Amine-functionalized biochar as a green and recyclable nanocatalyst for the efficient synthesis of 1,8-naphthyridine and 2-amino-4H-chromene derivatives.

Biochar is a renewable carbon-based material derived from biomass resources, which has attracted widespread attention due to its low production cost, environmental sustainability and high surface modification and functionalization capabilities. However, its potential application as an effective catalyst, especially for green multicomponent organic reactions, is still not fully explored. In this study, an amine-functionalized biochar nanocatalyst (biochar-NH2) is introduced as an effective, cost-effective, and environmentally friendly system for the green synthesis of 1,8-naphthyridine and 2-amino-4H-chromene derivatives. Biochar was prepared by pyrolysis of walnut shells and then modified by nitration and reduction to obtain biochar-NH2. The catalyst was characterized by SEM, EDX, FT-IR, BET and XRD analysis and was evaluated for its catalytic performance in multi-component reactions in aqueous media at ambient temperature. This catalyst exhibited very high efficiency for the synthesis of 1,8-naphthyridine and 2-amino-4H-chromene derivatives under mild conditions and using water as a green solvent. Under the optimized conditions, the model 1,8-naphthyridine derivative was obtained in 98% yield within 20 min using biochar-NH2 (0.04 g, 28.5 wt% relative to 4-chlorobenzaldehyde) in water at 25 °C. Under the same catalyst loading and reaction conditions, the model 2-amino-4H-chromene derivative was obtained in 97% yield within 7 min. The catalyst showed high recyclability and only a slight decrease in its corresponding activity was observed after several cycles of use, which indicates its structural stability and recyclability. Amine-enriched biochar is a highly effective, eco-friendly, and reusable (5-cycle) nanocatalyst for the synthesis of critical pharmaceutical heterocycles. It is a greener substitute for conventional catalysts and it helps researchers to move forward with green synthesis approaches.