Construction of a Dual-Functionalized Acid-Base Nanocatalyst via HEPES Buffer Functionalized on Fe 3 O 4 as a Reusable Catalyst for Annulation Reactions.

Herein, we present a highly efficient dual-functionalized acid-base nanocatalyst, denoted as Fe 3 O 4 @GLYMO-HEPES, featuring sulfuric acid and tertiary amines as its dual functional components. This catalyst is synthesized through the immobilization of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) as the source of these functionalities onto magnetite (Fe 3 O 4 ) using 3-glycidoxypropyltriethoxysilane (GLYMO) as a linker. Characterization studies confirm the integrity of the Fe 3 O 4 core, with the GLYMO-HEPES coating exhibiting no phase changes. Furthermore, Fe 3 O 4 @GLYMO-HEPES nanoparticles demonstrate a uniform size distribution without aggregation. Notably, the catalyst exhibits remarkable stability up to 200 °C and possesses a saturation magnetization value of 31.5 emu/g, facilitating easy recovery via magnetic separation. These findings underscore the potential of Fe 3 O 4 @GLYMO-HEPES as a versatile and recyclable nanocatalyst for various applications. Its catalytic ability was evaluated in the synthesis of various pyrano[2,3- c ]pyrazoles and 2-amino-3-cyano-4 H -chromenes through a tandem Knorr-Knoevenagel-Michael-Thorpe-Ziegler-type heterocyclization mechanism, using different aldehydes. A wide range of fused heterocycles was synthesized having good to excellent yields. The process is cost-effective, safe, sustainable, and scalable, and the catalyst can be reused up to five times. The prepared catalyst was found to be highly stable and heterogeneous and showed good recyclability.