Tehshik P. Yoon

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Name: Yoon, Tehshik P.

Organization: University of Wisconsin-Madison , USA

Department: Department of Chemistry

Title: Professor(PhD)

TOPICS

Organic Chemistry

Organic Photochemistry

Polymer

Atom Transfer Radical Polymerization

Chemicals
References

Photocatalytic Indole Diels–Alder Cycloadditions Mediated by Heterogeneous Platinum-Modified Titanium Dioxide

Co-reporter:Spencer P. Pitre, Juan C. Scaiano, and Tehshik P. Yoon

ACS Catalysis October 6, 2017 Volume 7(Issue 10) pp:6440-6440

Publication Date(Web):August 11, 2017

DOI:10.1021/acscatal.7b02223

Indole alkaloids represent an important class of molecules, with many naturally occurring derivatives possessing significant biological activity. One area that requires further development in the synthesis of indole derivatives is the Diels–Alder reaction. In this work, we expand on our previously developed heterogeneous protocol for the [4 + 2] cycloaddition of indoles and electron-rich dienes mediated by platinum nanoparticles supported on titanium dioxide semiconductor particles (Pt(0.2%)@TiO2) with visible-light irradiation. This reaction proceeds with broad scope and is more efficient per incident photon than the previous homogeneous method, and the catalyst can be easily recycled and reused.Keywords: Diels−Alder reactions; heterogeneous catalysis; indoles; photoredox catalysis; radical cation cycloadditions;

Enantioselective Excited-State Photoreactions Controlled by a Chiral Hydrogen-Bonding Iridium Sensitizer

Co-reporter:Kazimer L. Skubi, Jesse B. Kidd, Hoimin Jung, Ilia A. Guzei, Mu-Hyun Baik, and Tehshik P. Yoon

Journal of the American Chemical Society November 29, 2017 Volume 139(Issue 47) pp:17186-17186

Publication Date(Web):October 31, 2017

DOI:10.1021/jacs.7b10586

Stereochemical control of electronically excited states is a long-standing challenge in photochemical synthesis, and few catalytic systems that produce high enantioselectivities in triplet-state photoreactions are known. We report herein an exceptionally effective chiral photocatalyst that recruits prochiral quinolones using a series of hydrogen-bonding and π–π interactions. The organization of these substrates within the chiral environment of the transition-metal photosensitizer leads to efficient Dexter energy transfer and effective stereoinduction. The relative insensitivity of these organometallic chromophores toward ligand modification enables the optimization of this catalyst structure for high enantiomeric excess at catalyst loadings as much as 100-fold lower than the optimal conditions reported for analogous chiral organic photosensitizers.

Radical Cation Cycloadditions Using Cleavable Redox Auxiliaries

Co-reporter:Shishi Lin;Shane D. Lies;Christopher S. Gravatt

Organic Letters January 20, 2017 Volume 19(Issue 2) pp:368-371

Publication Date(Web):December 29, 2016

DOI:10.1021/acs.orglett.6b03545

The incorporation of an easily oxidized arylsulfide moiety facilitates the photocatalytic generation of alkene radical cations that undergo a variety of cycloaddition reactions with electron-rich reaction partners. The sulfide moiety can subsequently be reductively cleaved in a traceless fashion, affording products that are not otherwise directly accessible using photoredox catalysis. This approach constitutes a novel oxidative “redox auxiliary” strategy that offers a practical means to circumvent a fundamental thermodynamic limitation facing photoredox reactions.

Enantioselective Crossed Photocycloadditions of Styrenic Olefins by Lewis Acid Catalyzed Triplet Sensitization

Co-reporter:Dr. Zachary D. Miller;Byung Joo Lee; Tehshik P. Yoon

Angewandte Chemie International Edition 2017 Volume 56(Issue 39) pp:11891-11895

Publication Date(Web):2017/09/18

DOI:10.1002/anie.201706975

AbstractThe synthesis of unsymmetrical cyclobutanes by controlled heterodimerization of olefins remains a substantial challenge, particularly in an enantiocontrolled fashion. Shown herein is that chiral Lewis acid catalyzed triplet sensitization enables the synthesis of highly enantioenriched diarylcyclobutanes by photocycloaddition of structurally varied 2′-hydroxychalcones with a range of styrene coupling partners. The utility of this reaction is demonstrated through the direct synthesis of a representative norlignan cyclobutane natural product.

