(478h) Modified Catalytic Pathways In Supported Ionic Liquids

Ionic liquids are a novel class of chemicals with unique solvent properties due to their high dielectric constant. Immobilizing metal complexes in thin films of ionic liquids on silica supports is a novel concept for catalyst preparation combining the selectivity of homogeneous catalysts with the possibility of facile separation of heterogeneous catalysts.

The interaction of the immobilized complex [Pd(DPPF)(CF₃CO₂)₂] with a thin film of the ionic liquid 1-ethyl-3-methyl-imidazolium chloride (EMImCl) supported on SiO₂ was investigated by ¹H MAS NMR spectroscopy. It was shown that dissolution of the complex in the ionic liquid phase leads to the formation of structured solvent cages. The reason for this phenomenon is that the introduction of a complex into the liquid phase leads to disruption of the inter-ionic interactions. In an attempt to minimize the potential energy, the coordination sphere around the complex assumes a minimum size.

The effect of the solvent cages on the catalytic properties of the material was tested for the hydroamination of styrene with aniline, which yields N-(1-phenylethyl)aniline as Markownikoff product and N-(2-phenylethyl)aniline as anti-Markownikoff product. It was shown that the chemoselectivity of this system can be tuned over a wide range. The Markownikoff product was obtained under kinetic control, while the anti-Markownikoff product was formed preferentially under thermodynamic control.

In another series of experiments, acetophenone was selectively hydrogenated to (S)-1-phenylethanol using [Rh((S)-BINAP)(COD)]ClO₄^.thf / K₂CO₃ immobilized in a thin film of a phosphonium based ionic liquid on silica as catalyst. An enantiomeric excess of up to 74 % was achieved, while only 4 % ee were reported for the same catalyst-substrate pair in methanol. The improved enantioselectivity is attributed to enhancement of the substrate-catalyst interactions, which is caused by the solvent cages.