Ag@TiO₂ Nanocomposite as an Efficient Catalyst for Knoevenagel Condensation

Abstract

In the present study, a new series of different heterocycles was synthesized through base-free Knoevenagel condensation of various aldehydes and active methylene-containing compounds using the hydrothermal developed Ag@TiO 2 as a heterogeneous catalyst. The catalyst was synthesized by mixing TiO 2 (P25) with AgNO 3 and hydrothermally treated in ethanol at 180 °C for 12 h. The developed Ag@TiO 2 catalyst was directly applied for Knoevenagel condensation, and the optimized procedure involved stirring the aldehydes and active methylene-containing compounds with Ag@TiO 2 in ethanol at 65 °C. The reaction scope was investigated for various aromatic and heterocyclic aldehydes with active methylene-containing compounds, and the isolated yields were significantly high. The reusability of the catalyst was investigated for up to five cycles, where an insignificant decrease in the catalyst’s reactivity was observed. Also, the reaction could proceed in water as a solvent, and the isolated yield was 40%. Hence, this protocol features mild reaction conditions, a facile procedure, and clean reaction profiles.

A post-sulfonated one-pot synthesized magnetic cellulose nanocomposite for Knoevenagel and Thorpe–Ziegler reactions

“…7a depicts the proposed reaction mechanism for access to α,β-unsaturated nitrile derivatives based on the literature. 6,79 The key factor for the proposed mechanism is that acidic sulfonic sites enable the Knoevenagel coupling by polarizing the carbonyl group of benzaldehyde, where it can coordinate with oxygen and further be attacked by the pronucleophile-like malononitrile or ethyl cyanoacetate. Afterward, a water molecule was eliminated to yield the α,β-unsaturated product.…”

“…The heterocyclic derivatives benzothiazole 5h and thiazole 5i were tolerated to afford products 6h and 6i in 85% and 88% yields, respectively, and 7h and 7i in 75% and 80% yields, respectively. 6,79 The key factor for the proposed mechanism is that acidic sulfonic sites enable the Knoevenagel coupling by polarizing the carbonyl group of benzaldehyde, where it can coordinate with oxygen and further be attacked by the pronucleophile-like malononitrile or ethyl cyanoacetate. Aerward, a water molecule was eliminated to yield the a,b-unsaturated product.…”

“…Among the named reactions, Knoevenagel condensation is an essential condensation reaction, which involves a nucleophilic addition of active methylene to carbonyl compounds, which is followed by a dehydration reaction producing a,bunsaturated molecules via forming carbon-carbon double bonds. 6,7 Conventionally, different amines, organometallic catalysts, Lewis acids, such as, TiCl 4 , ZnCl 2 , and Al 2 O 3 , ionic liquids, and amino acids have been efficiently utilized as homogeneous catalysts for Knoevenagel condensation. [7][8][9] Due to the difficulty in recycling and harsh reaction conditions for homogeneous catalysts, heterogeneous catalysts, such as incorporating zeolites, 10,11 metal-organic frameworks (MOFs), 12,13 ionic liquids, hetero-poly acids, 14 functionalized mesoporous silica, 15 and carbon-based materials, 16,17 have been developed for Knoevenagel condensation.…”