Development of reagents and reactions to be used in visible light photocatalysis
Abstract
Visible light photocatalysis has become a powerful synthetic tool that can be used to promote various functional group transformations through the use of visible light as a green and traceless reagent. Recently, we have attempted to develop a reagent capable of promoting visible light prenylation. Prenylation is an essential reaction on which nature relies to modify properties of molecules and build terpenoids, but one which remains a challenging chemical reaction. Aiming to capitalize on recent advances in photocatalysis to cleanly generate a broad range of carbon based radicals, we have developed a prenyl transfer reagent that can capture transiently generated radicals. The reagent can be made in bulk, is bench stable, and broadly applicable such that it can be used with existing photocatalytic methods with very few changes to reaction conditions. In our next effort, we explored strategies to expand the scope of visible light mediated cross-couplings to alkyl halides. While aryl halides have proven to be competent precursors to aryl radicals, the extreme reduction potentials of unactivated alkyl halides limit their generality as radical precursors in organic synthesis via photocatalysis. To circumvent this limitation, we leveraged alkyl halides tendency towards substitution to install a functional group more inclined towards electron transfer and ultimately fragmentation to generate benzylic radicals from a variety of benzyl halides that would be sluggish, inert, or incompatible with current visible light photoredox catalysis. Applying this strategy, we demonstrate the use of collidinium salts as new reagents for formation of C-C bond which highlights the mild reaction conditions and high functional group tolerance. Finally, we demonstrated the visible light mediated photocatalyst free selective debromination of activated poly-bromides in the presence of amines. This visible light mediated alkyl bromide and chloride synthesis is a particularly convenient synthetic approach when coupled to perhalogenation reactions from the literature. This selective defunctionalization effectively separates the problems of bond formation and reaction selectivity and facilitates the synthesis of organo-bromides and -chlorides. We found that these photochemical reductions rely either on the formation of an electron donor-acceptor complex of the substrate and reductant, or alternatively on an auto-photocatalysis pathway.
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- OSU Dissertations [11222]