Crushed 3 Å, 4 Å, and 5 Å molecular sieves: a green heterogeneous catalyst for solvent-free aza-Michael additions via microwave, ultrasound, and mechanochemical activation

This study highlights the catalytic potential of commercially available molecular sieves (MS 3 Å, 4 Å, and 5 Å) as cost-effective, reusable, and environmentally benign catalysts for C–N bond formation via aza-Michael reactions. Ground zeolite beads were shown to efficiently promote the addition of both primary and secondary amines to various α,β-unsaturated compounds under solvent-free conditions, yielding β-amino carbonyl derivatives with high selectivity and excellent yields. Notably, monoaddition was exclusively observed with primary amines, and the catalyst retained its activity over multiple cycles. The versatility of this catalytic system was further demonstrated through activation methods such as microwave irradiation, ultrasonication, and mechanochemistry, which significantly reduced reaction times and aligned with the principles of green chemistry. Beyond intermolecular transformations, MS 4 Å was successfully applied to intramolecular aza-Michael reactions of functionalized chalcones, enabling the synthesis of 2-aryl-2,3-dihydroquinolin-4(1H)-ones. A novel one-pot domino strategy was also developed, involving imine formation followed by Mannich-type cyclization to access these heterocycles efficiently. The proposed mechanism involves Brønsted acid sites on the zeolite surface, which activate the Michael acceptor for nucleophilic attack. Overall, this work establishes molecular sieves as sustainable alternatives to conventional acid/base catalysts in organic synthesis, offering operational simplicity, reduced environmental impact, and broad applicability in heterocycle construction.

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