東華大學圖書館 |

A Radical Approach to Photochemical Transformations Using Earth-Abundant Metals /

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	A Radical Approach to Photochemical Transformations Using Earth-Abundant Metals // Kangjie Bian.
作者:	Bian, Kangjie,
面頁冊數:	1 electronic resource (799 pages)
附註:	Source: Dissertations Abstracts International, Volume: 86-10, Section: B.
Contained By:	Dissertations Abstracts International86-10B.
標題:	Organic chemistry. -
電子資源:	https://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=32010523
ISBN:	9798310386280

A Radical Approach to Photochemical Transformations Using Earth-Abundant Metals /
Bian, Kangjie,

A Radical Approach to Photochemical Transformations Using Earth-Abundant Metals /Kangjie Bian. - 1 electronic resource (799 pages)

Source: Dissertations Abstracts International, Volume: 86-10, Section: B.

The continuing emergence of visible light-mediated photochemistry in modern organic syntheses has allowed facile access to powerful, unconventional reaction manifolds to synthesize diverse small molecules. Conventional photocatalysis/photoredox heavily relies on noble-metal based, coordinatively-saturated mononuclear photoactive complexes to perform a bimolecular outer-sphere single electron transfer (OSET) process for the generation of open-shell radical species. While powerful, the efficiency of this approach is limited by the bimolecular diffusion rate of reactants and photocatalysts and a redox-potential matching requirement for productive oxidative/reductive quenching. By contrast, direct coordination of a metal and substrate can offer a complementary reaction manifold by facilitating inner-sphere single electron transfer (ISET) to promote the homolytic cleavage of this metal-ligand bond and generate an open-shell radical, which can bypass the OSET redox-potential matching prerequisite. Light-induced ligand-to-metal charge transfer (LMCT) is such reaction manifold, allowing for selective single electron oxidation of the coordinated 'ligand'. In general, this reaction scheme can convert anionic ligands to the corresponding radical forms which can function in various radical transformations including functionalization of unsaturated hydrocarbons and intermolecular C-H functionalization. Most importantly, a great synthetic advantage of this reaction manifold is that these processes are found in many early transition-metal (3d metals), which are significantly more earth-abundant than noble metals (e.g. Ir, Ru) used in traditional photoredox catalysts, presenting a low cost and sustainable alternative to noble metal photocatalysis. Apart from our exploration in LMCT catalysis, we also demonstrated radical ligand transfer (RLT) as an effective pathway to sequester transient alkyl radical species, introducing a powerful tool to utilize these reactive species for enhancing molecular complexity of feedstock chemicals. Herein, I will share my research of radical photochemical transformations enabled by earth-abundant metals and we hope our study of earth-abundant metal photocatalysis can inspire chemists to design sustainable pathways in pharmaceuticals and natural product syntheses.Radical difunctionalization is a powerful reaction scheme to incorporate useful functionalities onto unsaturated hydrocarbons, especially the prevalent unactivated alkene class. While atom transfer radical addition (ATRA) has been adopted in difunctionalization of unactivated alkenes to perform haloalkylation using the halide from the alkyl halide reagents, a more versatile reaction scheme that allows the incorporation of other functionalities by leveraging in situ generated transient alkyl radical intermediates is desirable. In chapter 1, we proposed bio-inspired radical ligand transfer (RLT) for taming transient alkyl radical species generated from radical addition to unactivated alkenes. Learning from the radical rebound process of the cytochrome P450 enzyme and non-heme iron-dependent oxygenases, we developed RLT catalysis to incorporate diverse functionalities to minimally functionalized alkenes. This efficient ligand transfer process outcompetes unproductive ATRA, indicating a powerful reaction manifold for functionalizing transient alkyl radical species. The RLT chemistry also inspires us to explore other alkene difunctionalization using earth-abundant metals.Vicinal diamine motifs are prevalent in bioactive molecules, pharmaceuticals, and molecular catalysts, underscoring their significance, and olefin diazidation has emerged as a promising strategy for synthesizing these motifs. Although synthetic precedents have