Preparation, characterization, and catalytic performance of a 2,3-dihydroxybenzoic acid-based deep eutectic solvent...

An ethyltriphenylphosphonium bromide/2,3-dihydroxybenzoic acid deep eutectic solvent (ETPPBr/2,3-DHBA-DES, 1 : 2 molar ratio) was prepared and characterized by eutectic phase behavior analysis, FT-IR spectroscopy, ¹H NMR spectroscopy, TGA/DTA, and density measurements, confirming the formation of a eutectic system. The DES was then employed as a recyclable catalyst for the one-pot three-component synthesis of 6H-benzo[h]chromeno[4,3-b]quinolin-6-ones via the condensation of 4-hydroxycoumarin, 1-naphthylamine, and aryl or heteroaryl aldehydes under solvent-free conditions. Under the optimized reaction conditions (0.50 mmol catalyst, 80 °C), ten target derivatives were obtained in 60.8-85.0% isolated yields within 45-90 min. The catalyst was readily recovered by aqueous extraction and reused for four consecutive cycles, affording product yields from 85% in the first run to 72% in the fourth. FT-IR analysis of the recovered catalyst indicated that its characteristic structural features were largely preserved after recycling. These results demonstrate that the ETPPBr/2,3-DHBA-DES is an effective and recyclable catalyst for the solvent-free synthesis of 6H-benzo[h]chromeno [4,3-b]quinolin-6-ones.