Ligand Design, Synthesis, and Reactivity of Chiral and Achiral tetra-NHC Macrocycle Complexes for Oxidative Transfer Catalysis

The exploration of catalytic methods that utilize lower toxicity metals, more benign reagents, and perform under mild conditions, is of critical importance. N-heterocyclic carbene (NHC) macrocycles have been constructive in advancing these investigations, especially in oxidative transfer catalysis. These multidentate, strong donor macrocycles stabilize difficult to access oxidation states for 1^st row transition metals and allow the use of more benign oxidants in important catalytic processes.

This dissertation discusses research focused on ligand design, synthesis, and reactivity of chiral and achiral macrocyclic tetracarbene complexes for oxidative transfer catalysis. A neutral D₂-symmetric tetra-N-heterocyclic carbene macrocycle was synthesized. The macrocycle was ligated to iron (II) via transmetalation from an isolated silver dimeric complex. This C₂-symmetric iron complex catalyzed the first stereospecific aziridination between aryl azides and aliphatic alkenes. The same complex catalyzed the first asymmetric epoxidation of unactivated aliphatic alkenes on iron using hydrogen peroxide. The catalyst gave high yields of epoxides and up to moderate enantiomeric excesses. A potential iron-oxygen intermediate was isolated as a product of the reaction between a C₂-symmetric iron (II) complex and various oxidants. Further exploration in transmetalation using a silver dimer for the synthesis of both a C₂-symmetric chromium (III) complex and a C₂-symmetric cobalt (II) complex is also discussed. Finally, a 16-atom di-anionic tetra-NHC macrocycle with a precedent for novel reactivity on iron and chromium was ligated to vanadium, manganese, and cobalt. Characterization of each of these complexes is presented.