Internal vs. external ionic functionality - A comparative study in the asymmetric hydrogenation in water as solvent

Abstract

Coupling of the benzoylsulfonate moiety to chiral hydroxy phosphines by acylation with o-sulfobenzoic anhydride (SBA) affords unique ligands bearing one or two sulfonate groups in distinguished positions in the molecular framework. Rhodium complexes based on the new ligands have been proven in the asymmetric hydrogenation of functionalized chelating olefins in methanol and in water. Results observed are compared to those featured by the corresponding non-sulfonated catalysts. In methanol as solvent only one of the complexes bearing a sulfonate group in a flexible ligand differed significantly from its parent complex, while in water all sulfonated complexes were superior. In the most cases, addition of the amphiphile sodium dodecylsulfate (SDS) improved the catalytic performance of the parent catalysts as well as that of the sulfonated complexes in water. Other ionic additives (e.g., Na2SO4, camphersulfonate, benzenesulfonate) which do not bear the long alkyl chain gave poor results illustrating the importance of the particular structure of SDS.