Scalable Continuous Flow Process for the Synthesis of Eflornithine Using Fluoroform as Difluoromethyl Source

A Continuous Flow Process for the Defluorosilylation of HFC-23 and HFO-1234yf

A continuous flow process has been developed for the defluorosilylation of trifluoromethane (HFC-23) and 2,3,3,3-tetrafluoropropene (HFO-1234yf) through reaction with lithium silanide reagents under inert conditions. Design of experiment optimization improved process conditions, including productivity, yields, reduction of solvent use, and gas destruction. The small chain fluorinated organosilane products R3SiCF2H and R3SiCH2C(F)═CF2 were competent nucleophiles in the fluoride-catalyzed difluoromethylation of aldehydes, and trifluoroallylation of aldehydes, ketones, and imines.

Multiphasic Continuous-Flow Reactors for Handling Gaseous Reagents in Organic Synthesis: Enhancing Efficiency and Safety in Chemical Processes

The use of reactive gaseous reagents for the production of active pharmaceutical ingredients (APIs) remains a scientific challenge due to safety and efficiency limitations. The implementation of continuous-flow reactors has resulted in rapid development of gas-handling technology because of several advantages such as increased interfacial area, improved mass- and heat transfer, and seamless scale-up. This technology enables shorter and more atom-economic synthesis routes for the production of pharmaceutical compounds. Herein, we provide an overview of literature from 2016 onwards in the development of gas-handling continuous-flow technology as well as the use of gases in functionalization of APIs.

Current State of Microflow Trifluoromethylation Reactions

The trifluoromethyl group is a powerful structural motif in drugs and polymers; thus, developing trifluoromethylation reactions is an important area of research in organic chemistry. Over the past few decades, significant progress has been made in developing new methods for the trifluoromethylation of organic molecules, ranging from nucleophilic and electrophilic approaches to transition-metal catalysis, photocatalysis, and electrolytic reactions. While these reactions were initially developed in batch systems, more recent microflow versions are highly attractive for industrial applications owing to their scalability, safety, and time efficiency. In this review, we discuss the current state of microflow trifluoromethylation. Approaches for microflow trifluoromethylation based on different trifluoromethylation reagents are described, including continuous flow, flow photochemical, microfluidic electrochemical reactions, and large-scale microflow reactions.

Repurposing of F-gases: challenges and opportunities in fluorine chemistry

Fluorinated gases (F-gases) are routinely employed as refrigerants, blowing agents, and electrical insulators. These volatile compounds are potent greenhouse gases and consequently their release to the environment creates a significant contribution to global warming. This review article seeks to summarise: (i) the current applications of F-gases, (ii) the environmental issues caused by F-gases, (iii) current methods of destruction of F-gases and (iv) recent work in the field towards the chemical repurposing of F-gases. There is a great opportunity to tackle the environmental and sustainability issues created by F-gases by developing reactions that repurpose these molecules.

Amino Acid Based Antimicrobial Agents - Synthesis and Properties

Structures of several dozen of known antibacterial, antifungal or antiprotozoal agents are based on the amino acid scaffold. In most of them, the amino acid skeleton is of a crucial importance for their antimicrobial activity, since very often they are structural analogs of amino acid intermediates of different microbial biosynthetic pathways. Particularly, some aminophosphonate or aminoboronate analogs of protein amino acids are effective enzyme inhibitors, as structural mimics of tetrahedral transition state intermediates. Synthesis of amino acid antimicrobials is a particular challenge, especially in terms of the need for enantioselective methods, including the asymmetric synthesis. All these issues are addressed in this review, summing up the current state‐of‐the‐art and presenting perspectives fur further progress.

Pentafluoroethylation of Carbonyl Compounds Using HFC-125 in a Flow Microreactor System

The protocol of micro-flow nucleophilic pentafluoroethylation using pentafluoroethane (HC₂F₅, HFC-125), a nontoxic, inexpensive, and commercially available greenhouse gas, is described. The micro-flow pentafluoroethylation by HFC-125 proceeded smoothly at room temperature or at -10 °C in DMF or toluene in the presence of a potassium base, namely, t-BuOK or KHMDS. A broad range of ketones, aldehydes, and chalcones with various substituted benzene rings were successfully converted to the corresponding pentafluoroethyl carbinols instantly with good to high yields.

