Microfluidic chip reactor and the stereoselective hydrogenation of methylacetoacetate over (R)-Ru-BINAP in the [N8222][Tf2N]/methanol/water mixed phase

Abstract

Homogeneous asymmetric catalysts represented by organometallics of transition elements reveal very high activity and stereoselectivity in asymmetric hydrogenation of β-ketoesters. Recently, it appeared efficient to use ionic liquids of the [NR222][Tf2N] type for accommodation of the chiral catalytic complex. Even on the industrial scale it is becoming attractive to carry out hydrogenations with such immobilised complexes in a continuous regime in microreactors. Attention was paid to the stereoselective hydrogenation of methylacetoacetate (MAA) to isomeric (R)−(+) or (S)−(−)-methyl-3-hydroxybutanoates over a chiral ([RuCl((R)-BINAP)(p-cymene)]Cl) complex immobilised in the mixed [N8222][Tf2N]/methanol/water solvent phase, and performed inside of a microfluidic chip reactor. It was shown that the reaction could be successfully carried out in such an arrangement with a very high enantioselectivity (above 99%) and at very high conversion of MAA (above 97%). It was proven that the participation of a specific solvent (methanol or the mixture [N8222][Tf2N]/methanol/water) essentially influences the mechanism of the reaction. The solvents actively participate in the re-coordination of the catalytic complex in a series of reactions which is reflected in the values of reaction enthalpies. The specific process output was also evaluated.