Síntese de nanopartículas de ouro e aplicação como catalisadores em reações orgânicas

Abstract

Gold nanoparticles (AuNPs) stabilized with different organic compounds have behaved as excellent catalysts for organic reactions such as oxidation. Thus, it is interesting to study the preparation and characterization of different AuNPs and evaluate their reactivity towards the catalysis of oxidation reactions. In this work, gold nanoparticles stabilized with the aminoacid N-acetyl-L-cysteine (NAC-AuNPs) were synthesized and their catalytic activity towards aromatic aldehydes and amines was investigated. NAC-AuNPs were characterized through ultraviolet and visible absorption spectroscopy UV-VIS, Raman scattering spectroscopy and Surface Enhanced Raman Scattering (SERS), zeta potential measurement through Dynamic Light Scattering (DLS) and scanning electron microscopy (SEM). In the present work, NAC-AuNPs were satisfactorily obtained with dimensions around 25 nm and firstly promoted the oxidation of benzaldehyde under mild conditions (dichloromethane as solvent, ambient temperature, 48 hours of reaction under stirring) to benzoic acid with a conversion of 97%. Subsequently, the attempt to obtain an amide by adding octylamine and 1-Ethyl-3- (3-dimethylaminopropyl) carbodiimide hydro (EDC, carboxylic acid activator) to the same reaction system under stirring for 24 hours (one-pot amidation) led amazingly to the formation of an imine with excellent conversion and yield (> 99% and 98% respectively). Thereafter, control experiments concluded that the presence of EDC was dispensable and that the presence of NAC-AuNPs was of extreme importance for the formation of an imine from the benzaldehyde having a carboxylic acid as intermediate. A new optimization containing the octylamine in the second stage of the reaction was executed in order to obtain imines in the new working system, and the results showed that DCM and NAC-AuNPs at 3.6 x 10-3% m / V were presented as the optimum reaction condition (conversion of > 99%). The substrate scope was evaluated under the optimum conditions set forth above with different aromatic aldehydes and aromatic and aliphatic amines, in which imines could be obtained in good to excellent yields (67% to 99%). All products were characterized by 1H and 13C NMR; products that were obtained as powders could also be characterized by powder X-ray diffraction (XRD).