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With ever-evolving light-emitting diode (LED) technology, classical photochemical transformations are becoming accessible with more efficient and industrially viable light sources. In combination with a triplet sensitizer, we report the detailed exploration of [2 + 2] cycloadditions, in flow, of various maleic anhydride derivatives with gaseous ethylene. By the use of a flow reactor capable of gas handling and LED wavelength/power screening, an in-depth optimization of these reactions was carried out. In particular, we highlight the importance of matching the substrate and sensitizer triplet energies alongside the light source emission wavelength and power. Initial triplet-sensitized reactions of maleic anhydride were hampered by benzophenone’s poor absorbance at 375 nm. However, density functional theory (DFT) calculations predicted that derivatives such as citraconic anhydride have low enough triplet energies to undergo triplet transfer from thioxanthone, whose absorbance matches the LED emission at 375...

A typical (Paterno-Buchi type) organic photoreaction is examined in flow microreactors. Compared to the results achieved for simple one-layer flow modes, slug flow conditions (with the unreactive reagent composed of organic solution and water) achieve higher efficiency (conversion and yield) irrespective of the light source, concentration, and solvent. Furthermore, remarkably high productivity is also observed when only 5% water is added as an unreactive reagent. This method is broadly applicable for achieving highly efficient organic photoreactions not only in the laboratory, but also on the industrial scale.