Continuous flow strategies for using fluorinated greenhouse gases in fluoroalkylations

Large quantities of fluorinated gases are generated as intermediates or byproducts from fluorinated polymer production annually, and they are effective ozone depleting substances or greenhouse gases. On the other hand, the incorporation of fluoroalkyl groups into drug molecules or bioactive compounds has been shown to enhance biological properties such as the bioavailability, binding selectivity, and metabolic stability. Extraction of fluoroalkyl sources, including trifluoromethyl and difluoromethyl groups, from the fluorinated gases is highly desirable, yet challenging under regular batch reaction conditions. Flow chemistry is an emerging and promising technique to address long-standing challenges in gas-liquid batch reactions such as insufficient interfacial contact and scalability issues. In this review, we highlight recent advances in continuous flow strategies toward enabling the use of fluorinated greenhouse gases in organic synthesis.

(2-Ethylhexyl)sodium: A Hexane-Soluble Reagent for Br/Na-Exchanges and Directed Metalations in Continuous Flow

We report the on-demand generation of hexane-soluble (2-ethylhexyl)sodium (1) from 3-(chloromethyl)heptane (2) using a sodium-packed-bed reactor under continuous flow conditions. Thus, the resulting solution of 1 is free of elemental sodium and therefore suited for a range of synthetic applications. This new procedure avoids the storage of an alkylsodium and limits the handling of metallic sodium to a minimum. (2-Ethylhexyl)sodium (1) proved to be a very useful reagent and undergoes in-line Br/Na-exchanges as well as directed sodiations. The resulting arylsodium intermediates are subsequently trapped in batch with various electrophiles such as ketones, aldehydes, Weinreb-amides, imines, allyl bromides, disulfides and alkyl iodides. A reaction scale-up of the Br/Na-exchange using an in-line electrophile quench was also reported.

Main Group Metal-Mediated Transformations of Imines

Main-group-metal-mediated transformations of imines have earned a valued place in the synthetic chemist's toolbox. Their versatility allows the simple preparation of various nitrogen containing compounds. This review will outline the early discoveries including metallation, addition/cyclisation and metathesis pathways, followed by the modern-day use of imines in synthetic methodology. Recent advances in imine C-F activation protocols are discussed, alongside revisiting "classic" imine reactivity from a sustainable perspective. Developments in catalytic methods for hydroelementation of imines have been reviewed, highlighting the importance of s-block metals in the catalytic arena. Whilst stoichiometric transformations in alternative reaction media such as deep eutectic solvents or water have been summarised. The incorporation of imines into flow chemistry has received recent attention and is summarised within.

Selective separation of amines from continuous processes using automated pH controlled extraction

An automated separation system is described for identifying the optimal conditions for purifying an amine from a mixture.

Late-stage difluoromethylation: concepts, developments and perspective

This review describes the conceptual advances that have led to the multiple difluoromethylation processes making use of well-defined CF₂H sources.

Defluorosilylation of trifluoromethane: upgrading an environmentally damaging fluorocarbon

The rapid, room-temperature defluorosilylation of trifluoromethane, a highly potent greenhouse gas, has been achieved using a simple silyl lithium reagent. An extensive computational mechanistic analysis provides a viable reaction pathway and demonstrates the unexpected electrophilic nature of LiCF₃. The reaction generates a bench stable fluorinated building block that shows promise as an easy-to-use difluoromethylating agent. The difluoromethyl group is an increasingly important bioisostere in active pharmaceutical ingredients, and therefore our methodology creates value from waste. The potential scalability of the process has been demonstrated by achieving the reaction on a gram-scale.