Enantioselective Crossed Photocycloadditions of Styrenic Olefins by Lewis Acid Catalyzed Triplet Sensitization

Co-reporter:Dr. Zachary D. Miller;Byung Joo Lee; Tehshik P. Yoon

Angewandte Chemie 2017 Volume 129(Issue 39) pp:12053-12057

Publication Date(Web):2017/09/18

DOI:10.1002/ange.201706975

Titanium dioxide visible light photocatalysis: surface association enables photocatalysis with visible light irradiation

Co-reporter:Spencer P. Pitre;Juan C. Scaiano

Chemical Communications 2017 vol. 53(Issue 31) pp:4335-4338

Publication Date(Web):2017/04/13

DOI:10.1039/C7CC01952A

Titanium dioxide (TiO2) is a widely employed and inexpensive photocatalyst, but its use in organic synthesis has been limited by the short-wavelength ultraviolet irradiation typically used. We have discovered that TiO2 particles efficiently mediate photocatalytic radical cation Diels–Alder cycloadditions using a simple visible light source, enabled by the formation of a visible light absorbing complex of the substrate on the semiconductor surface.

Dual Catalysis Strategies in Photochemical Synthesis

Co-reporter:Kazimer L. Skubi, Travis R. Blum, and Tehshik P. Yoon

Chemical Reviews 2016 Volume 116(Issue 17) pp:10035-10074

Publication Date(Web):April 25, 2016

DOI:10.1021/acs.chemrev.6b00018

The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Brønsted acids, organocatalysts, enzymes, and transition metal complexes.

Photochemical Stereocontrol Using Tandem Photoredox–Chiral Lewis Acid Catalysis

Co-reporter:Tehshik P. Yoon

Accounts of Chemical Research 2016 Volume 49(Issue 10) pp:2307

Publication Date(Web):August 9, 2016

DOI:10.1021/acs.accounts.6b00280

The physical, biological, and materials properties of organic compounds are determined by their three-dimensional molecular shape. The development of methods to dictate the stereochemistry of organic reactions has consequently emerged as one of the central themes of contemporary synthetic chemistry. Over the past several decades, chiral catalysts have been developed to control the enantioselectivity of almost every class of synthetically useful transformation. Photochemical reactions, however, are a conspicuous exception. Relatively few examples of highly enantioselective catalytic photoreactions have been reported to date, despite almost a century of research in this field. The development of robust strategies for photochemical enantiocontrol has thus proven to be a long-standing and surprisingly difficult challenge.For the past decade, our laboratory has been studying the application of transition metal photocatalysts to a variety of problems in synthetic organic chemistry. These efforts have recently culminated in the discovery of an effective system in which the activity of a visible light absorbing transition metal photoredox catalyst is combined with a second stereocontrolling chiral Lewis acid catalyst. This dual catalyst strategy has been applied to a diverse range of photochemical reactions; these have included highly enantioselective photocatalytic [2 + 2] cycloadditions, [3 + 2] cycloadditions, and radical conjugate addition reactions.This Account describes the development of the tandem Lewis acid photoredox catalysis strategy utilized in our laboratory. It provides an analysis of the factors that we believe to be particularly important to the success of this seemingly robust approach to photocatalytic stereocontrol. (1) The photocatalysts utilized in our systems are activated by wavelengths of visible light where the organic substrates are transparent, which minimizes the possibility of competitive racemic background photoreactions. (2) The high degree of tolerance that Ru(bpy)32+ and similar octahedral metal polypyridine complexes exhibit toward Lewis acids affords great flexibility in tuning the structure of the stereocontrolling chiral catalyst without perturbing the photoredox properties of the photocatalyst. (3) Synthetic chemists have amassed a substantial understanding of the features that are common in highly successful chiral Lewis acid catalyzed reactions, and these deep, well-validated insights are readily applied to the reactions of a variety of photogenerated intermediates. We hope that the recent success of this and similar dual catalytic systems will provide a useful foundation for the further development of powerful, stereocontrolled photochemical reactions.

