Probing Halogen Rebound in Manganese-Catalyzed C-H Activation

Abstract

The direct halogenation of unactivated aliphatic C-H bonds allows for the efficient preparation of medicinally important organohalide compounds. Manganese porphyrin and salen catalysts are capable of catalyzing chlorination, fluorination and bromination reactions; however, the mechanism by which these reactions occur is not fully understood. When radical probes were used to investigate the effect of trans-axial ligation on manganese complexes, it was discovered that trans-axial ligands are able to drastically increase the measured radical lifetime values and change the product distribution of a reaction. Additionally, these additives were found to affect the reactions in widely different ways, especially chloride. The effect of chloride compared to fluoride as a trans-axial ligand was investigated using radical trapping experiments. Additionally, the changes in coordination about the metal center due to trans-axial ligation, particularly the possibility of coordination of two different ligands on the same metal center, were investigated using UV-vis and XAS. The insight gained from the studies of trans-axial ligation on the manganese catalysts in this thesis will hopefully allow researchers to better tailor synthetic catalysts to perform selective halogenations in the future.