utilized highly oxidative azidobenziodoxolone (ABX, Zhdankin reagent) or electrochemical methods to prepare this privileged motif, these protocols were often confined to limited substrate scope and procedural complexity. Motivated by our development of radical ligand transfer (RLT), we introduced the photochemical diazidation enabled by ligand-to-metal charge transfer (LMCT) and radical ligand transfer (RLT)in chapter 2. Leveraging the merger of these two reaction manifolds, we utilize a stable, earth-abundant, and inexpensive iron salt to function as both radical initiator (LMCT) and terminator (RLT) to synthesize valuable diazidated products. Mechanistic understanding of this cooperative LMCT/RLT also motivated us to develop a photocatalytic diazidation protocol and further expand this chemistry to photocatalytic dichlorination and regioselective fluorochlorination, suggesting the versatility of this tandem scheme. This cooperative scheme can also be applied to photocatalytic decarboxylative C-N bond formation, further demonstrating diverse nucleophilic reactants can be utilized in open-shell radical generation via LMCT, which can subsequently participate in cooperating reaction pathways. This cooperative system prompted us to explore sustainable photocatalysis, with goals of eliminating the usage of exogenous oxidants/reductant, driven by the cooperation of LMCT and other pathways.The introduction of fluoroalkyl groups to parent molecules is a powerful tool to modulate biological and physiological activities of these compounds through enhancement of lipophilicity, bioavailability and metabolic stability. One direct way to introduce these fluoroalkyl groups is through hydrofluoroalkylation of alkenes. Early studies have explored the utilization of expensive and/or oxidative fluoroalkylating reagents and precious metals, which significantly restrict the application of these strategies. In many respects, fluoroalkyl carboxylic acids are the most ideal fluoroalkylating source due to low cost and availability, with trifluoroacetic acid (TFA, $9/mol) as an example representing a desirable CF3 source for hydrotrifluoromethylation. However, the decarboxylation of TFA (and other fluorocarboxylic acids) is known to be extremely challenging due to its high oxidation potential, with previous approaches tentatively utilizing TFA confined to pre-installation of redox moieties to overcome this barrier, drastically decreasing the atom/step economy in these methods. In chapter 3, we show how leveraging the synthetic advantage of LMCT, which can override OSET redox-potential mismatching, allows us to develop a photocatalytic hydrofluoroalkylation protocol using fluoroalkyl carboxylic acids including TFA and other feedstock fluorocarboxylic acids enabled by cooperative LMCT and hydrogen atom transfer (HAT). Critical to the success is the cooperation of earth-abundant iron LMCT and redox-active thiol HAT, offering a mild and redox-neutral protocol to synthesize fluorine-containing molecules without the preactivation of feedstock fluoroalkyl acids. Following the development of photocatalytic hydrofluoroalkylation, we took inspiration from our development of photocatalytic diazidation and dichlorination, reasoning these (pseudo)halide X-type ligands could be equally applied in an analogous hydrofunctionalization reaction manifold. Exciting, we found this to be true and further expand cooperative LMCT/HAT to achieve a photocatalytic anti-Markovnikov hydrochlorination of unsaturated hydrocarbons. Enabled by selective oxidation of anionic chloride using weakly oxidizing iron to promote LMCT reactivities, previous challenging substrates are tolerated in our protocol, giving high anti-Markovnikov regioselectivity.Moreover, this hydrochlorination strategy can be applied to diverse alkynes, offering facile routes to preparing alkenyl chlorides in high regioselectivities and good stereoselectivities. Additionally, with simple adjustment of deuterated co-solvent, both deuterochlorination of alkenes and alkynes behave well in our redox-neutral system, providing another strategy of isotopologue syntheses. Lastly, this cooperative LMCT/HAT also inspires us to develop photocatalytic hydroazidation where we observe a critical ligand-acceleration effect. The facile photochemical generation of azidyl radical also allows us to explore LMCT in combination with halogen atom transfer (XAT) to develop regioselective haloazidation. These azidation protocols can address previous limitations in oxidative/corrosive reagent usage, high loading of metal sources, or limited substrate scope.

