Metal-assisted reactions, Part 31 [1]: Adsorption isotherms and selective catalytic transfer reduction of aldehydes and ketones

Heterogeneous catalysis of transfer reduction was assessed by measurement of liquid-phase adsorption isotherms, enabling quantitative evaluation of H-donor, H-acceptor and solvent interplay at the catalyst surface so as to find the optimum reaction conditions. Liquid-phase isotherms were measured for adsorption of aromatic aldehydes (H-acceptors) onto a commercial Pd/C catalyst from a range of solvents. The resulting isotherms provided fundamental information on the competitive adsorption of aldehyde and solvent molecules at sites on the catalyst surface. These results suggested a novel solvent system for the heterogeneously catalysed transfer reduction of aromatic aldehydes to alcohols. This new solvent system performs heterogeneously catalysed transfer reduction of aromatic aldehydes at room temperature in a highly selective way, thus, avoiding the previously proposed unselective processes requiring high temperatures. The heterogeneous catalyst was re-usable at least five times without any loss of activity. A major role of oxygen as a reverting inhibitor of transfer reduction was demonstrated.

Carbonyl group in aromatic aldehydes and ketones was reduced using Pd/C as a catalyst and sodium phosphinate as a hydrogen donor in THF/water solvent system (see Scheme 1). The adsorption isotherms measured provide fundamental information on the competitive adsorption of aldehydes and solvent molecules at sites on the catalyst surface. These results allowed discovering an excellent solvent system for these reductions. Figure optionsDownload as PowerPoint slide