NMR reaction-monitoring systems
Figure 1: Basic NMR reaction monitoring setup.
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The use of NMR as a reaction-monitoring tool has increased in recent years (3, 4). The basic process involves removing a flowing
stream of the reaction mixture from the reactor vessel, which is then pumped to the magnet using tubing. There, it enters
a flow cell in the rf coil region of the probe before being returned to the reaction vessel (see Figure 1). The volume removed
from the reactor at any one time is typically less than 5 mL; h flow rates depend on the experimental setup, but should be
slow enough to give sufficient residence time in the high magnetic field region to assure adequate polarization and minimize
the T1 bias to peak areas (5).
Many of the reports on NMR reaction monitoring have employed the use of flow cells that are fixed inside the probe, but modified
NMR tubes have been used as a low cost alternative (6, 7). The replacement of these flow tubes is considered easier than
replacing flow cell probes, which have to be returned to the vendor for repairs and can result in instrument downtime if additional
flow probes are not available. Flow tubes offer the added advantage that any spectrometer with a conventional probe may be
readily converted to be used for NMR reaction monitoring, thereby allowing for the analysis of a wide range of NMR active
nuclei and temperatures.
Flow NMR can be used to investigate batch reactions. The technique also lends itself to monitoring flow and continuous reactions.
The output from a packed bed or thermal tube reactor can be simply diverted to the flow NMR for in-line analysis of the product
stream.
Data processing
Figure 2: Data processing of multiple NMR spectra, showing the hydrolysis of acetic anhydride as a simple model reaction.
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Advances in instrumentation facilitate the generation of large amounts of data and the potential to uncover vast amounts of
information. Processing of multiple individual spectra (typically > 100) proved time-consuming and labor-intensive; therefore
an efficient method of processing and mining the data was needed if this method of reaction monitoring was to be viable. The
development of automated processing software has expedited this procedure, allowing hundreds of 1-D NMR spectra to be processed
and useful information extracted in a matter of minutes (8). Peak and spectral alignment algorithms provide the ability to
accurately integrate a particular resonance of interest whose chemical shift may migrate across the spectrum during the course
of a reaction. Exploiting the fact that under the correct experimental conditions the integral for a particular signal can
be directly correlated to the concentration of that species, a graphical representation of concentration versus time is generated,
thereby providing an accurate picture of the reaction's progress (see Figure 2). The data generated may then be transferred
to various kinetic and modeling software packages to produce kinetic results for chemists and engineers. Offline, stopped
flow, or on-flow 2-D NMR experiments can be conducted to determine the nature of reactive intermediates formed during the
course of the reaction.
Application to the pharmaceutical industry
The value of NMR reaction monitoring has been increasingly documented in the literature and is being implemented in the development
of multiple synthetic steps by groups at Pfizer, Lilly, Roche, and AstraZeneca, to name but a few (4, 9–11). A recent report
on the optimization of the synthesis of an imidazole derivative demonstrated the value of flow NMR reaction monitoring for
understanding the formation of an undesired byproduct (11). The process proceeded through an intermediate that could not be
detected by other analytical techniques because of the instability of the intermediate. However, examination of the process
by flow NMR allowed the detection and characterization of this intermediate as well as elucidation of the mechanism of the
reaction. As a result, a method of producing the required imidazole as the sole product was found.
Conclusion
In summary, NMR technology is a valuable addition to the arsenal of PAT tools currently available in the pharmaceutical industry.
The detailed picture of a process that is gained from a single experiment provides valuable information and a deeper understanding
for scientists across a broad range of disciplines. This knowledge enables a robust process to emerge from the process research
and development stage of drug development.
David A. Foley is a postdoctoral fellow, Mark T. Zell is a senior principal scientist, and Brian L. Marquez* is an associate research fellow, all at Pfizer Global Research and Development, Groton/New London Laboratories, Brian.Marquez@pfizer.com
. Andreas Kaerner is a research advisor in analytical sciences R&D at Eli Lilly and Company.
*To whom all correspondence should be addressed.
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