Enantioselective Photocatalytic [3 + 2] Cycloadditions of Aryl Cyclopropyl Ketones

Co-reporter:Adrian G. Amador; Evan M. Sherbrook

Journal of the American Chemical Society 2016 Volume 138(Issue 14) pp:4722-4725

Publication Date(Web):March 25, 2016

DOI:10.1021/jacs.6b01728

Control of stereochemistry in photocycloaddition reactions remains a substantial challenge; almost all successful catalytic examples to date have involved [2 + 2] photocycloadditions of enones. We report a method for the asymmetric [3 + 2] photocycloaddition of aryl cyclopropyl ketones that enables the enantiocontrolled construction of densely substituted cyclopentane structures not synthetically accessible using other catalytic methods. These results show that the dual-catalyst strategy developed in our laboratory broadens synthetic chemists’ access to classes of photochemical cycloadditions that have not previously been feasible in enantioselective form.

Co-reporter:Ilia A. Guzei;Desiree M. Bates;Zachary D. Miller;Travis R. Blum

Science 2016 Volume 354(Issue 6318) pp:

Publication Date(Web):

DOI:10.1126/science.aai8228

Asymmetric catalysis by tuning triplets

Triplet excited states manifest a distinct mode of reactivity associated with their unpaired electrons. However, modulating this reactivity to select just one of two mirror-image products, or enantiomers, is difficult. Blum et al. found that Lewis acid coordination lowered the energy of a compound's triplet state, which allowed it to be accessed using an optically excited sensitizer. Because the Lewis acid was essential for triplet formation, it could also direct the reaction pathway toward a single enantiomer.

Science, this issue p. 1391

Hoch enantioselektive Photoreaktionen durch Photokatalysatoren mit chiralen Metallzentren

Co-reporter:Adrian G. Amador ; Tehshik P. Yoon

Angewandte Chemie 2016 Volume 128( Issue 7) pp:2348-2350

Publication Date(Web):

DOI:10.1002/ange.201511443

Spin-Selective Generation of Triplet Nitrenes: Olefin Aziridination through Visible-Light Photosensitization of Azidoformates

Co-reporter:Spencer O. Scholz;Elliot P. Farney;Sangyun Kim;Desiree M. Bates ; Tehshik P. Yoon

Angewandte Chemie 2016 Volume 128( Issue 6) pp:2279-2282

Publication Date(Web):

DOI:10.1002/ange.201510868

Abstract

Azidoformates are interesting potential nitrene precursors, but their direct photochemical activation can result in competitive formation of aziridination and allylic amination products. Herein, we show that visible-light-activated transition-metal complexes can be triplet sensitizers that selectively produce aziridines through the spin-selective photogeneration of triplet nitrenes from azidoformates. This approach enables the aziridination of a wide range of alkenes and the formal oxyamination of enol ethers using the alkene as the limiting reagent. Preparative-scale aziridinations can be easily achieved under continuous-flow conditions.

A Chiral Metal Photocatalyst Architecture for Highly Enantioselective Photoreactions

Co-reporter:Adrian G. Amador ; Tehshik P. Yoon

Angewandte Chemie International Edition 2016 Volume 55( Issue 7) pp:2304-2306

Publication Date(Web):

DOI:10.1002/anie.201511443

Spin-Selective Generation of Triplet Nitrenes: Olefin Aziridination through Visible-Light Photosensitization of Azidoformates

Co-reporter:Spencer O. Scholz;Elliot P. Farney;Sangyun Kim;Desiree M. Bates ; Tehshik P. Yoon

