Abstract
The visible-light photocatalytic E/Z isomerization of olefins can be mediated by a wide spectrum of triplet sensitizers (photocatalysts). However, the search for the most efficient photocatalysts through screenings in photo batch reactors is material and time consuming. Capillary and microchip flow reactors can accelerate this screening process. Combined with a fast analytical technique for isomer differentiation, these reactors can enable high-throughput analyses. Ion mobility (IM) spectrometry is a cost-effective technique that allows simple isomer separation and detection on the millisecond timescale. This work introduces a hyphenation method consisting of a microchip reactor and an infrared matrix-assisted laser desorption ionization (IR-MALDI) ion mobility spectrometer that has the potential for high-throughput analysis. The photocatalyzed E/Z isomerization of ethyl-3-(pyridine-3-yl)but-2-enoate (E-1) as a model substrate was chosen to demonstrate the capability of this device. Classic organic triplet sensitizers as well as Ru-, Ir-, and Cu-based complexes were tested as catalysts. The ionization efficiency of the Z-isomer is much higher at atmospheric pressure which is due to a higher proton affinity. In order to suppress proton transfer reactions by limiting the number of collisions, an IM spectrometer working at reduced pressure (max. 100 mbar) was employed. This design reduced charge transfer reactions and allowed the quantitative determination of the reaction yield in real time. Among 14 catalysts tested, four catalysts could be determined as efficient sensitizers for the E/Z isomerization of ethyl cinnamate derivative E-1. Conversion rates of up to 80% were achieved in irradiation time sequences of 10 up to 180 s. With respect to current studies found in the literature, this reduces the acquisition times from several hours to only a few minutes per scan.
Highlights
Preparative photoreactions carried out in stirred batch reactors are regularly characterized by reaction times on the scale of hours
We previously reported the integration of microflow reactions and chip HPLC [7,8,9] that allowed for the analysis of enantioselective transformations
Using a novel combination of photo flow reactors with an IRMALDI-Ion mobility (IM) spectrometer, we found catalyst M and K, L, and J to be effective and efficient sensitizers for the E/Z isomerization of ethyl cinnamate E-1
Summary
Preparative photoreactions carried out in stirred batch reactors are regularly characterized by reaction times on the scale of hours. The photocatalyzed E/Z isomerization of the ethyl-3-(pyridine-3-yl)but-2-enoate (E-1) in photo flow reactors was investigated by IR-MALDI-IM spectrometry This technique allows the real-time separation and detection of the E- and Z-isomer. The channel (70 nL) crossed a make-up flow channel where additional fluid (MeCN:H2O 1:1 (v/v), 1:79 up to 1:299 dilution of the reaction mixture) was delivered to form a liquid jet at the end of the microchip. The reaction mixture was diluted into a make-up flow of MeCN:H2O 1:1 (v/v) giving a total flow rate of 300 μL/min This terminated the reaction progress at a defined point in time and generated sufficient throughput to create a free-standing liquid beam of 20 μm in diameter by means of a glass nozzle (OD 1.0 mm, ID 0.235 mm, tapered to ID = 20 ± 2 μm at the nozzle tip, Biomedical Instruments, GER). It allows the calculation of the yield for each species by a simple cumulative Gaussian fit
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