Efficient and recyclable gold nanoparticles as catalysts for the cleaner production of 4-morpholinoanilines used as pharmaceutical building blocks

Gold nanoparticles (Au-NPs) stabilized with PEGylated Poly(amidoamine) (PEG-PAMAM) dendrimers were successfully prepared in mild aqueous conditions; they were evaluated as efficient and recyclable catalysts for the cleaner reduction of 4-morpholinonitrobenzenes (MNBs) to 4-morpholinoanilines (MANs), which have pharmaceutical significance. Transmission electron microscopy, UV–visible spectroscopy, X-ray photoelectron spectroscopy, and dynamic light scattering experiments were performed to confirm the formation of stable Au-NPs aggregates, with mean particles sizes of 1.6–4.0 nm. Catalytic reactions were conducted at room temperature using sodium borohydride as a reducing agent instead of hydrazine, representing a significant progress toward more environmentally friendly catalytic methods. The Au-NPs showed high catalytic efficiency in the reduction of MNBs to MANs, achieved maximum conversion rates >98% in less than 4 min, and no byproduct formation was detected. Pseudo-first order rate constants of 0.78–1.32 s−1 were obtained with a maximum turnover number of 964, which is 2.4-fold higher than that of the best catalyst for MNB reduction reported in literature. The best PEG-PAMAM-Au catalyst, recovered via dialysis, remained active after 7 consecutive cycles, with negligible metal leaching. This system exhibited enhanced activity in the reduction of substituted MNB derivatives, which are used as building blocks in the chemical industry. Thus, our results provide a valuable, practical, and green method by which aromatic amines of pharmaceutical relevance can be obtained via the reduction of MNBs using gold-based catalysts.