Angewandte Chemie International Edition 2016 Volume 55( Issue 6) pp:2239-2242

Publication Date(Web):

DOI:10.1002/anie.201510868

Abstract

Enantioselective Conjugate Additions of α-Amino Radicals via Cooperative Photoredox and Lewis Acid Catalysis

Co-reporter:Laura Ruiz Espelt; Iain S. McPherson; Eric M. Wiensch

Journal of the American Chemical Society 2015 Volume 137(Issue 7) pp:2452-2455

Publication Date(Web):February 10, 2015

DOI:10.1021/ja512746q

We report the highly enantioselective addition of photogenerated α-amino radicals to Michael acceptors. This method features a dual-catalyst protocol that combines transition metal photoredox catalysis with chiral Lewis acid catalysis. The combination of these two powerful modes of catalysis provides an effective, general strategy to generate and control the reactivity of photogenerated reactive intermediates.

Correction: Characterizing chain processes in visible light photoredox catalysis

Co-reporter:Megan A. Cismesia and Tehshik P. Yoon

Chemical Science 2015 vol. 6(Issue 10) pp:6019-6019

Publication Date(Web):28 Aug 2015

DOI:10.1039/C5SC90050F

Correction for ‘Characterizing chain processes in visible light photoredox catalysis’ by Megan A. Cismesia et al., Chem. Sci., 2015, DOI: 10.1039/c5sc02185e.

Characterizing chain processes in visible light photoredox catalysis

Co-reporter:Megan A. Cismesia and Tehshik P. Yoon

Chemical Science 2015 vol. 6(Issue 10) pp:5426-5434

Publication Date(Web):07 Jul 2015

DOI:10.1039/C5SC02185E

The recognition that Ru(bpy)32+ and similar visible light absorbing transition metal complexes can be photocatalysts for a variety of synthetically useful organic reactions has resulted in a recent resurgence of interest in photoredox catalysis. However, many of the critical mechanistic aspects of this class of reactions remain poorly understood. In particular, the degree to which visible light photoredox reactions involve radical chain processes has been a point of some disagreement that has not been subjected to systematic analysis. We have now performed quantum yield measurements to demonstrate that three representative, mechanistically distinct photoredox processes involve product-forming chain reactions. Moreover, we show that the combination of quantum yield and luminescence quenching experiments provides a rapid method to estimate the length of these chains. Together, these measurements constitute a robust, operationally facile strategy for characterizing chain processes in a wide range of visible light photoredox reactions.

Advances in the Chemistry of Oxaziridines

Co-reporter:Kevin S. Williamson, David J. Michaelis, and Tehshik P. Yoon

Chemical Reviews 2014 Volume 114(Issue 16) pp:8016

Publication Date(Web):April 22, 2014

DOI:10.1021/cr400611n

Iron-catalyzed kinetic resolution of N-sulfonyl oxaziridines

Co-reporter:Kevin S. Williamson, James W. Sawicki and Tehshik P. Yoon

Chemical Science 2014 vol. 5(Issue 9) pp:3524-3527

Publication Date(Web):16 Jun 2014

DOI:10.1039/C4SC01371A

We have developed a highly selective kinetic resolution of N-sulfonyl oxaziridines. This reaction utilizes an inexpensive and easily synthesized iron bis(oxazoline) catalyst to promote the efficient rearrangement of oxaziridines to the corresponding N-sulfonyl imides; no sacrificial reagents are required to effect this resolution. This process is readily translated to gram scale, which provides a practical method for the preparation of structurally diverse, enantiopure N-sulfonyl oxaziridines for use as reagents in organic synthesis.