The Concept of Chemical Generators: On-Site On-Demand Production of Hazardous Reagents in Continuous Flow

In recent years, a steadily growing number of chemists, from both academia and industry, have dedicated their research to the development of continuous flow processes performed in milli- or microreactors. The common availability of continuous flow equipment at virtually all scales and affordable cost has additionally impacted this trend. Furthermore, regulatory agencies such as the United States Food and Drug Administration actively encourage continuous manufacturing of active pharmaceutical ingredients (APIs) with the vision of quality and productivity improvements. That is why the pharmaceutical industry is progressively implementing continuous flow technologies. As a result of the exceptional characteristics of continuous flow reactors such as small reactor volumes and remarkably fast heat and mass transfer, process conditions which need to be avoided in conventional batch syntheses can be safely employed. Thus, continuous operation is particularly advantageous for reactions at high temperatures/pressures (novel process windows) and for ultrafast, exothermic reactions (flash chemistry).

In addition to conditions that are outside of the operation range of conventional stirred tank reactors, reagents possessing a high hazard potential and therefore not amenable to batch processing can be safely utilized (forbidden chemistry). Because of the small reactor volumes, risks in case of a failure are minimized. Such hazardous reagents often are low molecular weight compounds, leading generally to the most atom-, time-, and cost-efficient route toward the desired product. Ideally, they are generated from benign, readily available and cheap precursors within the closed environment of the flow reactor on-site on-demand. By doing so, the transport, storage, and handling of those compounds, which impose a certain safety risk especially on a large scale, are circumvented. This strategy also positively impacts the global supply chain dependency, which can be a severe issue, particularly in times of stricter safety regulations or an epidemic. The concept of the in situ production of a hazardous material is generally referred to as the “generator” of the material. Importantly, in an integrated flow process, multiple modules can be assembled consecutively, allowing not only an in-line purification/separation and quenching of the reagent, but also its downstream conversion to a nonhazardous product.

For the past decade, research in our group has focused on the continuous generation of hazardous reagents using a range of reactor designs and experimental techniques, particularly toward the synthesis of APIs. In this Account, we therefore introduce chemical generator concepts that have been developed in our laboratories for the production of toxic, explosive, and short-lived reagents. We have defined three different classes of generators depending on the reactivity/stability of the reagents, featuring reagents such as Br₂, HCN, peracids, diazomethane (CH₂N₂), or hydrazoic acid (HN₃). The various reactor designs, including in-line membrane separation techniques and real-time process analytical technologies for the generation, purification, and monitoring of those hazardous reagents, and also their downstream transformations are presented. This Account should serve as food for thought to extend the scope of chemical generators for accomplishing more efficient and more economic processes.

Recent advances in continuous-flow organocatalysis for process intensification

The progresses on continuous-flow organocatalysis from 2016 to early 2020 are reviewed with focus on transition from batch to flow.

Evolution of flow-oriented design strategies in the continuous preparation of pharmaceuticals

This review focuses on the flow-oriented design (FOD) in the multi-step continuous-flow synthesis of active pharmaceutical ingredients.

Multigram-scale flow synthesis of the chiral key intermediate of (-)-paroxetine enabled by solvent-free heterogeneous organocatalysis

The catalytic enantioselective synthesis of the chiral key intermediate of the antidepressant (-)-paroxetine is demonstrated as a continuous flow process on multi-gram scale. The critical step is a solvent-free organocatalytic conjugate addition followed by a telescoped reductive amination-lactamization-amide/ester reduction sequence. Due to the efficient heterogeneous catalysts and the solvent-free or highly concentrated conditions applied, the flow method offers key advances in terms of productivity and sustainability compared to earlier batch approaches.

Flow Technology for the Genesis and Use of (Highly) Reactive Organometallic Reagents

In the field of organic synthesis, the advent of flow chemistry and flow microreactor technology represented a tremendous novelty in the way of thinking and performing chemical reactions, opening the doors to poorly explored or even impossible transformations using batch methods. In this Concept article, we would like to highlight the impact of flow chemistry for exploiting highly reactive organometallic reagents, and how, alongside the well-known advantages concerning safety, scalability, and productivity, flow chemistry makes possible processes that are impossible to control by using the traditional batch approach.