What if a catalyst could make a complex medicinally relevant molecule without solvent, deliver a ninety-eight percent yield, and then be recovered for reuse? This paper tests exactly that idea with silica-supported iron Lewis acids.

Hantzsch-derived heterocycles, including polyhydroquinoline, have gained attention because of important biological activities and medicinal applications. For polyhydroquinoline-3-carboxamides, earlier four-component procedures suffered from complex synthetic paths, harsh reaction medium, longer reaction time, low product yield, and non-recyclable catalyst.

The demand, then, is for a simple, efficient, environment-friendly, and versatile synthetic protocol. Multi-component reactions address that need by making complex molecules from easy beginning substances without isolating intermediates.

The proposed route synthesizes hexahydroquinoline-3-carboxamides from aromatic aldehyde, dimedone, acetoacetanilide, and ammonium acetate. The condensation is one-pot and solvent-free at seventy degrees Celsius, with higher product yield and high catalyst recovery after a simple reaction workup.

The Lewis acid catalysts are reported to show magnificent catalytic efficiency under solvent-free conditions in a short time. Iron trifluoroacetate and iron trichloroacetate were prepared by direct reaction between iron three acetate, two grams, and an excess quantity of the corresponding acid, nine grams, in a one-to-three equivalent ratio.

The reaction mixture was heated at sixty degrees Celsius for six to eight hours. The reddish-brown product was separated by vacuum filtration to remove unreacted acid and acetic acid obtained as a side product. The silica-supported Lewis acids were prepared by a slight modification of literature-known procedures.

Two grams of synthesized iron trifluoroacetate or trichloroacetate Lewis acid were poured into ninety milliliters of methanol and stirred for ten to fifteen minutes at room temperature. Twenty grams of Kieselgel K100 or silica gel were added, and the resulting slurry was stirred for eight to ten hours at room temperature before the solvent was evaporated.

For the model reaction, benzaldehyde, dimedone, and acetoacetanilide were each used at one millimole, ammonium acetate at one point two millimoles, and silica-supported catalyst at fifty milligrams. The mixture was stirred at seventy degrees Celsius for eight minutes under solvent-free conditions, while thin-layer chromatography tracked reaction progress.

After the reaction, hot ethanol diluted the mixture and filtration removed the catalyst; the filtrate was added to crushed ice, and the crude solid was filtered and recrystallized with hot ethanol. The recovered catalyst was cleaned with ethanol and dried in a vacuum desiccator for two to three hours for reuse.

Figure one presents FTIR spectra for bulk and silica-supported iron trifluoroacetate and trichloroacetate catalysts. The labeled bands include carbon–oxygen asymmetric and symmetric vibrations, carbon–fluorine or carbon–chlorine features, and silica–oxygen and silica–oxygen–iron vibrations.

These signatures matter because they provide structural evidence for the iron carboxylate species and their interaction with the silica support, helping define the catalyst’s Lewis-acid framework. FTIR spectroscopy was used to investigate the primary structure of the bulk and silica-supported iron trifluoroacetate and iron trichloroacetate catalyst series.

Unsupported iron trifluoroacetate and trichloroacetate showed carbon dioxide asymmetric vibrations at one thousand six hundred thirty-seven and one thousand six hundred fifty-six inverse centimeters, respectively. The silica-supported catalysts showed asymmetric carbon dioxide vibration bands at one thousand six hundred thirty-one and one thousand six hundred twenty-two inverse centimeters, with symmetric bands at one thousand three hundred fifty-nine and one thousand three hundred seventy-seven inverse centimeters.

The interaction between silica and the iron carboxylates led to altered vibrational values in the supported catalysts compared with the unsupported catalysts. Figure three compares electron microscopy images of bulk and silica-supported iron trifluoroacetate and trichloroacetate.

The FE-SEM panels show soft, irregular particles in the bulk samples, while the supported materials appear distributed across a silica surface. HR-TEM further shows dark fine particles in the bulk catalysts and layered dark particles on the supporting material, with silica gel shown separately for reference.

