The compounds at the center of the work, tetrahydrobenzo[b]pyrans and related 2-amino-3-cyano-4H-pyrans, belong to a family of oxygen-containing ring systems with an impressive portfolio of biological activities. Published studies have linked members of this class to anticancer, antimicrobial, antioxidant, anti-inflammatory, antiviral, anticoagulant, antidiabetic, analgesic, and spasmolytic effects. Beyond pharmacology, these fused pyran frameworks serve as versatile intermediates for constructing more elaborate heterocyclic architectures and functional organic materials. That combination of therapeutic relevance and synthetic utility explains why finding faster, greener ways to make them has become one of the busiest corners of modern heterocyclic chemistry.

The reaction the team exploited is a classic of multicomponent chemistry. An aromatic aldehyde, malononitrile, and a cyclic 1,3-dicarbonyl compound such as dimedone, or alternatively ethyl acetoacetate, are combined in a single flask. The domino sequence that follows proceeds through a Knoevenagel condensation, in which the aldehyde and malononitrile form an activated alkene, followed by a Michael addition of the enolized dicarbonyl compound, and finally an intramolecular cyclization that closes the pyran ring. Because three or more starting materials merge in one operation, multicomponent reactions of this type offer excellent atom economy, operational simplicity, high bond-forming efficiency, short reaction times, low energy consumption, and minimal waste, all hallmarks of green chemistry.

What has historically held this transformation back is the catalyst. Most reported protocols rely on Brønsted acids, Lewis acids, transition-metal complexes, ionic liquids, deep eutectic solvents, metal-organic frameworks, carbon-based materials, or recyclable magnetic nanocatalysts. Many of these systems deliver good yields, but they often demand laborious, multistep catalyst preparation, carry high costs, resist large-scale recycling, or require elevated temperatures, long reaction times, and environmentally undesirable organic solvents. The Iranian team’s answer was to strip the catalyst out entirely and let the solvent do the work.

HFIP is an unusual liquid. It is highly polar, weakly nucleophilic, and possesses an exceptional capacity to donate hydrogen bonds, along with very high ionizing power. Those properties allow it to stabilize cationic intermediates and polar transition states, activate carbonyl compounds, and facilitate proton transfers, all without being consumed or incorporated into the product. In recent years HFIP has attracted growing attention as a medium for cyclizations, electrophilic additions, carbon-hydrogen functionalization, oxidative transformations, and multicomponent processes, and mounting evidence suggests it can reduce or even eliminate the need for conventional acid catalysts. What had been missing, the authors note, was a demonstration of HFIP acting alone as the promoter for this particular pyran synthesis.

The optimization experiments tell a striking story. Using benzaldehyde, malononitrile, and dimedone as the model reaction, the researchers varied the amount of HFIP and the temperature. At room temperature with 0.30 milliliters of HFIP, the isolated yield reached 87 percent in just ten minutes. Raising the temperature to 55 degrees Celsius cut the time to five minutes and pushed the yield to 92 percent. Adding more HFIP brought no further benefit. Most tellingly, when HFIP was removed altogether, only a trace of product appeared after thirty minutes under otherwise identical conditions. Because the yield and rate climb with HFIP concentration, the authors interpret the effect as promotional rather than catalytic, and they are careful not to claim a catalytic turnover cycle without dedicated kinetic evidence.

The mechanistic picture the team offers, explicitly framed as a plausible hypothesis rather than experimentally confirmed proof, rests on hydrogen bonding. HFIP can hydrogen-bond to the carbonyl oxygen of the aldehyde, increasing the electrophilicity of the carbonyl carbon and thereby accelerating the initial Knoevenagel condensation with malononitrile. The same hydrogen-bonding network is thought to stabilize the charged intermediates that appear during the subsequent Michael addition and the intramolecular cyclization, allowing the entire domino sequence to run quickly and cleanly at relatively low temperature. The authors acknowledge that no specific kinetic experiments or comparative studies with alternative hydrogen-bond-donating solvents were performed, so the separate contribution of hydrogen bonding cannot yet be distinguished from other solvent effects.

With the optimized conditions in hand, the researchers surveyed a representative set of aromatic aldehydes bearing both electron-donating and electron-withdrawing substituents. Methoxy, chloro, and nitro groups were all tolerated. When dimedone served as the dicarbonyl component, the tetrahydrobenzo[b]pyran products were isolated in 91 to 95 percent yields over reaction times of roughly 90 to 100 minutes. Swapping dimedone for ethyl acetoacetate delivered the corresponding 2-amino-3-cyano-4H-pyran derivatives in 83 to 94 percent yields. The authors are candid about the limits: heteroaromatic aldehydes, sterically hindered substrates, and more electronically extreme cases were not systematically examined, so the results demonstrate reliable compatibility within the tested set rather than a universally broad substrate scope.

