Cu doped CdS nanoparticles: A versatile and recoverable catalyst for chemoselective synthesis of indolo[2,3-b]quinoxaline derivatives under microwave irradiation

CdS and Cu doped CdS NPs has been obtained successfully using safe agents through a simple aqueous chemical method and characterized by XRD, TEM, SEM, EDAX, UV/VIS and ICP-AES. These investigations revealed that the particle size of the synthesized materials were uniformly distributed in the range of 2-4 nm and confirm the Cu doping in the lattice of CdS NPs. These nanoparticles were exploited to study their catalytic activities toward the chemoselective synthesis of indolo[2,3-blquinoxalines by the reaction of isatins with 1,2-diamines in ethylene glycol under microwave irradiation. The method showed remarkable selectivity for indolo[2,3-b]quinoxalines over 3-imino-isatin, spirobenzimidazole, and ring-opened quinoxalinone derivatives. The catalytic activity of Cu doped CdS NPs was found to be about 18-fold higher under microwave irradiation (MW) as compared to the conventional method. Nanocatalyst plays a dual role of catalyst as well as susceptor, and enhances the overall capacity to absorb MW in the reaction mixture. Doping of Cu promotes the activity and selectivity of CdS nanoparticles indicated by high TOF value with good chemoselectivity. The surface acidity of NPs was measured by FTIR spectra of chemisorbed pyridine. Simple workup, mild reaction conditions, low cost, easy separation, and reusability of the catalyst are some advantages of this method. (C) 2014 Elsevier B.V. All rights reserved.