Research

Our group is broadly interested in exploiting structure/function relationships of transition metal complexes to attain useful properties or reactivity. Our main approach is the design of new ligands that can be easily synthesized and readily modified to tune the steric, electronic, and asymmetric properties of their metal complexes. Current projects encompass new late metal catalysts for enantioselective organic reactions, early metal polymerization catalysts, and luminescent solid-state materials with potential sensing applications.

Our primary focus in recent years has been the design of new homogeneous catalysts containing modular, tunable chelating carbene ligands. N-heterocyclic carbene (NHC) ligands, most of them derived from imidazole (e.g. 1), have come to define the cutting edge of ligand design for catalysis in recent years due to their favorable combination of strong binding ability and robustness. However, NHCs still lag behind more established ligands such as phosphines in terms of the range of donor properties and chiral structures, among other features, that are accessible.

We have been exploring the synthesis of structurally diverse new chelating bis(carbene) ligands using a long-known reaction, the addition of amines or other nucleophiles to coordinated isocyanides. The resulting complexes contain chelating acyclic diaminocarbene (ADC) ligands, a ligand class that has received little attention. We have found that palladium bis(ADC) complexes like 3 derived from bulky isocyanides (RNC, R=tBu, iPr, Cy) are quite robust and can show comparable activity in Suzuki cross-coupling reactions to catalysts with the analogous bidentate imidazole-derived carbene ligands (e.g. 2, above). A small “library” of these catalysts has been synthesized by a general, two-step procedure and screened to identify optimal ligand substituents for catalysis.

Extending this strategy to chiral diamine precursors, we have reported one-step syntheses of palladium complexes containing the first chiral ADC ligands (4 and 5, below). Evidence suggests that the donor properties of the new bis(ADC) ligands are surprisingly weak compared with common bis(NHCs) and bis(phosphines), likely due to a combination of electronic effects and chelate ring size. This has allowed the use of these chiral bis(ADC) ligands in catalytic enantioselective aza-Claisen rearrangements, synthetically useful reactions that require an electrophilic metal catalyst. Variations of these ligands continue to take us into “new territory” for carbene-containing metal catalysts.