English

ISBN: 9798310386280Subjects--Topical Terms:

523952
Organic chemistry.
Subjects--Index Terms:

Radical chemistry

A Radical Approach to Photochemical Transformations Using Earth-Abundant Metals /
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520 $a The continuing emergence of visible light-mediated photochemistry in modern organic syntheses has allowed facile access to powerful, unconventional reaction manifolds to synthesize diverse small molecules. Conventional photocatalysis/photoredox heavily relies on noble-metal based, coordinatively-saturated mononuclear photoactive complexes to perform a bimolecular outer-sphere single electron transfer (OSET) process for the generation of open-shell radical species. While powerful, the efficiency of this approach is limited by the bimolecular diffusion rate of reactants and photocatalysts and a redox-potential matching requirement for productive oxidative/reductive quenching. By contrast, direct coordination of a metal and substrate can offer a complementary reaction manifold by facilitating inner-sphere single electron transfer (ISET) to promote the homolytic cleavage of this metal-ligand bond and generate an open-shell radical, which can bypass the OSET redox-potential matching prerequisite. Light-induced ligand-to-metal charge transfer (LMCT) is such reaction manifold, allowing for selective single electron oxidation of the coordinated 'ligand'. In general, this reaction scheme can convert anionic ligands to the corresponding radical forms which can function in various radical transformations including functionalization of unsaturated hydrocarbons and intermolecular C-H functionalization. Most importantly, a great synthetic advantage of this reaction manifold is that these processes are found in many early transition-metal (3d metals), which are significantly more earth-abundant than noble metals (e.g. Ir, Ru) used in traditional photoredox catalysts, presenting a low cost and sustainable alternative to noble metal photocatalysis. Apart from our exploration in LMCT catalysis, we also demonstrated radical ligand transfer (RLT) as an effective pathway to sequester transient alkyl radical species, introducing a powerful tool to utilize these reactive species for enhancing molecular complexity of feedstock chemicals. Herein, I will share my research of radical photochemical transformations enabled by earth-abundant metals and we hope our study of earth-abundant metal photocatalysis can inspire chemists to design sustainable pathways in pharmaceuticals and natural product syntheses.Radical difunctionalization is a powerful reaction scheme to incorporate useful functionalities onto unsaturated hydrocarbons, especially the prevalent unactivated alkene class. While atom transfer radical addition (ATRA) has been adopted in difunctionalization of unactivated alkenes to perform haloalkylation using the halide from the alkyl halide reagents, a more versatile reaction scheme that allows the incorporation of other functionalities by leveraging in situ generated transient alkyl radical intermediates is desirable. In chapter 1, we proposed bio-inspired radical ligand transfer (RLT) for taming transient alkyl radical species generated from radical addition to unactivated alkenes. Learning from the radical rebound process of the cytochrome P450 enzyme and non-heme iron-dependent oxygenases, we developed RLT catalysis to incorporate diverse functionalities to minimally functionalized alkenes. This efficient ligand transfer process outcompetes unproductive ATRA, indicating a powerful reaction manifold for functionalizing transient alkyl radical species. The RLT chemistry also inspires us to explore other alkene difunctionalization using earth-abundant metals.Vicinal diamine motifs are prevalent in bioactive molecules, pharmaceuticals, and molecular catalysts, underscoring their significance, and olefin diazidation has emerged as a promising strategy for synthesizing these motifs. Although synthetic precedents have utilized highly oxidative azidobenziodoxolone (ABX, Zhdankin reagent) or electrochemical methods to prepare this privileged motif, these protocols were often confined to limited substrate scope and procedural complexity. Motivated by our development of radical ligand transfer (RLT), we introduced the photochemical diazidation enabled by ligand-to-metal charge transfer (LMCT) and radical ligand transfer (RLT)in chapter 2. Leveraging the merger of these two reaction manifolds, we utilize a stable, earth-abundant, and inexpensive iron salt to function as both radical initiator (LMCT) and terminator (RLT) to synthesize valuable diazidated products. Mechanistic understanding of this cooperative LMCT/RLT also motivated us to develop a photocatalytic diazidation protocol and further expand this chemistry to photocatalytic dichlorination and regioselective fluorochlorination, suggesting the versatility of this tandem scheme. This cooperative scheme can also be applied to photocatalytic decarboxylative C-N bond formation, further demonstrating diverse nucleophilic reactants can be