Pushing Photochemistry into Water: Acceleration of the Di‐π‐Methane Rearrangement and the Paternó‐Büchi Reaction “On‐Water”

Two representative organic photoreactions, namely a bimolecular photocycloaddition and a monomolecular photorearrangement, are presented that are accelerated when the reaction is performed “on‐water”, that is, at the water‐substrate interface with no solvation of the reaction components. According to the established models of ground‐state reactions “on‐water”, the enhanced efficiency of the photoreactions is explained by hydrophobic effects (Paternó‐Büchi reaction) or specific hydrogen bonding (di‐π‐methane rearrangement) at the water‐substrate interface that decrease the energy of the respective transition state. These results point to the potential of this approach to conduct photoreactions more efficiently in an ecologically favorable medium. Organic photoreactions “on water”, namely in aqueous suspension or emulsion, are significantly accelerated in this ecologically favorable medium.