Photochemical Methods for Arene Functionalization and Lignin Depolymerization

[摘要] The current demands of synthetic organic chemistry necessitate the generation of new molecules, for applications as materials and medicines, in a sustainable and energy efficient manner. Visible light photochemistry as a synthetic technology offers unique benefits to organic synthesis by converting abundantly available light energy into chemical energy through the reliable photophysical processes of specific organic dyes and organometallic complexes. Chemoselectivity and operational simplicity are central reaction designs of visible light promoted photoredox catalysis, as the selective excitation of the catalyst can occur at ambient termperature and pressure in the presence of many organic molecules. From the photoexcited state, a catalyst can engage in redox or energy transfer processes with organic molecules to generate reactive intermediates useful in organic synthesis.Synthetically valuable functionalities such as the trifluoromethyl radical and aryl radicals have evaded the development of simple and environmentally benign synthetic methods. To render inert functionalities reactive, redox auxiliaries can be employed. Redox auxiliaries contain a redox labile functionality capable of interating with an excited state photocatalyst, as well as a predictably fragmentable pair of bonds; reagent design in this capacity harnesses both an enthalpic and entropic free energy contributions to generate the desired high energy intermiedate.The efforts described herein summarize the application of visible light photocatalysis, and photochemistry for the synthesis of functionalized arenes and chemoselective C–X bond reduction for biomass valorization. These methods are unified through similar mechanistic hypotheses that begin with a photo-promoted redox transformation between a catalyst and substrate and finish with a critical bond fragmentation event to realize the desired product. To provide a broad summary of the context of this reaction design, Chapter 1 is a summary of the design priciples of visible light photoredox catalysis and the relevance of fragmentable redox auxiliaries. Chapter 2 supports the broad background of Chapter 1 by describing a simple and rapid synthesis of one class of visible light photoredox catalysts, the Ir(III)+ polypyridyl complexes. Chapter 3 details the study of ketyl regulated C–X bond fragmentations as applied to the reduction of the β–O–4 bond in lignin biomass. Chapter 4 recounts a new method for alkene aminoarylation which employs arylsulfonamides as the sole reagent for amination and arylation. Lastly, Chapter 5 describes the development of a non-covalently associated electron donor-acceptor complex that is photoactivated for the trifluoromethylation of arenes.

[发布机构] University of Michigan

[效力级别] Chemistry

[关键词] photoredox catalysis;Chemistry;Science;Chemistry