Ag@TiO 2 Nanocomposite as an Efficient Catalyst for Knoevenagel Condensation.

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.