Dr. David Jenkins from the University of Tennessee will present “Developing small-ringed macrocyclic tetracarbene ligands for catalytic systems.”
Despite the numerous advances in homogeneous catalysis that have been achieved through the use of N-heterocyclic carbenes (NHCs) as auxiliary ligands, very few macrocyclic polydentate carbenes have been synthesized. Our group has synthesized a new ligand platform based on small-ringed (18 atoms) macrocyclic tetraimidazoliums as tetra-NHC precursors. These small-ringed tetracarbene macrocycles form novel and reactive complexes on group 5, 6, 7, 8, 9, 10, and 11 metals. Numerous novel tetracarbene complexes have been characterized by spectroscopic methods and X-ray crystallography, which demonstrates that the four carbenes bind to the metal in an equatorial configuration. Our group is currently pursuing catalytic oxidative group transfer reactions with group 8 and 9 metal complexes. One challenging catalytic reaction under investigation is the aziridination of unactivated aliphatic alkenes with simple organic azides. Since the aziridine functional group is found in natural products and pharmaceuticals, broadening the scope of the reaction is significant. The catalyst reacts with aryl azides and a wide variety of substituted aliphatic alkenes, including tetrasubstituted ones, to give aziridines in a C2 + N1 addition reaction. Furthermore, this novel iron catalyst can be recovered and reused up to three additional times without significant reduction in yield.