Visible-Light Mediate Redox Processes: Strategies and Applications in Organic Synthesis
Creators: name: Pitre, Spencer Paul
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2017
Subjects: Photoredox Catalysis
Organic Synthesis
Physical Organic Chemistry

description: Over the past decade, the field of photoredox catalysis has garnered increasing amounts of attention in the organic chemistry community due to its wide applicability in sustainable free radical-mediated processes. Several examples have demonstrated that under carefully optimized conditions, efficient and highly selective processes can be developed through excitation of a photosensitizer using inexpensive, readily available light sources. Furthermore, these reactions can generally be performed under milder conditions than thermal reactions, as all the energy required to overcome the reaction barrier is supplied by light. Despite all these recent advancements in the field, many of these discoveries often lack in depth investigations into the excited state kinetics and underlying mechanisms. Furthermore, the vast majority of these transformations are photocatalyzed by ruthenium and iridium polypyridyl complexes. Not only are these precious metal catalysts extremely costly, but these metals are also known to be toxic, limiting their potential use in the development of pharmaceutical protocols. Herein, we present our solutions to these shortcomings, which involve a three-prong approach in the development of novel protocols, understanding the underlying mechanisms through detailed kinetic analysis, and by the development of new tools to facilitate mechanistic investigation for practitioners who may not possess specialized photochemical equipment. In this work, we were the first to demonstrate that radicals derived from amines, commonly employed as “sacrificial” electron-donors, can also act as reducing agents in photoredox transformations. We also present examples in which Methylene Blue, an inexpensive, non-toxic organic dye, can be employed as a viable alternative to ruthenium complexes for photoredox transformations. By employing a photosensitizer with more favourable excited state kinetics for electron-transfer, we successfully demonstrated that Methylene Blue could be used to increase the efficiency of a previously developed photoredox transformation. While employing organic dyes is an excellent strategy to lowering the cost of photoredox transformations, another viable strategy is to employ heterogeneous semiconductors. Titanium dioxide is an example of a semiconductor which is often employed in photocatalytic applications due to its low cost, desirable redox properties, and high chemical stability which allows for continued use. However, titanium dioxide has seen limited use in organic synthesis due to the requirement of UV irradiation for excitation. Herein, we present a process which led to the discovery of visible light photochemistry with titanium dioxide, generated through the adsorption of indole substrates creating a new, visible light absorbing complex. Employing this strategy, we were able to promote the photocatalytic Diels–Alder reaction of indoles with electron-rich dienes, giving access to valuable tetrahydrocarbazole scaffolds. Finally, in order to facilitate the characterization of chain processes in photoredox catalysis, we have successfully developed a visible light actinometer based on the ubiquitous photocatalyst, Ru(bpy)3Cl2. This actinometer offers many advantages compared to other visible light actinometers, such as completely eliminating the need for spectral matching, as the actinometer is also the photocatalyst. This technique should provide researchers with a mechanistic tool to properly characterize chain propagation in the transformation of interest.