Fine‐Tuning Ligands for Catalysis Using Supramolecular Strategies

Abstract

AbstractCoordinative bonds have been used to prepare supramolecular ligands leading to well‐defined catalysts formed by assembly. The construction of these ligands is based on selective metal–ligand interactions between nitrogen donor atoms of phosphorus‐nitrogen building blocks and various zinc(II) porphyrins. The major advantage of this supramolecular approach of catalyst preparation is the simplification of ligand variation enabling straightforward modification of steric, electronic and chiral properties of the supramolecular ligand. A large number of new ligands becomes accessible by this modular variation of the building blocks. The ligand assembly based on pyridyl phosphites and zinc(II) porphyrin with electron‐withdrawing substituents led to a twelve‐fold increase in activity and an increase in enantioselectivity from 17 to 50 % in the rhodium‐catalyzed hydrogenation of dimethyl itaconate. The first examples of assemblies based on non‐chiral ligands and chiral zinc(II) porphyrin template molecules show, as proof of principle, an enantiomeric excess up to 18 % in the asymmetric palladium‐catalyzed allylic alkylation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)