Amines are paramount in industries such as pharmaceuticals, agrochemicals, and biomolecules. Reductive aminations yield substituted amines, but they rely on 1) carbonyl precursors (unstable and not largely available), and 2) molecular hydrogen for imine reduction. Alcohol amination is an alternative in which alcohols are used as precursors. It makes the reaction more sustainable because alcohols 1) are stable, available in bulk, and can come from renewable sources; and 2) provide hydrogen atoms to reduce the imine. This hydrogen transfer process scores high in atom economy, ideally forming only the desired amine and water. However, the presence of the corresponding imine in the final product is commonly observed in the literature. While highly efficient reaction conditions have been reported before, they usually rely on carefully crafted catalysts to consume the borrowed hydrogens at the same rate they are generated to avoid loss. We turned to confinement effects as a means to make this catalysis more resilient to reaction conditions. Catalysis in confined nanospaces is known to change diffusion rates and create local concentrations that can change reactivity. Our goal is to design yolk-shell nanosystems made of Ru(OH)x/Al2O3 yolks in mesoporous silica shells. We hypothesize that confining the reactants and catalysts inside silica shells will reduce the diffusion of hydrogen atoms liberated from the alcohol. The high hydrogen concentration will then favour the reduction of the imine, which will also be formed and concentrated inside the nanoreactor. The nuclei of our architecture were made by stirring Al2O3 NPs in an aqueous solution of RuCl3 at pH 13.2. Preliminary catalysis results for the unconfined reaction between aniline and benzyl alcohol give 54% yield of amine and 26% of imine. We synthesized the yolk-shell structures by using PVP as a sacrificial template. Silica shells were grown on top of the template and etched with NaOH, which also consumes the PVP. We are now testing the confined system, and we will study the influence of yolk-to-shell distance and pore size in reactivity.

Trapping hydrogens: Confined catalysis for improved alcohol amination selectivity

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ACS Fall 2022

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