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Synthetic Organic Chemistry

4. Homogeneous catalysis

Our work in this area, particularly in the field of organocatalysis, has led to a number of highly promising results.

In the field of homogeneous catalysis a bis 2-(2-(2-methoxyethoxy)ethoxy)ethyl functionalized Bicol-derived monodentate phosphoramidite ligand called PegPhos was prepared that gave ee’s up to 89% in the enantioselective Rh-catalysed hydrogenation of N-acyl dehydroalanine using water as the solvent (S. Leleu, NRSC-C-supported, collaboration Minnaard/Feringa, RUG).

Recently, a novel P,N-ligand class (ClickPhine) is disclosed in which the coordinating nitrogen atom is part of a triazole moiety made by “click” chemistry (R. Detz, NRSC-C-supported, collaboration Reek, UvA).

Our work in the area of organocatalysis has led to a number of highly promising results. Biaryl systems were applied as scaffolds for enantiopure protic acids to be used as catalyst in enantioselective iminium ion reactions (M. Wanner). Indole reacted with the readily accessible sulfenylimine to give an (S)-indolylglycine derivative in ca. 75% enantiomeric excess, when a binaphthylphosphoric acid (see picture) was used as enantiopure catalyst (1 mol%).

In other organocatalytic research the natural cinchona alkaloids were modified in order to develop novel catalysts for enantioselective organic reactions. Thus, a quinidine-derived thiourea appeared to be an excellent enantioselective catalyst in a Henry reaction, that is the addition of nitromethane to benzaldehyde, providing the product in over 90% ee. This result embellished the cover of the 6/2006 Angew. Chem Int. Ed. issue. The catalytic mechanism is of the bifunctional type, involving the activation of nitromethane by the basic quinuclidine nitrogen and the activation of benzaldehyde via hydrogen bonding by the thiourea (T. Marcelli, R. van der Haas, NRSC-C-supported).