Strategies and Applications for Visible Light Mediated Photoredox Transformations in Organic Chemistry

Abstract

The field of photoredox catalysis has emerged as a prominent approach to synthetic chemistry over the past decade, offering powerful strategies in the activation of small molecules. Many of these systems have been proven to perform efficiently and with high selectivity under milder conditions compared to thermally driven reactions. These systems are instead powered by cheap and widely available light sources which excite catalysts capable of converting visible light into useful chemical energy. In general, this process is mediated by the engagement of organic substrates with the excited catalyst via single electron transfer events to produce reactive intermediates. However, despite these advantages there are currently only a limited number of photoredox catalysts available, almost all of which are based on iridium and ruthenium complexes. These precious earth-scarce metal catalysts are extremely expensive, toxic and difficult to manufacture, in addition to usually offering nothing with regards to recyclability. The field is also lacking a full understanding of the complete underlying mechanistic pathways responsible for these transformations. Herein, examples of a cheap and widely available nickel complex as an all-in-one photocatalyst and cross-coupling agent will be introduced in the direct C-H coupling of ethers to boronic acids. To the best of our knowledge, this is the first example of this type of multifunctionality for photochemistry and cross-coupling in a nickel catalyzed system. Additionally, the translation of the important Heck reaction into a heterogeneous, photochemically driven system will be demonstrated. The inorganic composite semiconductor Au@TiO2 can be used to harvest visible light to facilitate the coupling of olefins at an unprecedented combination of low temperature and lower cost, while providing exceptionally high turnover rates and recycling capabilities. Finally, this work will illustrate the use of aluminum oxide as a heterogeneous catalyst for the coupling of aryl chlorides to amines. Although not photochemically catalyzed, this particular work illustrates the ability of a cheap and abundant material to promote reactivity of ubiquitous aryl chlorides in a fully heterogeneous matter. Overall, the contents of this thesis report the development and implementation of lower cost and bench-stable catalytic systems capable of contending with their predecessors while simultaneously offering simple, facile separation of catalysts and reagents.