Synthetic applications of visible-light photoredox catalysis
Tucker, Joseph Walter
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Reduction and oxidation reactions are some of the most common chemical transformations occurring both in nature and chemical laboratories. The vast majority of redox reaction systems utilized by synthetic chemists require stoichiometric quantities of redox reagents and generate a corresponding amount of chemical waste. Furthermore, the utilization of visible light irradiation to promote organic transformations has long been recognized as an attractive means to make synthetic chemistry more sustainable and environmentally benign. This work describes our efforts to merge the fields of visiblelight mediated photoredox reactions of metal complexes with synthetic organic chemistry to enable the synthesis of valuable reactive organic intermediates under mild reaction conditions using substoichiometric quantities of redox catalysts. The majority of the work presented utilizes the single electron reducing abilities of photoredox catalysts to mediate the reductive displacement of activated organohalides to generate carbon-centered free radical intermediates. A variety of transformations, including reductive hydrodehalogenation, heterocycle functionalization, radical cyclizations and intermolecular atom transfer radical addition, were developed. Mechanistic experiments and observations lead to a better understanding of the operative pathway for the reactions studied. In addition to the synthesis of reactive free radical intermediates, photoredox methodology was employed for the net carbon-hydrogen bond oxidation of electron rich benzylic ethers to form oxocarbenium ions. This led to the development a novel protocol for the oxidative cleavage of para-methoxybenzyl ethers to afford the corresponding deprotected alcohols. Observation of the crude reaction mixture showed the sole redox by-product of this transformation was chloroform. As a result, this method is an attractive alternative to the use of other oxidants, as the production of a stoichiometric quantity of persistent by-product is avoided. Finally, the overall efficiency of a number of transformations which are mediated by photoredox catalysis was enhanced by the use of a photochemical flow reactor designed in our laboratories as part of a collaboration with Prof. Timothy Jamison of MIT. The reactor designed was inexpensive and could easily be implemented in any laboratory without specialized equipment. Furthermore, the flow reactor allowed for efficient and convenient scale-up of photochemical transformations, a process that persistently proved difficult using traditional batch reactors.
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