Natural asphalt oxide-grafted carboxylic acid: a sustainable heterogeneous catalyst for synthesis of pyrano[2,3-<i>c</i>]pyrazoles and 2-amino-3-cyanopyridines in water

In this study, we successfully utilized natural asphalt as a natural carbon substrate for the synthesis of a novel heterogeneous Brønsted acid nanocatalyst, Re-NA-CH₂CO₂H. The -COOH functional groups present on the surface of Reduced Natural Asphalt Oxide (Re-NA-oxide) serve as catalytic sites for Brønsted acid. This arrangement, in addition to increasing acidity, also expands the surface area accessible for catalytic activity, positioning Re-NA-oxide as a viable option for a range of acid-catalyzed reactions. The synthesized catalyst was characterized using various methods, including FT-IR, TGA, SEM, EDX and TEM. This catalyst was employed in the synthesis of pyrano[2,3-<i>c</i>]pyrazole and 2-amino-3-cyanopyridine derivatives through four-component reactions involving ethyl acetoacetate, hydrazine hydrate, malononitrile, and various aldehydes, as well as ammonium acetate, malononitrile, aldehydes, and ketones, respectively. The final step of the reaction mechanism involved vinylogous anomeric-based oxidation. The high acidity of the Re-NA-CH₂CO₂H catalyst enhanced nucleophilic attacks on electrophiles, contributing to the efficiency of the reactions. It is noteworthy that this study uses a naturally derived catalytic support, emphasizing its sustainability. This research potentially enables the coupling of nucleophiles to natural asphalt for the development of new functional materials from this renewable resource. The reaction conversion rate is significantly influenced by the electron-donating and electron-accepting groups in the reactions of pyrano[2,3-<i>c</i>]pyrazole (90-97% yield in 20-50 min) and 2-amino-3-cyanopyridine (90-97% yield in 30-50 min). Furthermore, due to the use of water as the solvent, it is easy to separate and reuse, operational simplicity, and environmentally friendly. The catalyst exhibits exceptional recyclability and retains its activity for at least five cycles, outperforming currently available catalysts in terms of yield, reaction conditions, and overall efficiency.