Development of a Methodology for the Direct Activation of Alcohols Using Cobalt Photocatalysis

The utilization of light energy to drive reactions using precious metal catalysts has allowed significant progress in our ability to design new chemical reactions. It remains an ongoing challenge to reduce costs and waste formation by designing catalytic processes with earth-abundant metals, using simple non-toxic substrates and reagents, and generating only benign by-products. The development of new light-driven catalytic reactions to convert simple alcohol-containing chemicals into various functionalized products would be highly valuable. Using a novel cobalt-based radical pathway, we explored the use of cheap, abundant feedstocks to make valuable products while providing new insights into the mechanism of this important type of light-driven catalysis. This work explores the development of a cobalt-based catalyst system that harnesses light energy to perform the direct functionalization of alcohols via acyl and alkyl radical intermediates. Current methods typically require a pre-activation step, which produces undesirable, often toxic byproducts. The use of abundant cobalt catalysts inspired by the biochemistry of Vitamin B12 provides an alternative mechanism for in-situ activation which produces versatile radical intermediates that can participate in a wide variety of chemical transformations including catalytic deoxygenation, radical cyclizations and intermolecular cross-coupling. Stoichiometric pathways were studied for the proposed catalytic system to investigate these Co(II) and Co(III) complexes and the radical intermediates formed under photochemical conditions. A series of alkoxycarbonyl cobalt(III) complexes were prepared by carbonylation of aliphatic alcohols using protocols developed with three distinct ligand systems. Characterization provided structural details for the alkoxycarbonyl complexes previously unknown or uncharacterized. Homolysis-decarboxylation processes demonstrated cleavage of C–O bonds and trapping of the resulting alkyl radicals oxidatively or reductively. Homolysis-lactonization reactions established a method for cyclizations via the acyl radical intermediates, and subsequent application for the synthesis of the limonoid natural product fraxinellonone was investigated. Finally, development of these stoichiometric reactions into a catalytic methodology was explored