Supramolecular approaches to Homogeneous catalysis

Abstract

The development of homogeneous systems capable to mimic the enzymes activity is a fundamental topic for catalysis. For that purpose, in this thesis we reported the employment of supramolecular resorcin[4]arene capsule in organic solvents and micelles in water. The resorcin[4]arene capsule is a spherical hexamer. The high number of electron-rich aromatic rings on the surface of the capsule cause cation-π interactions that confer to the cavity the possibility to accommodate positively charged compounds. We demonstrated that the capsule is also capable to host formally neutral compounds with partial carbene-like electron poor character. In the presence of isocyanides, the empty capsule favoured the nucleophilic attack of either water leading to the formation of the corresponding N-formylamide or trimethylsilylazide leading to the corresponding 1H-tetrazole via the [3+2] cycloaddition. Similarly, in the presence of diazoacetate esters the cavity showed an interesting catalytic activity promoting the 1,3-dipolar cycloaddition with electron-poor alkenes forming N containing heterocyclic compounds. The interaction of dipolar compounds with capsule was investigated as well using N-oxides as guests. The strong H-bonding acceptor character of N-oxides caused a strong interaction with the hydroxyl groups of the resorcin[4]arene moieties that completely changed the reactivity of the guest with electron-rich aryl isocyanides. The formation of the corresponding diazo-compound as main product in the absence of the capsule moved to the urea derivative with supramolecular host in the reaction solution. The cation-π interactions within the cavity was exploited to catalyse certain reactions known to involve the formation of cationic intermediates, even though the substrates were not suitable gusts for the cavity. This was the case of the sulfoxidation reaction of thioethers with H2O2 as oxidant and the acid catalysed hydration of alkynes. In both cases the stabilization of the intermediate cationic species within the cavity turned out to be pivotal for the catalytic effect observed. Micelles are obtained from surfactants that self-assemble in water developing apolar nano-environment in polar media. Micelles made of commercial anionic sulfonated surfactants were employed to stabilize Pd-nanoparticles catalytic systems through a facile, economical and eco-friendly procedure. The achieved catalytic activity showed modulable depending on the surfactant employed, proving effective in hydrogenation, hydro-dehydration and hydrodechlorination reactions in water. An intensive study was realized in order to find a suitable micellar medium capable to promote the multicomponent synthesis of triazoles from organic bromides, sodium azide and alkynes in a single step reaction. The direct synthesis of triazoles was reached avoiding the employment of the preliminary preparation of dangerous organic azides. Finally, both supramolecular systems based on micelles in water and resorcin[4]arene in organic solvents were employed to achieve substrate selectivity. Micellar media allowed the preferential conversion of more hydrophobic substrate during competitive Heck and hydrogenation reactions. Conversely, the confined space within the cavity of the resorcin[4]arene capsule was employed to reach the preferential conversion of smaller substrates in the competitive condensation reaction between series of acids and amines with different lengths.