Graphene-Based Materials in Metal-, Carbo- & Organocatalysis

Resumen

Carbon is a very attractive material in sustainable chemistry, especially as support of metal catalysts. However, amorphous carbon materials suffer from different disadvantages, such as low stability and low oxidation resistance. During the last three decades, a large variety of nanocarbon materials have been developed, such as activated carbon, fullerenes, carbon nanotubes (CNTs), carbon nanofibers (CNFs) and graphene-derived materials. Graphene, a bidimensional material formed by a hexagonal monolayer network of sp2 hybridized carbon atoms, is 200 times harder than steel, very feathery and flexible. In addition, G offers the highest intrinsic carrier mobility at room temperature with a perfect atomic lattice and excellent thermal, electrical, mechanical and optical properties. Graphene was rediscovered, isolated and characterized in 2004 by Andre Geim and Konstantin Novoselov at the University of Manchester. In 2010 both professors were awarded with the Nobel Prize in Physics "for groundbreaking experiments regarding the two-dimensional material graphene”. G-mats have the highest surface area (2630 m2/g) in comparison to the rest of the nanostructured carbonaceous materials (100 to 1000 m2/g). Additionally, for example in the case of graphene oxide (GO), the high degree of oxygen functional groups present on the structure allows an easy covalent and non-covalent (hydrogen-bonding or π-stacking) functionalization of the material. These features make G-mats ideal carbonaceous supports for metal complexes, metallic nanoparticles and even chiral organocatalysts opening the door to the creation of new sustainable heterogeneous catalytic systems. Due to the previous background, was stablished as aim of the present doctoral thesis, the application of Graphene-Derived Materials in different fields such as metal-, carbo- and organocatalysis. Concerning metal catalysis, the Suzuki-Miyaura (S-M) reaction between aryl halides and potassium aryl trifluoroborates in aqueous conditions using palladium nanoparticles supported on graphene was performed giving moderate to very good results using 0.1 mol% of Pd in MeOH/H2O as solvent at 80 °C under microwaves (MW) or conventional heating conditions. The recovery of the catalyst was studied, observing good recyclability until the 8th reaction cycle under MW irradiation and until the 5th cycle under conventional heating. In this field and due to the experience in our research group in the use of oxime-derived palladacycles as efficient precatalysts in cross-coupling reactions, we decided to carry out the non-covalent support of different oxime palladacycles on graphene oxide. These new materials demonstrated to be efficient catalyst for the S-M coupling between aryl bromides and arylboronic acids using very low loadings (0.002 mol% of Pd) at room temperature under aqueous conditions. Regarding the recovery of the catalyst, it was possible to recover until the 3rd cycle due to the formation of some palladium oxide specie (confirmed by XPS) and the agglomeration of palladium nanoparticles determined by TEM. Regarding carbocatalysis, graphene-derived materials, due to the acidic nature of this material owing to the presence of oxygen functional groups such as hydroxyl, epoxides, carboxylic and sulfonated groups, among others, have been recently described as efficient promoter of several organic transformations. In this purpose, we decided to use graphene oxide to catalyzed the pinacol rearrangement of 1,2-diols (20 wt% of GO, neat/solvent, 100 °C, 20 h) and GO functionalized with acid groups (GO-COOH) as carbocatalyst in the direct nucleophilic substitution of allylic alcohols (20 wt% of GO-COOH, toluene or H2O, 80 °C, 20 h) obtaining from moderate or excellent results in both reactions. In the case of the allylic substitution, the carbocatalyst was efficiently reused until the 5th reaction cycle without a significant decrease in the catalytic activity. Finally, due to the experience in our group on the use of chiral 2-aminobenzimidazole derivatives as organocatalysts in the alpha functionalization of 1,3-dicarbonyl compounds, we studied the alpha-amination of such compounds under homogeneous (1 mol% of organocatalyst, Et2O, 10 h at room temperature) and heterogeneous conditions giving in the former case, moderate to good enantioselectivities. Currently, the synthesis of the non-covalently and covalently anchored chiral organocatalysts on graphene oxide and reduced graphene oxide was under study in order to test the reactivity, selectivity, and recovery of such catalysts in the alpha amination of 1,3-dicarbonyl compounds under heterogeneous conditions.