Sulfonated silica with magnetic properties was studied for dehydration of xylose to furfural and posteriorly for furfural valorization through the Biginelli and Hantzsch multicomponent reactions. Eleven polysubstituted heterocycles were obtained in excellent yields and without any side-product (81-91 %), using a multicomponent green methodology. The presence of Fe 3 O 4 particles allows an easy separation from the reaction medium giving a high yield in the reusability studies.
Lamellar double hydroxides (LDH) with double divalent cations were synthesized by the co-precipitation method and studied in the multicomponent synthesis of 4H-pyrans. The solids obtained were characterized by X-ray diffraction (XRD), infrared spectroscopy (FTIR), N2 adsorption isotherms, and thermogravimetric analysis (TGA). The XRD patterns confirmed the formation of LDHs in which the incorporation of Ni2+ or Co2+ improves their crystalline and textural properties. The results of catalytic activity showed that the synthesis of 4H-pyrans is favored in solvent-free conditions with the LDH–Ni catalyst, avoiding calcination processes. In addition, it was found that hydrotalcite with double divalent cations can conduct this reaction through multicomponent synthesis or by the Michael addition reaction, which can be performed by different types of basicity that depend on the composition of another divalent cation in the brucite layer or a calcination process.
Magnetic Fe 3 O 4 -based hydrotalcites were used as bifunctional catalysts for the microwave-assisted solvent-free synthesis of 4H-chromene derivatives by a three-component reaction from (hetero)aromatic aldehydes, malononitrile, and naphthol derivatives. Structures of 4H-chromenes 4 d and 4 g were studied and confirmed by single-crystal X-ray diffraction analysis. The heterogeneous catalysts were synthesized by the co-precipitation method incorporating divalent metal cations such as Ni 2 + or Co 2 + in LDHÀ Mg. This multicomponent protocol allows the synthesis of diverse 4H-chromenes in 88-95 % yields, reduced reaction time, high atom economy, broad substrate scope, and operational simplicity. The reusability of the catalyst up to five recycles without appreciable loss of its catalytic activity, make the present protocol sustainable and advantageous compared to conventional methods.
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