How does the HFIP protocol stack up against the competition? The team compared its results with representative literature procedures, including magnetic nanocatalysts operating under microwave irradiation or grinding, ammonium hydroxide in water, and urea in aqueous ethanol. For the benchmark products 4a through 4c, the HFIP method delivered 92 to 95 percent isolated yields at 55 degrees Celsius with reaction times similar to or shorter than most of the reference procedures, some of which required reflux, extended stirring, or elaborate catalyst synthesis. The comparison rests on reaction time and isolated yield alone, since parameters such as catalyst loading, solvent quantity, and recycling efficiency are not uniformly reported across studies. The clear advantage of the new protocol is its formulation: a homogeneous medium, a one-pot operation, mild temperature, and no added metal or acid catalyst, with products isolated by simple crystallization from ethanol rather than chromatography.

The authors also weigh the environmental ledger honestly. HFIP is a fluorinated solvent, and although it is readily distilled and was recovered by distillation for reuse in subsequent runs in the laboratory, the study did not experimentally evaluate recovery efficiency, energy costs, or solvent losses at scale. The environmental burden of fluorinated compounds, occupational hazards, and the energy required for solvent recovery all must be counted against the benefits of catalyst elimination, fast reactions, and high atom economy. The team concludes that the method is a valuable addition to the synthetic toolbox for the substrates tested, but that future work should expand the substrate scope, probe the mechanism with kinetic and comparative solvent studies, and rigorously assess HFIP recovery. For now, the message is clear and likely to resonate across medicinal chemistry and green synthesis alike: sometimes the most powerful catalyst in the flask is the solvent itself.

Subject of Research: HFIP-promoted catalyst-free one-pot multicomponent synthesis of tetrahydrobenzo[b]pyran and 2-amino-3-cyano-4H-pyran derivatives

Article Title: HFIP-promoted Lewis acid-free one-pot synthesis of pyran derivatives under mild conditions

Article References: Nouri, F. Z., Maleki, B., Baharfar, R., Alinezhad, H., & Ashrafi, S. S. (2026). HFIP-promoted Lewis acid-free one-pot synthesis of pyran derivatives under mild conditions. Results in Chemistry, 31, Article 103961. https://doi.org/10.1016/j.rechem.2026.103961

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103961

Keywords: HFIP, multicomponent reaction, pyran synthesis, green chemistry, hydrogen bonding, Knoevenagel condensation, Michael addition, heterocyclic chemistry, fluorinated solvents, catalyst-free synthesis, tetrahydrobenzo[b]pyrans, organic synthesis

The molecules in question are far from obscure laboratory curiosities. Ortho-aminocarbonitrile tetrahydronaphthalenes are densely functionalized heterocycles featuring adjacent amino and nitrile groups on a partially saturated naphthalene ring. That combination gives them a rigid three-dimensional architecture, donor-acceptor electronic properties, and abundant handles for further chemical modification. They serve as precursors to dicyanoanilines, compounds prized for their optical and electronic behavior, and as building blocks for fluorescent systems, functional materials, and agents investigated for antiparasitic, anticancer, antioxidant, and antimicrobial activity. The cyano groups can be hydrolyzed to carboxylic acids or deployed in cyclization and reduction chemistry, while the amino group supports hydrogen bonding and nucleophilic transformations, making the scaffold a versatile launch point for medicinal and materials chemistry alike.

Traditionally, assembling such molecules requires multistep sequences that generate significant waste. The Kashan team instead turned to multicomponent reactions, or MCRs, a strategy in which three or more starting materials combine in a single operational step. MCRs are celebrated for their atom economy and operational simplicity, and they have become mainstays of drug discovery and green chemistry because they compress lengthy synthetic routes into one pot. In this work, the researchers combined cyclohexanone, malononitrile, and a range of substituted aromatic aldehydes, letting the catalyst orchestrate a cascade of bond-forming events that culminates in the fully assembled tetrahydronaphthalene framework.

The catalyst itself is a carefully engineered core-shell structure. The team began with nickel ferrite, NiFe2O4, nanoparticles produced by co-precipitating nickel nitrate and iron nitrate in a strongly alkaline solution at pH 13, stabilizing the particles with oleic acid, heating them at 80 degrees Celsius, and finally calcining them at 600 degrees Celsius to lock in a robust crystalline structure. Nickel ferrite was a deliberate choice: the material is known for strong magnetic properties, chemical stability, and proven catalytic performance across heterocycle synthesis, coupling reactions, oxidations, and reductions. A silica layer was then grafted onto the particles using 3-chloropropyl triethoxysilane in anhydrous toluene at 105 degrees Celsius, providing a chemically versatile surface that prevents the magnetic core from clumping.

Two further modification steps installed the catalytic engine. The chloro-functionalized surface reacted with 3-mercapto-1,2,4-triazole under reflux, introducing thiol-triazole functionalities, and a final coupling with cyanuric chloride at 110 degrees Celsius anchored triazine rings onto the scaffold. The resulting material, designated NiFe2O4@SiO2@TT, carries an abundance of basic nitrogen sites that drive the reaction chemistry, while the magnetic core allows the entire catalyst to be snatched out of solution with an ordinary external magnet rather than filtered or centrifuged. That simple recovery step eliminates catalyst loss and avoids energy-intensive separation techniques, a hallmark of green chemistry thinking.

