Ethyl diazoacetate synthesis in flow

Stereoselective Synthesis of Either Exo- or Endo-3-Azabicyclo[3.1.0]hexane-6-carboxylates by Dirhodium(II)-Catalyzed Cyclopropanation with Ethyl Diazoacetate under Low Catalyst Loadings

Although cyclopropanation with donor/acceptor carbenes can be conducted under low catalyst loadings (<0.001 mol %), such low loading has not been generally effective for other classes of carbenes such as acceptor carbenes. In this current study, we demonstrate that ethyl diazoacetate can be effectively used in the cyclopropanation of N-Boc-2,5-dihydropyrrole with dirhodium(II) catalyst loadings of 0.005 mol %. By appropriate choice of catalyst and hydrolysis conditions, either the exo- or endo-3-azabicyclo[3.1.0]hexanes can be formed cleanly with high levels of diastereoselectivity with no chromatographic purification.

Synthesis of Substituted Pentafluorosulfanylpyrazoles Under Flow Conditions

The development of flow conditions for the synthesis of pentafluorosulfanylpyrazoles is reported. A range of alkyl- and aryl-substituted SF5-alkynes were used in combination with different diazoacetates for this transformation. The corresponding substituted SF5-pyrazoles were obtained in up to 90% yield (average of 74% for 21 examples) as a mixture of isomers (up to 73:27 ratio). Synthetic transformations starting from an SF5-containing pyrazole were also demonstrated.

Optimization of continuous-flow diphenyldiazomethane synthesis: an integrated undergraduate chemistry experiment

We present a challenging flow-chemistry experiment concerning the synthesis of diphenyldiazomethane using the Omura-Sharma-Swern oxidation, that we have developed and used in our second-year undergraduate lab classes over the past seven years. The experiment integrates a number of different aspects and concepts of chemistry that are traditionally taught as separate subjects in undergraduate chemical education: organic synthesis, quantitative chemical analysis, design of experiments, optimization, statistical modelling, computer programming and continuous-flow processes.

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.

The Generation of Diazo Compounds in Continuous-Flow

Toxic, cancerogenic and explosive-these attributes are typically associated with diazo compounds. Nonetheless, diazo compounds are nowadays a highly demanded class of reagents for organic synthesis, yet the concerns with regards to safe and scalable transformations of these compounds are still exceptionally high. Lately, the research area of the continuous-flow synthesis of diazo compounds attracted significant interest and a whole variety of protocols for their "on-demand" preparation have been realized to date. This concept article focuses on the recent developments using continuous-flow technologies to access diazo compounds; thus minimizing risks and hazards when working with this particular class of compounds. In this article we discuss these concepts and highlight different pre-requisites to access and to perform downstream functionalization reaction.

Iron-catalysed carbene-transfer reactions of diazo acetonitrile

Herein we report the flow synthesis of hazardous diazo acetonitrile to enable X–H insertion reactions with a readily available iron catalyst.

Taming hazardous chemistry by continuous flow technology

Over the last two decades, flow technologies have become increasingly popular in the field of organic chemistry, offering solutions for engineering and/or chemical problems. Flow reactors enhance the mass and heat transfer, resulting in rapid reaction mixing, and enable a precise control over the reaction parameters, increasing the overall process selectivity, efficiency and safety. These features allow chemists to tackle unexploited challenges in their work, with the ultimate objective making chemistry more accessible for laboratory and industrial applications, avoiding the need to store and handle toxic, reactive and explosive reagents. This review covers some of the latest and most relevant developments in the field of continuous flow chemistry with the focus on hazardous reactions.

Exploring Flow Procedures for Diazonium Formation

The synthesis of diazonium salts is historically an important transformation extensively utilized in dye manufacture. However the highly reactive nature of the diazonium functionality has additionally led to the development of many new reactions including several carbon-carbon bond forming processes. It is therefore highly desirable to determine optimum conditions for the formation of diazonium compounds utilizing the latest processing tools such as flow chemistry to take advantage of the increased safety and continuous manufacturing capabilities. Herein we report a series of flow-based procedures to prepare diazonium salts for subsequent in-situ consumption.

Ethyl Lithiodiazoacetate: Extremely Unstable Intermediate Handled Efficiently in Flow

Ethyl diazoacetate (EDA) is one of the most prominent diazo reagents. It is frequently used in metal-carbene-type reactions. However, EDA can also be used as a nucleophile under base catalysis. Whilst the addition of EDA to aldehydes can be performed using organic bases, the addition of EDA to other carbonyl electrophiles requires the use of organometallics such as lithium diisopropylamide (LDA). The generated ethyl lithiodiazoacetate is highly reactive and decomposes rapidly, even at low temperatures. Herein, we report a continuous flow protocol that overcomes the problems associated with the instantaneous decomposition of ethyl lithiodiazoacetate. The addition of ethyl lithiodiazoacetate to ketones provides direct access to tertiary diazoalcohols in good yields.

Fluoroalkyl-Substituted Diazomethanes and Their Application in a General Synthesis of Pyrazoles and Pyrazolines

A novel continuous-flow approach for the synthesis of fluoroalkyl-substituted diazomethanes has been developed. Utilizing a cheap, self-made microreactor fluoroalkyl-substituted amines were transformed into the corresponding diazomethanes using tert-butyl nitrite and acetic acid as catalyst. These diazomethanes were employed in [2+3] cycloaddition reactions with olefins and alkynes, yielding valuable pyrazolines and pyrazoles in good to excellent yields.

Generation and Synthetic Application of Trifluoromethyl Diazomethane Utilizing Continuous Flow Technologies

A continuous process for the synthesis and inline separation of anhydrous trifluoromethyl diazomethane in a single continuous flow process is presented. The diazo building block is generated from the corresponding amine and NaNO2 under acidic, aqueous conditions and subsequently diffuses through a gas-permeable membrane into an organic stream. To avoid storage and transportation of the hazardous compound, a representative downstream process in a packed-bed reactor yielding highly functionalized building blocks was developed.

Diazo compounds in continuous-flow technology

Diazo compounds are very versatile reagents in organic chemistry and meet the challenge of selective assembly of structurally complex molecules. Their leaving group is dinitrogen; therefore, they are very clean and atom-efficient reagents. However, diazo compounds are potentially explosive and extremely difficult to handle on an industrial scale. In this review, it is discussed how continuous flow technology can help to make these powerful reagents accessible on large scale. Microstructured devices can improve heat transfer greatly and help with the handling of dangerous reagents safely. The in situ formation and subsequent consumption of diazo compounds are discussed along with advances in handling diazomethane and ethyl diazoacetate. The potential large-scale applications of a given methodology is emphasized.

Chemoenzymatic flow cascade for the synthesis of protected mandelonitrile derivatives

Integrated two-step chemoenzymatic continuous flow process for the synthesis of protected cyanohydrins from aldehydes.