FE-SEM and HR-TEM were employed to analyze the surface texture and morphology of the prepared Lewis acid catalysts. Bulk unsupported iron trifluoroacetate and trichloroacetate had soft, irregularly shaped particles with smooth surfaces, while the supported samples were shown separately in the corresponding images.

The morphology of the supported Lewis catalysts was substantially comparable to that of the bulk unsupported samples. The intact surface morphology authenticated thorough dispersion of the iron trifluoroacetate and trichloroacetate functionalities in the pores of mesoporous silica, with no distinct bulk crystallites seen in either supported catalyst.

Figure six shows ammonia temperature-programmed desorption profiles for bulk and silica-supported iron trifluoroacetate and trichloroacetate catalysts. Peak positions indicate the strength of acidic sites, while the area under each curve estimates total acidity.

The separated profiles therefore provide a comparison of how silica support and iron carboxylate composition affect the distribution and strength of Lewis-acid sites. Ammonia temperature-programmed desorption determined acid strength and acid quantity: desorption peak temperature indicates acid concentration, while the area under the curve estimates acid quantity.

Bulk iron trifluoroacetate and iron trichloroacetate each showed a single ammonia desorption peak, at two hundred thirty-four and two hundred four degrees Celsius, corresponding to weak acidic sites. By contrast, both silica-supported catalysts showed two ammonia desorption peaks corresponding to weak and strong acidic sites.

For the supported catalysts, weak-site peaks occurred at two hundred forty-two and two hundred thirty-six degrees Celsius, while strong-site peaks occurred at three hundred twenty-eight and three hundred twenty-two degrees Celsius. The main objective was to evaluate the synthesized Lewis acid catalysts and optimize conditions for the one-pot, four-component synthesis of hexahydroquinoline-3-carboxamide derivatives.

The model reaction used benzaldehyde, dimedone, acetoacetanilide, and ammonium acetate as substrates. The model reaction first compared unsupported iron trifluoroacetate and iron trichloroacetate with their silica-supported counterparts under solvent-free conditions.

The supported Lewis acid catalysts showed higher catalytic activity, measured by reaction time and product yield, than similar catalysts without silica support. Table three compares silica gel with unsupported and silica-supported iron Lewis acid catalysts in the solvent-free synthesis of compound five a.

With fifty milligrams of each catalyst, silica gel gives a reaction time of three hundred minutes and a product yield of thirty-four percent, while the supported catalysts report reaction times of nine and eight minutes with product yields of ninety-eight percent. The authors use these results to show how silica support modifies catalytic activity.

The reaction conditions were monitored across different solvents, temperatures, and catalyst quantities to identify the ideal protocol using silica-supported iron trifluoroacetate or trichloroacetate. Polar and non-polar solvents were tested to determine the suitable reaction medium.

The best reaction time and product yield were obtained in a solvent-free environment. The proposed explanation is that the porous catalyst structure is more effective without solvent because increased reactant availability provides simple access to active sites through the pores.

Under solvent-free conditions, reaction time and product yield improved gradually and steadily as temperature increased from fifty to seventy degrees Celsius. Seventy degrees Celsius was determined to be the optimal temperature for the one-pot, solvent-free synthesis of hexahydroquinoline-3-carboxamides.

Increasing the temperature above seventy degrees Celsius had no discernible effect on reaction progress time or product yield. Table six tests silica-supported iron trifluoroacetate and trichloroacetate catalysts under solvent-free conditions at seventy degrees Celsius, using ten to seventy milligrams.

For both catalysts, reaction times remain eight and nine minutes, while product yield rises from sixty-three and sixty-one percent at ten milligrams to ninety-eight percent for each catalyst at fifty milligrams. Increasing the amount to sixty or seventy milligrams leaves the reported yields unchanged, identifying fifty milligrams as the effective amount tested.

As the amount of silica-supported Lewis acid catalyst increased gradually, the product yield increased in the reaction. At seventy degrees Celsius, fifty milligrams, or zero point zero five grams, of either silica-supported iron trifluoroacetate or trichloroacetate gave the highest product yield of ninety-eight percent.

