Supported gold nanoparticles as promising catalysts in sustainable organic synthesis

Abstract

Gold nanoparticles (AuNPs) emerged as very promising catalysts in a lot of organic reactions and industrial applications. These catalysts are in fact capable of catalysing chemical processes under mild conditions and according to the principles of Green Chemistry. Considering these peculiar features, during the years, scientific research has been addressed toward the design of more and more performing heterogeneous gold catalysts. To this aim, very relevant is the role of the support; as a matter of fact this allows preventing the sintering of the AuNPs but can also affect the catalyst performances by e.g. tuning the selectivity of the reaction. Among the heterogeneous gold catalysts, AuNPs embedded in a porous crystalline polymer matrix consisting of polystyrene‐co‐cis‐1,4‐polybutadiene (sPSB) appeared of relevant research interest because of the peculiar reactivity patterns observed in a number of organic reactions, such as the selective aerobic oxidation of alcohols, nitroarenes reduction and aerobic oxidative esterification of cinnamyl alcohol. This PhD thesis has been mainly addressed to the applications of the AuNPs‐sPSB catalyst in Green Chemistry to assess the potential of this gold catalyst in organic reactions of interest in sustainable catalysis, such as the intermolecular and intramolecular hydroamination of arylacetylenes with anilines, and selective oxidation and oxidative esterification of HMF. The intermolecular hydroamination of anilines with phenylacetylenes allowed the synthesis of aromatic ketimines in high yields and with excellent regio‐ and stereo‐selectivity; the AuNPs‐sPSB catalyst appeared thermally robust and recyclable. The kinetic investigation of hydroamination reaction, supported by DFT calculations, highlighted a new reaction pathway where the nucleophilic attack of the aniline occurs at the coordinated/activated phenylacetylene onto gold surface and is assisted by the formation of aniline aggregates stabilized via hydrogen bondings. Cascade reactions are hot topic in Green Chemistry. In situ intramolecular hydroamination of 2‐(2‐ propynyl)anilines followed by asymmetric transfer hydrogenation with the Hantzsch ester to produce chiral tetrahydroquinolines were successfully investigated using the AuNPs‐TiO2 catalyst; excellent regio‐ and enantioselectivity toward chiral tetrahydroquinolines were achieved under mild reaction conditions. The introduction of an electron‐donating group (EDG = OMe) on the aromatic ring of the phenylacetylene moiety allowed an excellent control of the regioselectivity of the nucleophilic attack of the nitrogen atom to the ‐ system of the alkyne. The selective oxidation and oxidative esterification of HMF were tuned by means of a fine control of the reaction parameters (crystalline phase of the support, temperature, solvent, oxygen pressure); the HMF conversion into the desired products, namely 2,5‐diformylfuran (DFF), 5‐formyl‐2‐furancarboxylic acid (FFCA), 5‐hydroxymethyl methylfuroate (HMMF), furan‐2,5‐dimethylcarboxylate (FDMC) and 2,5‐ furandicarboxylic acid (FDCA) was obtained in good yields with high selectivity under moderate experimental conditions. [edited by Author]