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Creators/Authors contains: "Ngai, Ming-Yu"

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  1. Allyl carboxylates are useful synthetic intermediates in a variety of organic transformations, including catalytic nucleophilic/electrophilic allylic substitution reactions and 1,2-difunctionalization reactions. However, the catalytic 1,3-difunctionalization of allyl carboxylates remains elusive. Herein, we report the first photoinduced, phosphine-catalyzed 1,3-carbobromination of allyl carboxylates, affording a range of valuable substituted isopropyl carboxylates (sIPC). The transformation has broad functional group tolerance, is amenable to the late-stage modification of complex molecules and gram-scale synthesis, and expands the reaction profiles of allyl carboxylates and phosphine catalysis. Preliminary experimental and computational studies suggest a non-chain-radical mechanism involving the formation of an electron donor–acceptor complex, 1,2-radical migration (RaM), and Br-atom transfer processes. We anticipate that the 1,2-RaM reactivity of allyl carboxylates and the phosphine-catalyzed radical reaction will both serve as a platform for the development of new transformations in organic synthesis. 
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    Excited-state catalysis, a process that involves one or more excited catalytic species, has emerged as a powerful tool in organic synthesis because it allows access to the excited-state reaction landscape for the discovery of novel chemical reactivity. Herein, we report the first excited-state palladium-catalyzed 1,2-spin-center shift reaction that enables site-selective functionalization of carbohydrates. The strategy features mild reaction conditions with high levels of regio- and stereoselectivity that tolerate a wide range of functional groups and complex molecular architectures. Mechanistic studies suggest a radical mechanism involving the formation of hybrid palladium species that undergoes a 1,2-spin-center shift followed by the reduction, deuteration, and iodination to afford functionalized 2-deoxy sugars. The new reactivity will provide a general approach for the rapid generation of natural and unnatural carbohydrates. 
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  3. Intermolecular C–H difluoromethoxylation of (hetero)arenes remains a long-standing and unsolved problem in organic synthesis. Herein, we report the first catalytic protocol employing a redox-active difluoromethoxylating reagent 1a and photoredox catalysts for the direct C–H difluoromethoxylation of (hetero)arenes. Our approach is operationally simple, proceeds at room temperature, and uses bench-stable reagents. Its synthetic utility is highlighted by mild reaction conditions that tolerate a wide variety of functional groups and biorelevant molecules. Experimental and computational studies suggest single electron transfer (SET) from excited photoredox catalysts to 1a forming a neutral radical intermediate that liberates the OCF 2 H radical exclusively. Addition of this radical to (hetero)arenes gives difluoromethoxylated cyclohexadienyl radicals that are oxidized and deprotonated to afford the products of difluoromethoxylation. 
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  4. Abstract

    Visible‐light photocatalysis has advanced as a versatile tool in organic synthesis. However, attaining precise stereocontrol in photocatalytic reactions has been a longstanding challenge due to undesired photochemical background reactions and the involvement of highly reactive radicals or radical ion intermediates generated under photocatalytic conditions. To address this problem and expand the synthetic utility of photocatalytic reactions, a number of innovative strategies, including mono‐ and dual‐catalytic approaches, have recently emerged. Of these, exploiting chiral organocatalysis, such as enamine catalysis, iminium‐ion catalysis, Brønsted acid/base catalysis, andN‐heterocyclic carbene catalysis, to induce chirality transfer of photocatalytic reactions has been widely explored. This Review aims to provide a current, comprehensive overview of asymmetric photocatalytic reactions enabled by chiral organocatalysts published through June 2021. The substrate scope, advantages, limitations, and proposed reaction mechanisms of each reaction are discussed. This review should serve as a reference for the development of visible‐light‐induced asymmetric photocatalysis and promote the improvement of the chemical reactivity and stereoselectivity of these reactions.

     
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  5. Abstract

    Applications of TEMPO.catalysis for the development of redox‐neutral transformations are rare. Reported here is the first TEMPO.‐catalyzed, redox‐neutral C−H di‐ and trifluoromethoxylation of (hetero)arenes. The reaction exhibits a broad substrate scope, has high functional‐group tolerance, and can be employed for the late‐stage functionalization of complex druglike molecules. Kinetic measurements, isolation and resubjection of catalytic intermediates, UV/Vis studies, and DFT calculations support the proposed oxidative TEMPO./TEMPO+redox catalytic cycle. Mechanistic studies also suggest that Li2CO3plays an important role in preventing catalyst deactivation. These findings will provide new insights into the design and development of novel reactions through redox‐neutral TEMPO.catalysis.

     
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  6. Abstract

    Applications of TEMPO.catalysis for the development of redox‐neutral transformations are rare. Reported here is the first TEMPO.‐catalyzed, redox‐neutral C−H di‐ and trifluoromethoxylation of (hetero)arenes. The reaction exhibits a broad substrate scope, has high functional‐group tolerance, and can be employed for the late‐stage functionalization of complex druglike molecules. Kinetic measurements, isolation and resubjection of catalytic intermediates, UV/Vis studies, and DFT calculations support the proposed oxidative TEMPO./TEMPO+redox catalytic cycle. Mechanistic studies also suggest that Li2CO3plays an important role in preventing catalyst deactivation. These findings will provide new insights into the design and development of novel reactions through redox‐neutral TEMPO.catalysis.

     
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  7. Abstract

    Heterocycles are one of the largest groups of organic moieties with significant medicinal, chemical, and industrial applications. Herein, we report the discovery and development of visible‐light‐induced, synergistic excited‐state copper catalysis using a combination of Cu(IPr)I as a catalyst andrac‐BINAP as a ligand, which produces more than 10 distinct classes of heterocycles. The reaction tolerates a broad array of functional groups and complex molecular scaffolds, including derivatives of peptides, natural products, and marketed drugs. Preliminary mechanistic investigation suggests in situ generations of [Cu(BINAP)2]+and [Cu(IPr)2]+catalysts that work cooperatively under visible‐light irradiation to facilitate catalytic carbo‐aroylation of unactivated alkenes, affording a wide range of useful heterocycles.

     
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  8. Abstract

    Heterocycles are one of the largest groups of organic moieties with significant medicinal, chemical, and industrial applications. Herein, we report the discovery and development of visible‐light‐induced, synergistic excited‐state copper catalysis using a combination of Cu(IPr)I as a catalyst andrac‐BINAP as a ligand, which produces more than 10 distinct classes of heterocycles. The reaction tolerates a broad array of functional groups and complex molecular scaffolds, including derivatives of peptides, natural products, and marketed drugs. Preliminary mechanistic investigation suggests in situ generations of [Cu(BINAP)2]+and [Cu(IPr)2]+catalysts that work cooperatively under visible‐light irradiation to facilitate catalytic carbo‐aroylation of unactivated alkenes, affording a wide range of useful heterocycles.

     
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