Increasing the catalyst quantity further to sixty and seventy milligrams left the product yields unchanged. Table 7 shows a one-pot, solvent-free route to hexahydroquinoline-3-carboxamide derivatives labeled 5a through 5l. The scheme combines three organic components with ammonium acetate and either silica-supported iron trifluoroacetate or iron trichloroacetate, using zero point zero five grams of catalyst at seventy degrees Celsius.

The authors use this table to present the derivative synthesis under optimized conditions and report that the trifluoroacetate-supported catalyst exhibits higher catalytic activity across the optimization parameters. Hexahydroquinoline-3-carboxamide derivatives five a through five l were synthesized using the catalytic system under the ideal reaction conditions.

The paper reports excellent outcomes and provides their detailed description in Table 7. Silica-supported iron trifluoroacetate showed higher catalytic activity than silica-supported iron trichloroacetate across the optimization parameters used for derivative synthesis.

The proposed reason is that highly electronegative fluorine atoms have more withdrawing character than chlorine atoms, which increases Lewis acid catalyst acidity. Table eight compares catalysts used to synthesize hexahydroquinoline-three-carboxamide derivative five-a, listing conditions, reaction time, and yield.

The two current silica-supported iron Lewis acids, iron trichloroacetate and iron trifluoroacetate, both report ninety-eight percent yield under solvent-free conditions at seventy degrees Celsius, in nine and eight minutes respectively. The authors present this comparison as evidence that these catalysts are strong alternatives to previously reported systems, considering catalyst amount, reaction time, and product yield.

Compared with other literature-reported catalysts, the silica-supported iron trifluoroacetate and trichloroacetate Lewis acid catalysts performed superiorly in catalyst amount, reaction times, and product yield. The current Lewis acid-promoted synthesis is presented as an excellent alternative to those catalysts.

The route is described as more economically convenient and environment friendly. Figure nine summarizes the measured green-chemistry metrics for hexahydroquinoline-three-carboxamides five-a through five-l in a radar chart.

It displays optimum efficiency, atom economy, effective mass yield, reaction mass yield, and E-factor on a zero-to-one-hundred scale, allowing the sustainability profile of each derivative to be viewed across several criteria. The authors use these recognized metrics to assess the protocol’s environmental performance alongside its synthetic outcomes.

Green chemistry metrics were calculated and demonstrated for hexahydroquinoline-3-carboxamides five a through five l. The reported atom economy, E-factor, optimum efficiency, reaction mass efficiency, and effective mass yield values were close to their ideal values.

Figure eleven tracks catalyst reuse across seven runs, plotting product yield for silica-supported iron trifluoroacetate and trichloroacetate catalysts. The bars remain close to the high-yield region throughout the sequence, showing that regenerated catalysts retain broadly consistent activity.

This matters because the authors recover the solids by hot-ethanol filtration, wash them with an ethanol–water mixture, and reactivate them before reuse, supporting a simple recycling procedure without a significant loss of catalytic performance. The regenerated catalysts showed almost consistent catalytic activity.

The studies demonstrate reusable Lewis acids without a significant reduction in catalytic activity. The recycled catalysts were recovered and identified by FTIR and PXRD analysis after seven cycles. Silica-supported iron trifluoroacetate and trichloroacetate were developed as recyclable and water-competent green Lewis acid catalysts through a novel, environment-friendly approach.

Their efficiency was assessed in the synthesis of hexahydroquinoline-3-carboxamides five a through five l, and the catalysts worked without losing catalytic activity in water and organic solvents compared with conventional Lewis acid catalysts. The optimized protocol was solvent-free at seventy degrees Celsius, with superb product yield, short reaction time, and easy work-up procedure.

The study identifies silica-supported iron trifluoroacetate and trichloroacetate as recyclable Lewis acid catalysts for a one-pot, solvent-free synthesis, with the best conditions at seventy degrees Celsius and a reported ninety-eight percent yield.