents or activated acyl sources that need to be independently prepared. Herein, a specific magnesium amide additive is used to activate and control the addition of more tolerant Grignard reagents to carboxylate anions. This strategy enables the modular synthesis of ketones from CO₂ and the preparation of isotopically labeled pharmaceutical building blocks in a single operation.

Reactive nanomessengers for artificial chemical communication

Artificial chemical communication is an emerging field of study driven by the need of exchanging information in delicate environments where standard procedures based on electromagnetic waves cannot be used. A non-synchronized artificial chemical communication system, based on a new modulation technique, namely reaction shift keying (RSK), is presented. The RSK implies that the quenchers are injected into the transmitter, the chemical messenger reacts and a chemically modified messenger travels towards the receiver. Encoding of "0" is obtained by means of the emission of a messenger that reaches the receiver once chemically modified. To encode the value "1", the messenger is not subjected to chemical reaction. Fluorescent carbon nanoparticle molecular messengers that exploit the reaction with Cu(ii) ions for signal modulation were synthesized. A prototypal RSK modulated chemical communication system is developed, from simulations of the communication platform to an operating prototypal system.

Continuous flow synthesis of diaryl ketones by coupling of aryl Grignard reagents with acyl chlorides under mild conditions in the ecofriendly solvent 2-methyltetrahydrofuran

An efficient continuous flow sequential synthesis of diaryl ketones was achieved by coupling of aryl Grignard reagents with acyl chlorides in the bio-derived “green” solvent 2-methyltetrahydrofuran (2-MeTHF) under mild reaction conditions (ambient temperature, 1 hour), allowing a safe and on-demand generation of 2-(3-benzoylphenyl)propionitrile with a productivity of 3.16 g hour⁻¹.

Continuous flow method for parallel synthesis of diaryl ketones using commercially available reagents.

Continuous flow chemistry: where are we now? Recent applications, challenges and limitations

A general outlook of the changing face of chemical synthesis is provided in this article through recent applications of continuous flow processing in both industry and academia. The benefits, major challenges and limitations associated with the use of this mode of processing are also given due attention as an attempt to put into perspective the current position of continuous flow processing, either as an alternative or potential combinatory technology for batch processing.

Synthesis of protected glucose derivatives from levoglucosan by development of common carbohydrate protecting group reactions under continuous flow conditions

Common carbohydrate protecting group reactions under continuous flow processes are reported in the context of producing partially-protected glucose building blocks from levoglucosan. Benzyl ether protection was demonstrated without the use of NaH using barium oxide, which, however, pointed to the need for forms of this catalyst not as susceptible to close packing under flow. Acylation conditions were developed under continuous flow in acetonitrile and avoiding pyridine. Ring-opening the derivatized levoglucosan with propanethiol was also demonstrated producing S-alkyl 2,4-di-O-benzyl-glucopyranoside building block in 2 rather than 12 steps in increased overall yield.

Unsymmetrical difunctionalization of cyclooctadiene under continuous flow conditions: expanding the scope of ring opening metathesis polymerization

Functionalized cyclooctenes (FCOEs) are important monomers in ring-opening metathesis polymerization (ROMP). Herein, a new library of disubstituted FCOEs bearing adjacent heteroatoms were synthesized and applied in ROMP. To address the issues associated with the handling of the reactive thienyl chloride intermediate, a two-step continuous flow method has been developed to prepare 5-thio-6-chlorocyclooctene compounds from abundant cyclooctadiene starting materials. These newly synthesized FCOE monomers were subsequently polymerized through ROMP, giving rise to a range of functionalized polymers with high molecular weights. Furthermore, we demonstrated that the thermal properties of these polymers could be fine-tuned by changing the functional groups in the FCOE monomers. We expect that this functionalization-polymerization strategy will enable the preparation of a range of polymeric materials with complex structures.

Flow reactors integrated with in-line monitoring using benchtop NMR spectroscopy

The state-of-the-art flow reactors integrated with in-line benchtop NMR are thoroughly discussed with highlights on the strengths and weaknesses of this emerging technology.

The assembly and use of continuous flow systems for chemical synthesis

The adoption of and opportunities in continuous flow synthesis ('flow chemistry') have increased significantly over the past several years. Continuous flow systems provide improved reaction safety and accelerated reaction kinetics, and have synthesised several active pharmaceutical ingredients in automated reconfigurable systems. Although continuous flow platforms are commercially available, systems constructed 'in-lab' provide researchers with a flexible, versatile, and cost-effective alternative. Herein, we describe the assembly and use of a modular continuous flow apparatus from readily available and affordable parts in as little as 30 min. Once assembled, the synthesis of a sulfonamide by reacting 4-chlorobenzenesulfonyl chloride with dibenzylamine in a single reactor coil with an in-line quench is presented. This example reaction offers the opportunity to learn several important skills including reactor construction, charging of a back-pressure regulator, assembly of stainless-steel syringes, assembly of a continuous flow system with multiple junctions, and yield determination. From our extensive experience of single-step and multistep continuous flow synthesis, we also describe solutions to commonly encountered technical problems such as precipitation of solids ('clogging') and reactor failure. Following this protocol, a nonspecialist can assemble a continuous flow system from reactor coils, syringes, pumps, in-line liquid-liquid separators, drying columns, back-pressure regulators, static mixers, and packed-bed reactors.

The Hitchhiker's Guide to Flow Chemistry ∥

Flow chemistry involves the use of channels or tubing to conduct a reaction in a continuous stream rather than in a flask. Flow equipment provides chemists with unique control over reaction parameters enhancing reactivity or in some cases enabling new reactions. This relatively young technology has received a remarkable amount of attention in the past decade with many reports on what can be done in flow. Until recently, however, the question, "Should we do this in flow?" has merely been an afterthought. This review introduces readers to the basic principles and fundamentals of flow chemistry and critically discusses recent flow chemistry accounts.

Safety assessment in development and operation of modular continuous-flow processes

Improved safety is one of the main drivers for microreactor application in chemical process development and small-scale production.

Photoredox activation of carbon dioxide for amino acid synthesis in continuous flow

Although carbon dioxide (CO₂) is highly abundant, its low reactivity has limited its use in chemical synthesis. In particular, methods for carbon–carbon bond formation generally rely on two-electron mechanisms for CO₂ activation and require highly activated reaction partners. Alternatively, radical pathways accessed via photoredox catalysis could provide new reactivity under milder conditions. Here we demonstrate the direct coupling of CO₂ and amines via the single-electron reduction of CO₂ for the photoredox-catalyzed, continuous flow synthesis of α-amino acids. By leveraging advantages for utilizing gases and photochemistry in flow, a commercially available organic photoredox catalyst effects the selective α-carboxylation of amines bearing various functional groups and heterocycles. Preliminary mechanistic studies support CO₂ activation and carbon–carbon bond formation via single-electron pathways, and we expect that this strategy will inspire new perspectives on using this feedstock chemical in organic synthesis.