Opportunities in Photocatalytic Synthesis

Co-reporter:Tehshik P. Yoon;Corey R. J. Stephenson

Advanced Synthesis & Catalysis 2014 Volume 356( Issue 13) pp:

Publication Date(Web):

DOI:10.1002/adsc.201400871

Redox Mediators in Visible Light Photocatalysis: Photocatalytic Radical Thiol–Ene Additions

Co-reporter:Elizabeth L. Tyson, Zachary L. Niemeyer, and Tehshik P. Yoon

The Journal of Organic Chemistry 2014 Volume 79(Issue 3) pp:1427-1436

Publication Date(Web):January 15, 2014

DOI:10.1021/jo500031g

Synthetically useful radical thiol–ene reactions can be initiated by visible light irradiation in the presence of transition metal polypyridyl photocatalysts. The success of this method relies upon the use of p-toluidine as an essential additive. Using these conditions, high-yielding thiol–ene reactions of cysteine-containing biomolecules can be accomplished using biocompatibile wavelengths of visible light, under aqueous conditions, and with the thiol component as the limiting reagent. We present evidence that p-toluidine serves as a redox mediator that is capable of catalyzing the otherwise inefficient photooxidation of thiols to the key thiyl radical intermediate. Thus, we show that co-catalytic oxidants can be important in the design of synthetic reactions involving visible light photoredox catalysis.

[3+2] Photooxygenation of aryl cyclopropanes via visible light photocatalysis

Co-reporter:Zhan Lu, Jonathan D. Parrish, Tehshik P. Yoon

Tetrahedron 2014 70(27–28) pp: 4270-4278

Publication Date(Web):

DOI:10.1016/j.tet.2014.02.045

Solar Synthesis: Prospects in Visible Light Photocatalysis

Co-reporter:Danielle M. Schultz

Science 2014 Volume 343(Issue 6174) pp:

Publication Date(Web):28 Feb 2014

DOI:10.1126/science.1239176

Structured Abstract

Background

Interest in photochemical synthesis has been motivated in part by the realization that sunlight is effectively an inexhaustible energy source.Chemists have also long recognized distinctive patterns of reactivity that are uniquely accessible via photochemical activation. However, most simple organic molecules absorb only ultraviolet (UV) light and cannot be activated by the visible wavelengths that comprise most of the solar energy that reaches Earth’s surface. Consequently, organic photochemistry has generally required the use of UV light sources.

Visible light photocatalysis. (A) Transition metal photocatalysts, such as Ru(bpy)₃²⁺, readily absorb visible light to access reactive excited states. (B) Photoexcited Ru*(bpy)₃²⁺ can act as an electron shuttle, interacting with sacrificial electron donors D (path i) or acceptors A (path ii) to yield either a strongly reducing or oxidizing catalyst toward organic substrates S. Ru*(bpy)₃²⁺can also directly transfer energy to an organic substrate to yield electronically excited species (path iii). bpy, 2,2'-bipyridine; MLCT, metal-to-ligand charge transfer.

Advances

Over the past several years, there has been a resurgence of interest in synthetic photochemistry, based on the recognition that the transition metal chromophores that have been so productively exploited in the design of technologies for solar energy conversion can also convert visible light energy into useful chemical potential for synthetic purposes. Visible light enables productive photoreactions of compounds possessing weak bonds that are sensitive toward UV photodegradation. Furthermore, visible light photoreactions can be conducted by using essentially any source of white light, including sunlight, which obviates the need for specialized UV photoreactors. This feature has expanded the accessibility of photochemical reactions to a broader range of synthetic organic chemists. A variety of reaction types have now been shown to be amenable to visible light photocatalysis via photoinduced electron transfer to or from the transition metal chromophore, as well as energy-transfer processes. The predictable reactivity of the intermediates generated and the tolerance of the reaction conditions to a wide range of functional groups have enabled the application of these reactions to the synthesis of increasingly complex target molecules.

Outlook

This general strategy for the use of visible light in organic synthesis is already being adopted by a growing community of synthetic chemists. Much of the current research in this emerging area is geared toward the discovery of photochemical solutions for increasingly ambitious synthetic goals. Visible light photocatalysis is also attracting the attention of researchers in chemical biology, materials science, and drug discovery, who recognize that these reactions offer opportunities for innovation in areas beyond traditional organic synthesis. The long-term goals of this emerging area are to continue to improve efficiency and synthetic utility and to realize the long-standing goal of performing chemical synthesis using the sun.

