This work demonstrates the in‐line monitoring of a flow photochemical reaction using 1D and ultrafast 2D NMR methods at high magnetic field. The reaction mixture exiting the flow reactor is flown through the NMR spectrometer and directly analyzed. In the case of simple substrates, suitable information can be obtained through 1D 1H spectra, but for molecules of higher complexity the use of 2D experiments is key to address signal overlaps and assignment issues. Here we show the usefulness of ultrafast 2D COSY experiments acquired in 70 s or less, for the in‐line monitoring of photochemical reactions, and the possibility to obtain reliable quantitative information. This is a powerful framework to, for example, efficiently screen reaction conditions.
Real-time monitoring methods are essential to the understanding and control of chemical reactions. NMR spectroscopy is particularly well suited for this purpose. It can provide quantitative information and offers great versatility in the analytical setup, making it possible to monitor reactions in chemically relevant conditions and to access sub-second timescales. 2D NMR experiments can play a crucial role in cases of peak overlap in the spectra or when structural information is sought for species transiently formed during the reaction. However, in their conventional form, 2D experiments typically require several minutes or more to acquire a full data set. In this chapter, we show how fast 2D NMR methods make it possible to push the limits of what can be monitored with NMR spectroscopy. First, we describe the motivation to use fast 2D NMR methods for reaction monitoring. Several methods for fast 2D NMR experiments have been exploited for reaction monitoring, in particular non-uniform sampling (NUS) and ultrafast 2D NMR. We also describe and compare the key features of experimental setups for reaction monitoring, such as benchtop spectrometers, flow-NMR setups, and fast-mixing apparatus. We then discuss applications in chemical synthesis and catalysis, which illustrate the potential of fast 2D NMR methods.
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