Abstracts Category : Other

Surprises and Discoveries in Visible-Light Photocatalysis

by Santosh K Pagire

This thesis demonstrates the development of new and unprecedented activation methods in visiblelight mediated photoredox catalysis. The established transformations not only unlocked new avenues in organic synthesis but holds enormous potential for the future advancements and sustainable developments in this field. In chapter 1, we have outlined a short overview of visible-light-mediated processes that have been established over the years, starting with Ciamicians idea of using sunlight as a clean and unlimited energy source for organic synthesis. Therein, we have also highlighted four principal activation modes associated with visible-light photocatalysis (i.e, oxidative quenching, reductive quenching, energy transfer, and a photocascade process). Accordingly, we stated the diversity of various synthetic methods relies upon the ability of photoredox catalysts such as [Ru(bpy)3]Cl2, [Ir{dF(CF3)ppy}2(dtbbpy)]PF6, [Ir(ppy)2(dtbbpy)]PF6, or fac-Ir(ppy)3 to undergo a number of productive quenching pathways from the excited state. The flexibility of these catalysts has allowed for the development of a series of transformations such as diradical formation by triplet photosensitization, cycloaddition reactions, vinyl radical formation by reductive radical dehalogenations, 1,6-HAT process or -amino functionalization, sulfonylation as well as arylation reactions, etc. Notably, these photocatalysts certainly possess a series of advantages over the classical radical chemistry in light of sustainable developments. In chapter 2, we have summarized the visible-light mediated activation of C-X bonds, generation of vinyl radical as a key intermediate, and its applications in organic synthesis that are developed in last few years, mostly from our group. In chapter 3, we have presented the unprecedented activation of vinyl-bromides with molecular oxygen utilizing dual energy and electron transfer modes: Ortho-alkynylated -bromocinnamates can be converted by a visible-light-mediated photocascade reaction with molecular oxygen into either indenones or dihydroindeno[1,2-c]chromenes. The one-step process features key photochemical steps; thus, the initial activation of vinyl bromides through energy transfer to give -ketoradicals in a reaction with molecular oxygen, followed by -oxidation of an arene moiety by 6- electrocyclization, and subsequent hydroxylation by an electron-transfer process from the Abstract same photocatalyst leads to the dihydroindeno[1,2-c]chromenes. For this chapter, we have conducted a series of experiments in attempts of understanding the detailed reaction mechanism. While finding the mechanistic details, some of the attempts also ended up in disclosing the new reactions; for examples, visible-light mediated [2+2]-photocycloaddition of cinnamates with visible-light was unknown. As a consequence, we have subsequently developed the latter method and extended the scope of the stated transformation, which is separately described in chapter 4. In chapter 4, an efficient method for the synthesis ofAdvisors/Committee Members: Reiser, Oliver (advisor), Knig, Burkhard (advisor), Pfitzner, Arno (advisor).