Table 1. Comparative summary of metal-catalyzed C–N bond formation in water.

Metal	Typical reactions	Advantages	Limitations	Representative role of watera
Cu	Ullmann-type cross-coupling, C–H functionalization, aerobic oxidation, and intramolecular N-arylation	Low cost, versatile, and amenable to micellar and heterogeneous systems	Requirement for ligands, need for strong bases, high temperature requirement, and limited scope for unactivated aryl chlorides	Solvent, Cu-species stabilizer, proton-transfer medium, and interfacial promoter
Pd	Buchwald–Hartwig amination, C–H amination, and cascade cyclization	Broad substrate scope, high functional-group tolerance, and high activity	High cost, specialized ligand requirement, and potential metal contamination	Solvent, micellar organizer, catalyst stabilization, and substrate dispersant
Ir	Transfer hydrogenation, borrowing-hydrogen amination, and reductive hydroamination	High selectivity, mild operating conditions, and water as hydrogen source	High cost, water-soluble ligand requirement, and limited substrate scope	pH regulator, proton-transfer medium, and hydrogen source
Rh	C–H amidation, C–H amination, nitrene transfer, annulation, and aminohydroxylation	High reactivity, broad C–H substrate scope, and regioselectivity	Precious metal status, requirement for oxidants/nitrene precursors, and necessity of directing groups	Interfacial activator, hydrogen-bonding promoter, proton-transfer mediator, and nucleophile source
Fe	N-Arylation, ammoxidation, C–H activation, and nitrile synthesis	Earth-abundance, low toxicity, and synthetic feasibility of heterogeneous single-atom catalysts	Limited reaction diversity and requirement for engineered catalysts	Substrate dispersion, Fe-site stabilization, and proton-transfer mediation
Au	Reductive amination, cascade hydrogen transfer, and quinazoline synthesis	Heterogeneous, recyclable, and operable under mild conditions	High cost, limited reaction scope, supported catalyst requirement	Green medium and hydrogen donor
Ni	N-Arylation and reductive amination	Earth-abundance, low cost, magnetic recoverability	Limited examples and requirement for specialized conditions	Solvent and hydrogen generation through Al–H2O process
Ru	Borrowing-hydrogen amination and reductive amination	Versatility and heterogeneous catalytic systems	Precious metal and limited examples	Solvent
Ag	Nitrene-transfer C–H amination	Reactive intermediates and mild conditions	High cost and limited scope	Solvent
I2 (metal-free)	Oxidative cyclization and C–H functionalization	No metal required, air as oxidant, and high atom economy	Limited examples	Solvent

^aThe role of water depends on the specific reaction system and should not be generalized across all catalytic processes. In some cases, its involvement has been experimentally demonstrated, whereas in others, it has been proposed based on mechanistic studies. Sustainability advantages should be evaluated using comprehensive green chemistry metrics, including atom economy, E-factor, and process mass intensity, rather than solvent selection alone.