utilized in open-shell radical generation via LMCT, which can subsequently participate in cooperating reaction pathways. This cooperative system prompted us to explore sustainable photocatalysis, with goals of eliminating the usage of exogenous oxidants/reductant, driven by the cooperation of LMCT and other pathways.The introduction of fluoroalkyl groups to parent molecules is a powerful tool to modulate biological and physiological activities of these compounds through enhancement of lipophilicity, bioavailability and metabolic stability. One direct way to introduce these fluoroalkyl groups is through hydrofluoroalkylation of alkenes. Early studies have explored the utilization of expensive and/or oxidative fluoroalkylating reagents and precious metals, which significantly restrict the application of these strategies. In many respects, fluoroalkyl carboxylic acids are the most ideal fluoroalkylating source due to low cost and availability, with trifluoroacetic acid (TFA, $9/mol) as an example representing a desirable CF3 source for hydrotrifluoromethylation. However, the decarboxylation of TFA (and other fluorocarboxylic acids) is known to be extremely challenging due to its high oxidation potential, with previous approaches tentatively utilizing TFA confined to pre-installation of redox moieties to overcome this barrier, drastically decreasing the atom/step economy in these methods. In chapter 3, we show how leveraging the synthetic advantage of LMCT, which can override OSET redox-potential mismatching, allows us to develop a photocatalytic hydrofluoroalkylation protocol using fluoroalkyl carboxylic acids including TFA and other feedstock fluorocarboxylic acids enabled by cooperative LMCT and hydrogen atom transfer (HAT). Critical to the success is the cooperation of earth-abundant iron LMCT and redox-active thiol HAT, offering a mild and redox-neutral protocol to synthesize fluorine-containing molecules without the preactivation of feedstock fluoroalkyl acids. Following the development of photocatalytic hydrofluoroalkylation, we took inspiration from our development of photocatalytic diazidation and dichlorination, reasoning these (pseudo)halide X-type ligands could be equally applied in an analogous hydrofunctionalization reaction manifold. Exciting, we found this to be true and further expand cooperative LMCT/HAT to achieve a photocatalytic anti-Markovnikov hydrochlorination of unsaturated hydrocarbons. Enabled by selective oxidation of anionic chloride using weakly oxidizing iron to promote LMCT reactivities, previous challenging substrates are tolerated in our protocol, giving high anti-Markovnikov regioselectivity.Moreover, this hydrochlorination strategy can be applied to diverse alkynes, offering facile routes to preparing alkenyl chlorides in high regioselectivities and good stereoselectivities. Additionally, with simple adjustment of deuterated co-solvent, both deuterochlorination of alkenes and alkynes behave well in our redox-neutral system, providing another strategy of isotopologue syntheses. Lastly, this cooperative LMCT/HAT also inspires us to develop photocatalytic hydroazidation where we observe a critical ligand-acceleration effect. The facile photochemical generation of azidyl radical also allows us to explore LMCT in combination with halogen atom transfer (XAT) to develop regioselective haloazidation. These azidation protocols can address previous limitations in oxidative/corrosive reagent usage, high loading of metal sources, or limited substrate scope.
520 $a Importantly,the cooperation of iron LMCT and thiol HAT has showcased a mild and general solution to hydrofunctionalization of unsaturated hydrocarbons.In this thesis, I have investigated the photochemical transformations enabled by earth-abundant metals, exploring the process of radical ligand transfer (RLT), ligand-to-metal charge transfer (LMCT), hydrogen atom transfer (HAT) and most importantly, the cooperation of these reaction manifolds which allows diverse transformations, establishing earth-abundant metal (photo)catalysis as a competitive synthetic manifold in accessing molecules of high value. These studies have been enabled by increasing mechanistic understanding of each reaction, fueling our continuous efforts in earth-abundant metal catalysis. We hope these studies enabled by earth-abundant metals communicate the importance of promoting sustainable (photo)catalysis in synthetic chemistry and serve as a powerful tool to synthetic chemists. We expect sustainable metal photocatalysis will keep enabling exciting chemistry!
546 $a English
590 $a School code: 0187
650 4 $a Organic chemistry. $3 523952
650 4 $a Chemistry. $3 516420
653 $a Radical chemistry
653 $a Photocatalysis
653 $a Earth-abundant metals catalysis
653 $a Unsaturated hydrocarbons
690 $a 0490
690 $a 0485
710 2 $a Rice University. $b Chemistry. $e degree granting institution. $3 3824635
720 1 $a West, Julian $e degree supervisor.
773 0 $t Dissertations Abstracts International $g 86-10B.
790 $a 0187
791 $a Ph.D.
792 $a 2024
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