Encapsulated transition metal catalysts are presented that are formed by templated self-assembly processes of simple building blocks such as porphyrins and pyridylphosphine and phosphite ligands, using selective metal-ligand interactions. These ligand assemblies coordinate to transition metals, leading to a new class of transition metal catalysts. The assembled catalyst systems were characterized using NMR and UV-vis spectroscopy and were identified under catalytic conditions using high-pressure infrared spectroscopy. Tris-3-pyridylphosphine binds three mesophenyl zinc(II) porphyrin units and consequently forms an assembly with the phosphorus donor atom completely encapsulated. The encapsulated phosphines lead exclusively to monoligated transition metal complexes, and in the rhodium-catalyzed hydroformylation of 1-octene the encapsulation of the catalysts resulted in a 10-fold increase in activity. In addition, the branched aldehyde was formed preferentially (l/b = 0.6), a selectivity that is highly unusual for this substrate, which is attributed to the encapsulation of the transition metal catalysts. An encapsulated rhodium catalyst based on ruthenium(II) porphyrins and tris-meta-pyridyl phosphine resulted in an even larger selectivity for the branched product (l/b = 0.4). These encapsulated catalysts can be prepared easily, and various template ligands and porphyrins, such as tris-3-pyridyl phosphite and ruthenium(II) porphyrins, have been explored, leading to catalysts with different properties.

Abstract:The use of cross-coupling reactions for the preparation of alkylated and arylated heteroaromatic compounds has increased tremendously over the past two decades. This has been driven on the one hand by the increasingly complex structures of new drugs, most of which contain one or more heterocyclic motifs. On the other hand, the development of new catalysts and reaction conditions for these reactions has rendered even the most unreactive of heteroarenes amenable to cross-coupling chemistry. Not only have new bulky electron-donating ligands been created that allow the coupling of aryl chlorides under mild conditions, but also the use of ligand-free palladium, in particular at very low doses, sometimes called homeopathic palladium, has served to bring down the cost of these reactions. More recent and enabling developments are the use of catalysts based on cheap metals such as nickel, copper, and iron. Scale-up issues are availability and cost of starting materials, cost of the catalysts (related to cost of the metal and the ligand, intrinsic activity and stability of the catalyst), solvent choice, and removal of the metal to <10 ppm from the final product. This latter point is aggravated with heteroaromatics as they tend to be good ligands for the transition metal. For the same reason substrate and product inhibition are quite common.

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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).

Selectivity from a rigid sandwich‐type complex: A supramolecular approach was used to prepare chelating ligands that give high selectivity for the linear aldehyde in the rhodium‐catalyzed hydroformylation of 1‐octene. The multicomponent assembly consists of two tris(zinc(II) porphyrin)phosphite ligands (blue) and three ditopic templates (red) with the rhodium complex (green) in the middle of the structure.