Co-reporter:Juana Du;Kazimer L. Skubi;Danielle M. Schultz

Science 2014 Volume 344(Issue 6182) pp:

Publication Date(Web):

DOI:10.1126/science.1251511

A Dual Approach to 2 + 2

Asymmetric catalysis generally accelerates the pathway to one specific product geometry that can be manipulated by reducing the temperature to slow down competing reactions. It is more difficult to be selective in photochemical reactions, but in the [2 + 2] coupling of olefins to make four-membered rings, Du et al. (p. 392; see the Perspective by Neier) used a ruthenium catalyst that absorbs visible light to activate the substrates below the frequency threshold where they absorb intrinsically. Then a second—a chiral Lewis acid—catalyst directs the product stereochemistry. A major advantage of the dual reactions is that each catalyst can be tuned independently.

Photocatalytic Synthesis of Dihydrobenzofurans by Oxidative [3+2] Cycloaddition of Phenols

Co-reporter:Travis R. Blum;Dr. Ye Zhu;Sarah A. Nordeen ; Tehshik P. Yoon

Angewandte Chemie 2014 Volume 126( Issue 41) pp:11236-11239

Publication Date(Web):

DOI:10.1002/ange.201406393

Abstract

We report a protocol for oxidative [3+2] cycloadditions of phenols and alkenes applicable to the modular synthesis of a large family of dihydrobenzofuran natural products. Visible-light-activated transition metal photocatalysis enables the use of ammonium persulfate as an easily handled, benign terminal oxidant. The broad range of organic substrates that are readily oxidized by photoredox catalysis suggests that this strategy may be applicable to a variety of useful oxidative transformations.

[2+2] Cycloaddition of 1,3-Dienes by Visible Light Photocatalysis

Co-reporter:Anna E. Hurtley;Dr. Zhan Lu ; Tehshik P. Yoon

Angewandte Chemie 2014 Volume 126( Issue 34) pp:9137-9140

Publication Date(Web):

DOI:10.1002/ange.201405359

Abstract

[2+2] Photocycloadditions of 1,3-dienes represent a powerful yet synthetically underutilized class of reactions. We report that visible light absorbing transition metal complexes enable the [2+2] cycloaddition of a diverse range of 1,3-dienes. The ability to use long-wavelength visible light is attractive because these reaction conditions tolerate the presence of sensitive functional groups that might be readily decomposed by the high-energy UVC radiation required for direct photoexcitation of 1,3-dienes. The resulting vinylcyclobutane products are poised for a variety of further diversification reactions, and this method is consequently expected to be powerfully enabling in the synthesis of complex organic targets.

Visible-Light Sensitization of Vinyl Azides by Transition-Metal Photocatalysis

Co-reporter:Elliot P. Farney ; Tehshik P. Yoon

Angewandte Chemie 2014 Volume 126( Issue 3) pp:812-816

Publication Date(Web):

DOI:10.1002/ange.201308820

Abstract

Irradiation of vinyl and aryl azides with visible light in the presence of Ru photocatalysts results in the formation of reactive nitrenes, which can undergo a variety of CN bond-forming reactions. The ability to use low-energy visible light instead of UV in the photochemical activation of azides avoids competitive photodecomposition processes that have long been a significant limitation on the synthetic use of these reactions.

Photocatalytic Synthesis of Dihydrobenzofurans by Oxidative [3+2] Cycloaddition of Phenols

Co-reporter:Travis R. Blum;Dr. Ye Zhu;Sarah A. Nordeen ; Tehshik P. Yoon

Angewandte Chemie International Edition 2014 Volume 53( Issue 41) pp:11056-11059

Publication Date(Web):

DOI:10.1002/anie.201406393

Abstract

[2+2] Cycloaddition of 1,3-Dienes by Visible Light Photocatalysis

Co-reporter:Anna E. Hurtley;Dr. Zhan Lu ; Tehshik P. Yoon

Angewandte Chemie International Edition 2014 Volume 53( Issue 34) pp:8991-8994

Publication Date(Web):

