Real-time Monitoring of Reactions Performed Using Continuous-flow Processing: The Preparation of 3-Acetylcoumarin as an Example

Summary

Real-time monitoring allows for fast optimization of reactions performed using continuous-flow processing. Here the preparation of 3-acetylcoumarin is used as an example. The apparatus for performing in-situ Raman monitoring is described, as are the steps required to optimize the reaction.

Abstract

By using inline monitoring, it is possible to optimize reactions performed using continuous-flow processing in a simple and rapid way. It is also possible to ensure consistent product quality over time using this technique. We here show how to interface a commercially available flow unit with a Raman spectrometer. The Raman flow cell is placed after the back-pressure regulator, meaning that it can be operated at atmospheric pressure. In addition, the fact that the product stream passes through a length of tubing before entering the flow cell means that the material is at RT. It is important that the spectra are acquired under isothermal conditions since Raman signal intensity is temperature dependent. Having assembled the apparatus, we then show how to monitor a chemical reaction, the piperidine-catalyzed synthesis of 3-acetylcoumarin from salicylaldehyde and ethyl acetoacetate being used as an example. The reaction can be performed over a range of flow rates and temperatures, the in-situ monitoring tool being used to optimize conditions simply and easily.

Introduction

By using continuous-flow processing, chemists are finding that they can perform a range of chemical reactions safely, effectively, and with ease^1,2. As a result, flow chemistry equipment is becoming an integral tool for running reactions both in industrial settings as well as research labs in academic institutions. A wide variety of synthetic chemistry transformations have been carried out in flow reactors^3,4. In select cases, reactions that do not work in batch have been shown to proceed smoothly under continuous-flow conditions⁵. For both reaction optimization and quality control, incorporation of in-line reaction monitoring with flo....

Protocol

1. Find Suitable Signals for Reaction Monitoring

1. Obtain Raman spectra for all starting materials and the product.
2. Overlay spectra and identify an intense band that is unique to the product.
3. Use this Raman band to monitor the progress of the reaction. A band at 1,608 cm^-1 was selected in this case.

2. Set up the Flow Cell

1. Obtain a suitable flow cell. Here use one with the following dimensions: width of 6.5 mm, height of 20 mm, and a path length of 5 mm (Figure 2A).
2. Place the flow cell in a container that provides an environment free of ambient light.

Results

The continuous-flow preparation of 3-acetylcoumarin was chosen as a representative reaction for in-line monitoring. In batch, the reaction proceeds well when using ethyl acetate as the solvent. However, the product (1) is not completely soluble at RT. To prevent potential clogging of the back-pressure regulator, as well as mitigate the risk of having solid particles in the flow cell which would perturb signal acquisition, we used a technique we developed previously for this and other reactions²²

Discussion

The ease in which the Raman spectrometer can be interfaced with the flow unit makes this inline technique valuable for reaction monitoring. A number of reaction variables can be probed in an expedited manner, allowing the user to arrive at optimized reaction conditions faster than when using offline methods. Application of the techniques described herein also allows for monitoring of the formation of side products, assuming a suitable band can be found. Conditions can be screened and selected, which allow both for the hi.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Salicylaldehyde	Sigma-Aldrich	S356	Reagent Grade, 98%
Ethyl acetoacetate	Acros Organics	117970010	99%
Piperidine	Sigma-Aldrich	104094	Reagent Plus, 99%
Hydrochloric acid	Sigma-Aldrich	320331	ACS Reagent, 37%
Ethyl acetate	Sigma-Aldrich	34858	CHROMASOLV, for HPLC, >99.7%
Acetone	Sigma-Aldrich	650501	CHROMASOLV, for HPLC, >99.9%
Flow cell	Starna Cells		583.65.65-Q-5/Z20
Flow unit	Vapourtec		E-series system
Raman spectrometer	Enwave Optronics Inc		Model EZRaman-L