Abstract
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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.
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