Characterization confirmed the design worked as intended. X-ray diffraction showed the cubic spinel structure of the nickel ferrite core remained intact after functionalization, with reflections matching the reference pattern and no impurity phases. Applying the Scherrer equation to the broadened diffraction peaks gave an average crystallite size of roughly 26 nanometers. Field-emission scanning electron microscopy resolved discrete particles between 27 and 33 nanometers, forming porous aggregates of 120 to 280 nanometers, a hierarchical structure that enhances mass transport and exposes reactive sites. Energy-dispersive X-ray spectroscopy and elemental mapping verified the presence and even distribution of iron, nickel, silicon, carbon, nitrogen, and sulfur throughout the composite.

Other measurements filled in the picture of a material built for demanding service. Vibrating sample magnetometry recorded a saturation magnetization of 23.8 emu per gram, ample for swift magnetic extraction. Nitrogen adsorption analysis revealed a Type IV isotherm typical of mesoporous materials, with a BET surface area of 50.8 square meters per gram and a bimodal pore distribution combining micropores, which boost surface area, with larger mesopores that ease molecular traffic to active sites. Thermogravimetric analysis showed a total weight loss of just 9.8 percent up to 800 degrees Celsius, with the organic functional groups decomposing around 340 degrees Celsius, confirming the hybrid framework remains thermally stable under realistic reaction conditions.

Performance testing delivered striking numbers. After optimizing solvent, temperature, and catalyst loading, the team found that just 5 milligrams of the nanocatalyst in refluxing ethanol converted the model reaction mixture to product in 95 percent yield within ten minutes. Toluene, by contrast, gave the worst results, while ethanol’s polarity proved ideal for shuttling reactants to the catalyst’s active sites. The protocol then swept through sixteen different aromatic aldehydes bearing electron-donating and electron-withdrawing substituents, including chloro, bromo, fluoro, nitro, methyl, hydroxy, methoxy, and dimethylamino groups. Every substrate delivered its product in 86 to 95 percent yield within ten to twenty minutes, demonstrating broad substrate tolerance and practical reliability. Turnover frequencies reached as high as 397 per hour.

The proposed mechanism reads like a molecular relay race. Basic sites on the catalyst first deprotonate malononitrile and promote a Knoevenagel condensation with cyclohexanone, generating an unsaturated intermediate after dehydration. In parallel, another malononitrile molecule condenses with the aromatic aldehyde to form an arylidene malononitrile. The two fragments then meet in a Michael-type addition, followed by intramolecular cyclization, proton transfer, and catalyst-assisted tautomerization that delivers the final tetrahydronaphthalene while regenerating the catalyst for the next cycle. A hot filtration test, in which the catalyst was removed at roughly half conversion, showed no further product formation in the filtrate, proving the activity truly resides on the solid surface rather than in leached species. ICP-OES analysis quantified metal leaching at less than 0.004 percent for nickel and 0.007 percent for iron.

Perhaps most impressively, the catalyst survived seven consecutive reaction cycles with only marginal loss of activity, and post-use FE-SEM, FT-IR, XRD, and VSM analyses confirmed its morphology and structure were essentially unchanged. Compared with previously reported systems, which required anywhere from an hour to a full day and sometimes costly ionic liquids or stoichiometric organic bases, the new protocol matches or beats the best yields in a fraction of the time. A green chemistry assessment using the ComplexMoGAPI metric credited the method’s one-pot design, low catalyst loading, short reaction time, ethanol solvent, and magnetic recyclability. The researchers suggest their strategy of marrying functionalized magnetic supports with nitrogen-rich active groups could inspire similar platforms for other catalytic transformations, pointing toward a future where complex drug-like molecules are assembled quickly, cleanly, and with a magnet doing the cleanup.

Subject of Research: Development of a triazine-triazole functionalized magnetic nickel ferrite silica nanocatalyst for the green multicomponent synthesis of ortho-aminocarbonitrile tetrahydronaphthalene derivatives

Article Title: Triazine-triazole decorated magnetic nickel ferrite as a novel nanocatalyst for green synthesis of ortho-aminocarbonitrile tetrahydronaphthalenes

Article References: Mohammadi, M., Khorasani, M., & Naeimi, H. (2026). Triazine-triazole decorated magnetic nickel ferrite as a novel nanocatalyst for green synthesis of ortho-aminocarbonitrile tetrahydronaphthalenes. Results in Chemistry, 31, Article 103859. https://doi.org/10.1016/j.rechem.2026.103859

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103859

Keywords: nanocatalyst, nickel ferrite, magnetic separation, multicomponent reaction, green chemistry, tetrahydronaphthalene, heterogeneous catalysis, triazine, triazole, Knoevenagel condensation, Michael addition, catalyst recyclability