Pushing C-N bond formation in water: sustainable strategies, mechanistic roles and future perspectives.

C-N bonds are fundamental structural motifs in pharmaceuticals, natural products, agrochemicals, dyes, polymers, and functional organic materials. Consequently, the development of efficient and sustainable methods for C-N bond construction has become a central objective in modern synthetic chemistry. In this context, water has emerged as an attractive reaction medium owing to its abundance, low toxicity, environmental compatibility, unique hydrogen-bonding ability, and capacity to promote distinct reactivity under both homogeneous and heterogeneous conditions. Nevertheless, the use of water in C-N bond formation offers more than solvent replacement; it represents a mechanistically distinctive approach in which aqueous environments actively reshape reaction pathways. This review summarizes recent progress in C-N bond formation and covers both "on water" and "in water" phenomena, with particular emphasis on current achievements, mechanistic understanding, and future opportunities. The discussed transformations are organized according to reaction strategy, including transition-metal-free processes, transition-metal-catalysed systems, and emerging electrochemical and photochemical approaches. Particular emphasis is placed on how water changes reaction pathways through hydrogen bonding, interfacial effects, proton transfer, hydrophobic aggregation, and catalyst stabilization. By integrating synthetic scope with mechanistic insights, this review provides a coherent framework for the rational design of sustainable aqueous C-N bond-forming reactions.