Guanine-functionalized MWCNT-supported Cu(I) magnetic nanocatalyst for green synthesis of isoquinolines in PEG/H₂O

This study reports the development of a guanine-functionalized, multi-walled carbon nanotube (MWCNT)–supported copper(I) magnetic nanocatalyst designed to enable a rapid, green, three-component synthesis of isoquinolines. The reaction combines substituted 4-bromobenzaldehyde, acetamide, and substituted phenylacetylene in the presence of potassium acetate as the base, using a PEG/water solvent system at 100 °C for three hours. The guanine moiety enhances the activity of the Cu(I) centers and facilitates magnetic separation of the catalyst, promoting sustainability and convenient recovery. The catalyst was prepared via a stepwise sequence of MWCNT modification, deposition of Fe3O4 magnetic nanoparticles, guanine functionalization, and coordination of Cu(I), yielding a robust, magnetically retrievable material with uniform dispersion of Fe3O4 and CuI on the CNT framework. Comprehensive characterization by FT-IR, XRD, TGA, BET, SEM, TEM, EDX, and VSM confirmed successful functionalization, strong thermal and chemical stability, and efficient nanoparticle distribution on the CNT support. Under optimized conditions, KOAc as the base and a PEG/H2O (2:1) solvent system afforded isoquinolines in high yields within three hours, with broad substrate tolerance across electron-donating, electron-withdrawing, and heteroaryl functionalities. Mechanistic insights point to a Cu(I)-mediated sequence involving oxidative addition, alkyne activation, C–C coupling, cyclization, and aromatization on the catalyst surface, with guanine functionalization stabilizing the Cu(I) center and facilitating substrate activation. Notably, the catalyst demonstrated recyclability for at least seven cycles with minimal loss of activity and preserved magnetic and structural integrity, underscoring its practical potential. Overall, this work introduces the first guanine-anchored, MWCNT-supported Cu(I) magnetic nanocatalyst for fast, high-yielding, and recyclable multicomponent isoquinoline synthesis under green solvent conditions, highlighting the promise of nucleobase-functionalized nanomaterials in advancing environmentally friendly organic synthesis.