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

Abstract

Conspectus 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 Br 2 , HCN, peracids, diazomethane (CH ...

How to approach flow chemistry

“…The ''generator'' concept is commonly used to indicate a module for the continuous in situ production of hazardous materials. 25 Here, we expand this concept to include all reactive intermediates that can be generated in situ and on-demand.…”

A modular, low footprint and scalable flow platform for the expedient α-aminohydroxylation of enolizable ketones

“…65 With increasingly restrictive legislations both at the R&D and commercial scales, the use of reactive and potentially hazardous chemicals has become limiting if not impossible with conventional batch reactors. The concept of chemical generators under continuous flow conditions has been extensively documented by Kappe, [66][67][68][69][70][71][72] hence providing a robust answer for exploiting reactive intermediates yet benefiting from all the safety and process advantages of continuous flow operation. A continuous flow chemical generator actually relies on stable and widely available precursors and on the concatenation of upstream operations aiming at the preparation of unstable, toxic or reactive chemicals, which are next immediately consumed within downstream operations (Fig.…”

Coupling biocatalysis with high-energy flow reactions for the synthesis of carbamates and β-amino acid derivatives

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