Combination of near-infrared spectroscopy and a transient flow method for efficient kinetic analysis of the Claisen rearrangement†

Kinetic analysis of the Claisen rearrangement of allyl phenyl ether (APE) to 2-allylphenol was conducted in pressurized N-methylpyrrolidone (NMP) at various temperatures from 240 to 280 °C using an automated flow reactor. Rapid inline analysis using a compact near-infrared (NIR) spectrometer coupled with a flow rate ramp as a reciprocal function of the experimental time allowed high-density data acquisition (900 points in 15 min) of the conversion of APE over residence times ranging from 0.8 to 10.3 min. Inline NIR monitoring was also employed to measure the residence time of the NMP solution in the reactor. The residence time was shown to decrease by 26% with increasing temperature from 20 to 300 °C due to the thermal expansion of the solution. The APE conversion exhibited first-order kinetics with an activation energy of 137 ± 1 kJ mol⁻¹ and a pre-exponential factor of 7.3 × 10¹⁰ s⁻¹. The result of the flow rate ramp experiment was consistent with that of the temperature ramp experiment, while the latter gave a continuous Arrhenius plot in a wider temperature range from 230 to 290 °C. The rate constant in NMP was found to be 10 and 1.5 times smaller than those reported in subcritical water and alcohol solvents, respectively.