Frontispiece: Reactions in Water Involving the “On‐Water” Mechanism

Ever‐evolving advances in water‐compatible catalysts have enriched the current understanding of sustainable organic chemistry; nevertheless, the lack of a clearly defined classification system has led to confusion. Upon categorizing them into seven types, in their Minireview on page 9408 ff., T. Kitanosono and S. Kobayashi propose that reactions should be collectively called “in water” as long as water is used as a reaction medium. When accelerated in water as heterogeneous mixtures, the reactions can be regarded as following an “on‐water” mechanism. The modified “on‐water” model defines unique water layers formed at the interface. In light of the latitudinous outlook on the role of water at the interface, this Minireview covers selected examples of catalytic reactions that are to follow an “on‐water” mechanism, in conformity with this classification.