J.N.H. Reek

Supraphos: A Supramolecular Strategy To Prepare Bidentate Ligands

We report a new strategy for the preparation of chelating bidentate ligands, which involves just the mixing of two monodentate ligands functionalized with complementary binding sites. In the current example, the assembly process is based on selective metal-ligand interactions, using phosphite zinc(II) porphyrins 1-6 and the nitrogen donor ligands b-i. From only 16 monodentate ligands, a library of 60 palladium catalysts based on 48 bidentate ligand assemblies has been prepared. The relatively small catalyst library gave a large variety in the selectivity of the alkylation of rac-1,3-diphenyl-2-propenyl acetate. Importantly, small variations in the building blocks lead to large differences in the enantioselectivity imposed by the catalyst (up to 97% ee).

The bite angle makes the catalyst

Catalytic reactions are described for metal complexes containing bidentate phos- phine ligands that enforce wide bite angles in the complexes. The calculated natural bite angles are in the range 100-1208. Three applications will be discussed: (i) nickel catalysed hydro- cyanation, for which the first active phosphine catalyst was found, (ii) palladium catalysed allylic alkylation, for which the selectivity also strongly depends on the bite angle, and (iii) rhodium catalysed hydroformylation, which leads to highly linear products.

Diphosphines: Xantphos Ligands in Transition Metal Complexes and Catalysis.

Homogeneous catalysts can decompose in a variety of ways: metal deposition, ligand decomposition, reaction with impurities, dimer formation, and reaction of the metal center with the ligand. Sometimes these side reactions lead to temporary deactivation only and the catalyst activity can be restored. Without attempting to be complete, I have collected a range of examples of catalyst decomposition. Our examples will concentrate on homogeneous reactions of commercial interest, such as polymerization, oxidation, oligomerization, hydroformylation, carbonylation, hydrocyanation, and cross-coupling reactions.