Scientists synthesize Z-allylamines using masked alkenyl anions from alkynyl tetracoordinate boron compounds

Chemists have reported a new way to build stereodefined allylamines, a class of molecules that sits at the intersection of drug discovery, agrochemistry, organic synthesis and advanced materials. The method, described by An, Zhu, Ma and colleagues, constructs these valuable structures through a metal-free cascade reaction that controls both the position and geometry of a carbon–carbon double bond. Its most striking feature is the selective formation of the less easily obtained Z isomer: under the reported conditions, the reaction produces allylamines with exclusive Z configuration. The approach could give researchers a more direct route to molecular architectures that have traditionally required carefully predesigned starting materials, specialized amine partners or precious-metal catalysts.

Allylamines contain an amine group attached to an alkene, and their combination of functional groups makes them unusually versatile. The nitrogen atom can participate in hydrogen bonding, acid–base chemistry and further bond-forming reactions, while the alkene provides a platform for selective chemical modification. In pharmaceutical molecules, these features can influence solubility, binding interactions, metabolic stability and three-dimensional shape. Similar structures appear in compounds used to protect crops and in molecular components designed for functional materials. Yet making allylamines with a precisely defined double-bond geometry remains difficult. The two principal arrangements, known as E and Z, can have different physical and biological properties, and mixtures of the two often complicate purification, characterization and downstream synthesis.

Existing strategies commonly impose constraints on the molecular pieces that can be joined. A reaction may require an amine with a particular substitution pattern, an alkene that has already been prefunctionalized, or a catalyst based on a noble metal such as palladium, rhodium or iridium. Such requirements can restrict the range of products accessible to synthetic chemists and add preparation steps before the key bond-forming event. The new platform addresses this challenge by using alkynyl tetracoordinate boron compounds as starting materials. These boron-containing molecules act not merely as passive carriers of an alkyne, but as precursors to a reactive intermediate that is generated during the reaction itself. The intermediate is described as a “masked alkenyl anion”—a species with the reactivity of a negatively charged alkenyl fragment, but whose behavior is temporarily embedded within a more manageable molecular framework.

The central transformation begins with a 1,2-metallate shift. In broad terms, this is a rearrangement in which a group attached to a tetracoordinate boron center migrates as the boron-containing system reorganizes. Here, the shift converts an alkynyl boron compound into an intermediate capable of forming a new carbon–carbon bond with an imine-derived electrophile. The process is conducted without a metal catalyst and in the presence of a common Brønsted acid, a proton donor that helps activate the participating molecules and regulate the sequence of events. Rather than isolating the reactive alkenyl species, the researchers produce it in situ, allowing it to react immediately with the other components. This one-pot design is important because highly reactive intermediates can be difficult to store, handle or prepare independently.

The reaction’s self-regulated sequence begins with the formation of an imine from an amine and an aldehyde. Imines contain a carbon–nitrogen double bond and are commonly used as electrophilic partners in carbon–carbon bond-forming chemistry. Once the imine forms, the masked alkenyl anion engages it in a stereodefined alkenylation step. The order matters: imine formation creates the appropriate reaction partner before the carbon fragment is delivered, helping prevent competing pathways. According to the reported findings, this sequence enforces both chemo- and regioselectivity. Chemoselectivity means that the reaction favors the intended functional group over other potentially reactive sites, while regioselectivity determines which atom in a molecule forms the new bond. Together, these controls allow the method to install the allylamine framework at a defined location rather than generating a collection of constitutional isomers.

The geometry of the resulting double bond is the defining achievement. Alkene geometry is determined by the relative positions of substituents around the rigid carbon–carbon double bond. In an E isomer, the principal groups lie on opposite sides; in a Z isomer, they are on the same side. Because rotation around the double bond is restricted, this distinction persists in the product and can alter its shape and reactivity. The researchers report exceptional Z/E ratios, with exclusive formation of the Z isomer under the described conditions. Such selectivity is especially valuable when the Z arrangement is the desired architecture, because conventional methods may deliver mixtures that need separation or may isomerize under later reaction conditions. The reported stereochemical control therefore represents more than a cosmetic difference: it provides a way to encode molecular shape directly during bond construction.

The scope of the method extends across a broad collection of amine partners. The reaction accommodates primary and secondary amines, including both aromatic amines and aliphatic amines. Aromatic amines contain nitrogen attached to an aromatic ring, whereas aliphatic amines are built around non-aromatic carbon frameworks; these classes differ in electronic properties and steric demands. The platform also works with enamines, which are nitrogen-containing compounds featuring a carbon–carbon double bond, and with an ammonia equivalent called tert-butyloxycarbonyl-NH₂. The latter can serve as a protected nitrogen source, allowing the amine functionality to be introduced in a form that may be manipulated later in a synthetic sequence. The reported compatibility with drug molecules, chiral amines and amino acids or esters expands the method beyond simple model substrates and suggests that it can operate in the presence of molecular complexity.

Chiral amines and amino-acid-derived partners bring an additional level of stereochemical information to the reaction. A chiral molecule exists in a form that cannot be superimposed on its mirror image, and its presence can influence which of several possible three-dimensional products is favored. When the new allylamine framework is created alongside an existing stereocenter, the products can occur as diastereomers—stereoisomers that are not mirror images of one another. The researchers report excellent diastereomeric ratios greater than 20:1 for the chiral substrates examined. This means that one diastereomer was formed in much greater proportion than its alternatives. Combining control of alkene geometry with control of relative stereochemistry could be particularly useful in medicinal chemistry, where the orientation of substituents can determine whether a candidate molecule interacts productively with a biological target or fails to do so.

The aldehyde component is similarly flexible. The reported reaction accepts substrates ranging from formaldehyde to aliphatic aldehydes, heteroaromatic and aromatic aldehydes, alpha,beta-unsaturated aldehydes and formyl formates. Formaldehyde represents a minimal carbonyl building block, while aliphatic aldehydes introduce non-aromatic carbon chains. Aromatic and heteroaromatic aldehydes bring ring systems containing carbon or heteroatoms such as nitrogen, oxygen or sulfur, often found in pharmaceuticals and agrochemicals. Alpha,beta-unsaturated aldehydes contain an additional alkene conjugated with the carbonyl group, creating the possibility of competing reactions at multiple unsaturated sites. Compatibility with this range indicates that the cascade can distinguish the intended aldehyde-derived imine chemistry from other reactive features. Formyl formates add another type of carbonyl-containing partner and further demonstrate the breadth of the aldehyde component.

The significance of the work lies in the combination of modularity, metal-free reactivity and stereochemical precision rather than in any single substrate example. By assembling an amine, an aldehyde and an alkynyl tetracoordinate boron compound in one cascade, the method offers a framework for varying several parts of the final allylamine independently. The use of a common Brønsted acid also points toward a comparatively accessible catalytic environment, although the source material does not establish how the protocol will perform at industrial scale, across all functional groups or in a manufacturing setting. As with any synthetic advance, practical adoption will depend on factors such as substrate cost, reaction concentration, purification, waste generation and robustness in larger reactors. Even so, the ability to generate Z-selective allylamines from diverse partners addresses a persistent problem in molecular construction. It gives chemists a new strategy for creating structurally varied, stereodefined compounds without relying on prefunctionalized alkenes or noble-metal catalysts, potentially accelerating the search for new medicines, crop-protection agents and functional molecules.