46754-GB3
Bowl-shaped Ligands for Secondary-sphere Control of Selectivity in Homogeneous Catalysis
Anthony R. Chianese, Colgate University
The goal of the proposed project is to design and synthesize late transition metal catalysts that exhibit unique selectivity through control of the secondary coordination sphere. Catalysts supported by phosphine, bipyridine, and N-heterocyclic carbene (NHC) ligands were originally targeted. A convergent synthesis based on common intermediates was developed, and series of bowl-shaped bipyridine and N-heterocyclic carbene ligands were synthesized, with bowl depth varying by the level of extension of meta-phenylene-linked dendritic side groups.
For the bipyridine-based ligands, cationic palladium-allyl complexes were synthesized, and were shown to be active catalysts for allylic substitution reactions, using nucleophiles such as sodium dimethylmalonate and benzylamine. These catalysts were sufficiently active, but in situ monitoring of the reaction mixtures indicated that the bipyridine ligands were displaced early in the course of the reaction: free ligand was observed by NMR spectroscopy. Since the active catalyst was not bound to our designed ligands in this case, we have temporarily put the study of bipyridine-based bowl-shaped ligands on hold.
N-heterocyclic carbenes are known for binding strongly to transition metal ions, and we expected that ligand dissociation during catalytic reactions would be less of a concern. Three NHC ligands of varying size were synthesized, and were metalated to produce complexes of the formula IrCl(cod)(NHC), where cod = 1,5-cyclooctadiene. The complexes are all active for the hydrosilylation of aryl methyl ketones with diphenylsilane; full conversion is reached in less than a day at room temperature. Competition experiments were performed to determine if catalysts derived from the larger bowl-shaped NHCs exhibited specificity toward smaller ketone substrates. The hypothesis was confirmed, though modestly: using the largest ligand, acetophenone is consumed 3.7 times more quickly than 3-(3,5-di-tert-butylphenyl)acetophenone. A smaller control ligand gave a ratio of 1.8 for this competition experiment. A report based on this work was submitted to the ACS journal Organometallics and was recently accepted for publication, with two Colgate undergraduate students as coauthors.
These initial results are encouraging, and efforts during the remainder of the grant period will be directed at improving the current system and discovering new ones. We expect that greater substrate specificity can be obtained using the iridium-NHC system if the ligands are more structurally rigid; synthetic efforts targeting ligands that address this concern are underway. We are also adjusting the original ligand synthetic scheme to increase convergence, and modifying it to develop a series of bowl-shaped NHC-pyridine bidentate ligands to envelop a greater portion of the space surrounding the catalytically active metal center.
