Abstract
Herein, we present a green system fulfilling several of the scope of green chemistry principles: the reactions are performed using heterogeneous catalysts, green solvents, under air and with mild reaction conditions. Water was used as the primary solvent in all synthesis methods. The aim was to promote safer and cheaper synthesis with faster reactions and easier work-up. A cellulose derivative was used to eliminate solubility problems of various reagents in water. The heterogeneous catalytic systems were palladium nanoparticles supported on mesoporous metal oxides that were synthesized in-house. A sol-gel technique was used to synthesize the various solid supports and mixed metal oxides. Prior to catalytic evaluations, properties of the synthesized heterogeneous catalysts such as morphology, surface area, crystallinity and thermal stability were monitored with the characterization techniques (SEM, TEM, BET, p-XRD and TGA, respectively).
Suzuki-Miyaura cross-coupling reactions were performed using water as a solvent and palladium nanoparticles supported on iron oxide. The reactions achieved high yields of the bi-aryl products. Compared with various solvents such as DMSO, DMF, DCM and THF, the HPMC/H2O system provided faster and safer reactions higher yields. The Pd-Fe2O3 catalyst was easily recoverable and recyclable more than five times without losing much activity.
Palladium nanoparticles (approx. 1%) loaded into several first-row transition metal solid supports (Fe2O3, NiO, CeO2, Co3O4, Mn3O4 and TiO2) were evaluated for potential application in Sonogashira and Buchwald Hartwig cross-coupling reactions. It was discovered that the Pd-Co3O4 was the best catalyst for Sonogashira coupling, while the Pd-CeO2 was the best for Buchwald Hartwig coupling reactions. These cross-coupling reactions are traditionally performed in the presence of ligand complexes which are expensive, complex, and not easily recoverable. However, supported palladium nanoparticles successfully the traditional palladium-ligand systems for these cross-coupling reactions in this study. Also, the reactions were quite successful, with high yields and selectivity.
Mesoporous zirconia and silica metal oxides were doped with MoO3 and WO3 (approx. 5%) to produce mixed metal oxides. These Lewis acid catalysts were applied to acetylation reactions of amines, anilines and alcohols using water as a solvent. The reaction conditions resulted in short reaction times, reusability of catalysts, and moderate to excellent yields of the desired products. Keywords
Heterogeneous catalysis; organic synthesis; green chemistry; hydroxypropyl methylcellulose; palladium nanoparticles; mesoporous metal oxides; soft templating method; Suzuki-Miyaura coupling; Buchwald-Hartwig amination, Sonogashira coupling; acetylation; alcohols; amines; anilines.