Finding the Perfect Match: A Combined Computational and Experimental Study toward Efficient and Scalable Photosensitized [2+2] Cycloadditions in Flow

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 nm. This observation held true experimentally, allowing optimization and further exemplification in a larger-scale reactor, whereby >100 g of material was processed in 10 h. These straightforward DFT calculations were also applied to a number of other substrates and showed a good correlation with experimental data, implying that their use can be a powerful strategy in targeted reaction optimization for future substrates.