Photoredox catalysis harnessing oxygen-centered radicals

Abstract

Visible light provides an abundant and renewable source of energy with an ample capacity to mediate chemical transformations. Photoredox catalysis utilizes this energy for the generation of high-energy radical intermediates and strategically employs these reactive species in versatile chemical transformations. Moreover, a platform of photoredox catalysis enables chemical reactions that would otherwise be difficult, if not impossible, with traditional thermal chemistry. It allows to carry out various types of reactions under relatively mild and environmentally friendly conditions. Oxygen-centered radicals are of particular interest in this thesis, as these species offer rich possibilities in a synthetic context if harnessed effectively. The generation of oxygen- centered radicals is a long-standing challenge, thus, the goal of the work presented in this thesis is to develop new and reliable strategies for accessing these species by photoredox catalysis. Herein presented investigations on oxygen-centered radicals, specifically alkoxy radicals, utilized precursors employing hypervalent iodine (III), N-tiazolethione, and carbonate redox-active moieties. Oxygen-centered radicals were generated upon the photomediated cleavage of a particular moiety. Recognizing the capacity of oxygen- centered radical species to initiate hydrogen atom transfer processes enabled the development of three distinct photocatalytic methods for the direct and selective functionalization of ubiquitous aliphatic C(sp3)- and C(sp2)-H bonds. Furthermore, this thesis presents CO2-enriched methanol as a novel alkoxy radical precursor that facilitates a previously unexplored strategy for the generation and utilization of the corresponding alkoxycarbonate radical in a photocatalytic manner. This work also demonstrates an unparalleled use of a metallic needle as a source of iron for dual photoredox/iron catalysis. This strategy enabled the achievement of chemoselectively controlled transformations of alcohols to cyclic ethers and bromoketone derivatives. Moreover, this thesis discloses a protocol that utilizes the simple Fe(acac)3 as a photocatalyst with an unconventionally short- lived excited-state for the oxidative fragmentation of ethers and acetals to bromoketones. Thus, the research presented in this thesis not only expands the repertoire of mild and robust photocatalytic methods for the generation of oxygen-centered radicals but also provides the means of their synthetic engagement in transformations yielding valuable sets of products, such as diversely functionalized cyclic ethers, acetals, chromanes, and bromoketones. These research results further contribute to the understanding of the fundamental reactivity of oxygen-centered radicals in hydrogen atom transfer processes. Furthermore, it showcased the possibility of employing earth-abundant iron as a more sustainable alternative to common photocatalysts, thereby creating new dimensions for photoredox catalysis.