Real-Time Raman Monitoring of an iEDDA Click Reaction Using an Immersion Probe

Click chemistry has become an indispensable tool in modern chemical research because it enables the rapid, selective, and high-yield coupling of molecular building blocks under mild conditions. Since its introduction by Sharpless and co-workers, click chemistry has transformed fields including polymer science, materials engineering, pharmaceuticals, and bioconjugation.[1-3] 

For researchers developing click chemistry processes, the primary objective is to optimize reaction conditions while ensuring rapid and complete conversion. This requires accurate determination of reaction kinetics, identification of the reaction endpoint, and comparison of different formulations or catalysts. However, conventional offline analysis is time-consuming, interrupts the reaction, and may introduce contamination or alter reaction conditions. Fast reactions such as the iEDDA ligation are particularly challenging to investigate because important kinetic information can be lost between sampling intervals.

Immersion Raman spectroscopy provides a direct solution to these challenges by enabling continuous, in-situ monitoring of the reaction without sampling. An immersion probe placed directly into the reaction vessel records molecular fingerprints of reactants and products in real time. This approach provides immediate insight into reaction progress, enables accurate kinetic analysis, and supports efficient optimization of click chemistry protocols while preserving the integrity of the reaction mixture. 

Among the different types of click reactions, the inverse electron-demand Diels–Alder (iEDDA) reaction between tetrazines and strained alkenes is particularly attractive due to its exceptional reaction rates, catalyst-free conditions, high chemoselectivity, and nitrogen as the only by-product.[2,3] The reaction of pyridyl tetrazine (pyTz) with norbornene is therefore widely used for rapid functionalization of polymers, hydrogels, and advanced materials as well as for bioorthogonal labeling.[2,3]

This application report demonstrates the use of an immersion Raman probe for real-time monitoring of the iEDDA reaction between pyTz and two different norbornenes. The two substrates were selected to demonstrate the applicability of the approach to different reaction partners and to enable a direct comparison of their reaction kinetics. The observed differences in reaction kinetics are compared with previously reported kinetic studies of the pyTz–norbornene system [4], demonstrating how Raman spectroscopy provides a rapid and non-destructive tool for reaction development.