Interactions of the (R) Ru-BINAP catalytic complex with an inorganic matrix in stereoselective hydrogenation of methylacetoacetate: kinetic, XPS and DRIFT studies

Abstract

Stereoselective hydrogenation of methyl acetoacetate to optical isomers of methyl-3-hydroxybutanoate over various forms of supported [RuCl((R)-BINAP)(p-cymene)]Cl is reported. The catalysts were prepared via two alternative methods: direct immobilization, and surface anchorage with heteropolyacids. The kinetic data (rate constants, optical yields, overall selectivity to methyl-3-hydroxybutanoate) were discussed and interpreted with help of XPS and DRIFT to assess bonding surface interactions. In all cases montmorillonite as a model support was employed. It was shown that the stereoselective course was dependent on the mode of [RuCl((R)-BINAP)(p-cymene)]Cl immobilization. The procedure based on the commonly used impregnation did not provide a catalyst with a stable anchorage of the active complex. Strong surface anchorage was found for a catalystproduced with help of H₄SiMo₁₂O₄₀ heteropolyacid, but due to structural changes upon the reaction the catalyst performance in the reaction was poor. When H₃PW₁₂O₄₀ was employed, the immobilization provided a catalyst with a very good performance. The two-step immobilization via the dimer [RuCl₂(p-cymene)]₂ instead of the one-step with [RuCl((R)-BINAP)(p-cymene)]Cl yielded a catalyst with the performance parameters comparable to the homogeneous experiments with [RuCl((R)-BINAP)(p-cymene)]Cl dissolved in the reaction mixture. It was due to breaking up the original dimer structure and accomplishing the surface interaction through the adsorbed monomeric units.