Transition-metal chemistry has transformed the way scientists modify complex molecules, but two of the simplest and most abundant substances on Earth—water and ammonia—have remained surprisingly difficult partners in direct C–H functionalization. A new study reported in Nature describes a solution that moves beyond conventional transition-metal catalysis. Researchers have found that ordinary triarylphosphines can promote the direct coupling of azine C–H bonds with water and ammonia, creating new carbon–oxygen and carbon–nitrogen bonds in molecules relevant to drug discovery and agrochemical development.
The advance targets azines, a broad family of nitrogen-containing aromatic compounds that includes pyridines, quinolines, and diazines. These structures appear throughout pharmaceuticals, crop-protection agents, and biologically active natural products. Their aromatic C–H bonds are generally stable and chemically similar to one another, which makes selective functionalization challenging. Yet the ability to replace one of those hydrogen atoms directly with a hydroxyl or amino group could significantly streamline medicinal chemistry, allowing researchers to alter advanced molecules without rebuilding them from the beginning.
Directly using water as a hydroxyl source is especially attractive because water is inexpensive, abundant, and relatively safe. Ammonia offers a similarly simple route to nitrogen-containing functionality. In practice, however, both molecules can interfere with transition-metal catalysts. Water may coordinate strongly to metal centers or alter their oxidation states, while ammonia can bind to metals and disrupt the coordination environments needed for catalytic C–H activation. The elementary steps required to form C–O and C–N bonds can also be incompatible with the metal complexes typically used in these reactions.
The researchers instead identified a role for phosphorus outside the transition-metal block. Their method uses simple triarylphosphines, compounds traditionally recognized as ligands that bind to metals, but which in this case function directly as reaction-promoting agents. Under the reported conditions, the phosphine enables an azine substrate to react with water or ammonia, converting an aromatic C–H bond into a C–O or C–N bond. This approach challenges the assumption that such transformations must rely on a transition-metal center to organize and activate every key step.
The chemistry depends on an unusual form of neighboring group participation. During the reaction, functional groups positioned within the reacting system do not remain chemically static. Pendant aldehyde and imine groups interconvert into acetal-like and aminal-like structures. An acetal contains a carbon atom connected to two oxygen-based groups, while an aminal contains a carbon atom connected to two nitrogen-based groups. These temporary arrangements act as internal organizational elements, bringing the small molecules involved in the reaction into the correct environment for bond formation.
According to the study, this molecular choreography effectively delivers water and ammonia into a phosphorus(V) coordination environment. The phosphorus center can then support the key bond-forming events through ligand-coupling reactions. In ligand coupling, two groups attached to a central atom combine with one another to form a new bond and depart as a connected product. Here, the process provides a pathway for oxygen or nitrogen derived from water or ammonia to become attached to an azine carbon, even though the reaction does not follow the more familiar sequence of metal-mediated C–H activation and reductive elimination.
The proposed mechanism was examined through both experiments and computational studies. This combination allowed the researchers to track the unusual interconversion of aldehyde and imine groups and to evaluate how the acetal- and aminal-type intermediates contribute to the reaction. Mechanistic insight is particularly important in this case because the phosphine is not simply serving as a passive ligand. Its behavior, together with the neighboring functional groups, creates a reactive phosphorus(V) environment capable of carrying out chemistry commonly associated with transition-metal complexes.
The reported scope extends across a broad selection of pyridines and includes quinolines and diazines, demonstrating that the method is not limited to a single azine framework. More importantly for practical synthesis, the researchers show that the reaction can be applied at a late stage to complex pharmaceutical and agrochemical structures. Late-stage functionalization is a powerful strategy in structure–activity relationship studies because it allows chemists to introduce or compare molecular changes rapidly. A single advanced compound can potentially be converted into several analogues by selectively replacing an azine C–H bond with a hydroxyl or amino group.
The discovery could make abundant, simple reagents more useful in the rapid diversification of biologically important molecules. Hydroxylated and aminated azines can display altered solubility, polarity, hydrogen-bonding behavior, metabolic stability, and biological activity, all of which are central considerations in drug and agrochemical design. By showing that triarylphosphines can mediate these transformations through a mechanism involving neighboring group participation and phosphorus-based ligand coupling, the work broadens the conceptual toolkit for C–H functionalization. It also illustrates how reactions that appear incompatible with transition metals may become accessible when the central role is reassigned to a main-group element.
Subject of Research: Direct azine C–H functionalization with water and ammonia using triarylphosphines to form C–O and C–N bonds.
Article Title: Phosphine-mediated azine C–H couplings with water and ammonia
Article References: Nottingham, K.G., Brunner, D.A., Dalmau, D. et al. “Phosphine-mediated azine C–H couplings with water and ammonia.” Nature (2026). https://doi.org/10.1038/s41586-026-10991-w
Image Credits: AI Generated
DOI: 10.1038/s41586-026-10991-w
Keywords: C–H functionalization, azines, pyridines, quinolines, diazines, triarylphosphines, water coupling, ammonia coupling, phosphorus(V), ligand coupling, late-stage functionalization, medicinal chemistry, agrochemical chemistry