DOI:10.1002/anie.201405359

Abstract

Visible-Light Sensitization of Vinyl Azides by Transition-Metal Photocatalysis

Co-reporter:Elliot P. Farney ; Tehshik P. Yoon

Angewandte Chemie International Edition 2014 Volume 53( Issue 3) pp:793-797

Publication Date(Web):

DOI:10.1002/anie.201308820

Abstract

Visible Light Photocatalysis: The Development of Photocatalytic Radical Ion Cycloadditions

Co-reporter:Tehshik P. Yoon

ACS Catalysis 2013 Volume 3(Issue 5) pp:895

Publication Date(Web):April 1, 2013

DOI:10.1021/cs400088e

Photochemistry has the potential to significantly impact multiple aspects of chemical synthesis, in part because photoinduced reactions can be used to construct molecular architectures that would otherwise be difficult to produce. Nevertheless, organic chemists have been slow to embrace photochemical synthesis because of technical complications associated with the use of ultraviolet light. Our laboratory has been part of an effort to design synthetically useful reactions that utilize visible light. This strategy enables the synthesis of a diverse range of organic structures by generation of a variety of reactive intermediates under exceptionally mild conditions. This Perspective article describes the reasoning that led to the conception of our first experiments in this area, the features of our reaction design that have been most powerful in the discovery of new processes, and a few of the possible future areas in which visible light photocatalysis might have a large impact.Keywords: cycloadditions; natural products; photocatalysis; photochemistry; radical ions

Brønsted Acid Cocatalysts in Photocatalytic Radical Addition of α-Amino C–H Bonds across Michael Acceptors

Co-reporter:Laura Ruiz Espelt, Eric M. Wiensch, and Tehshik P. Yoon

The Journal of Organic Chemistry 2013 Volume 78(Issue 8) pp:4107-4114

Publication Date(Web):March 28, 2013

DOI:10.1021/jo400428m

In marked contrast to the variety of strategies available for oxidation and nucleophilic functionalization of methylene groups adjacent to amines, relatively few approaches for modification of this position with electrophilic reaction partners have been reported. In the course of an investigation of the reactions of photogenerated α-amino radicals with electrophiles, we made the surprising observation that the efficiency of radical photoredox functionalization of N-aryl tetrahydroisoquinolines is dramatically increased in the presence of a Brønsted acid cocatalyst. Optimized conditions provide high yields and efficient conversion to radical addition products for a range of structurally modified tetrahydroisoquinolines and enones using convenient household light sources and commercially available Ru(bpy)3Cl2 as a photocatalyst. Our investigations into the origins of this unexpected additive effect have demonstrated that the carbon–carbon bond-forming step is accelerated by TFA and is a rare example of Brønsted acid catalysis in radical addition reactions. Moreover, a significant conclusion arising from these studies is the finding that product formation is dominated by radical chain processes and not by photocatalyst turnover. Together, these findings have important implications for the future design and mechanistic evaluation of photocatalytic radical processses.

Transition Metal Photoredox Catalysis of Radical Thiol-Ene Reactions

Co-reporter:Elizabeth L. Tyson, Michael S. Ament, and Tehshik P. Yoon

The Journal of Organic Chemistry 2013 Volume 78(Issue 5) pp:2046-2050

Publication Date(Web):October 24, 2012

DOI:10.1021/jo3020825

We describe the anti-Markovnikov hydrothiolation of olefins using visible-light-absorbing transition metal photocatalysts. The key thiyl radical intermediates are generated upon quenching of photoexcited Ru*(bpz)32 with a variety of thiols. The adducts of a wide variety of olefins and thiols are formed in excellent yield (73–99%).

Crossed intermolecular [2 + 2] cycloaddition of styrenes by visible light photocatalysis

Co-reporter:Michael A. Ischay, Michael S. Ament and Tehshik P. Yoon

Chemical Science 2012 vol. 3(Issue 9) pp:2807-2811

Publication Date(Web):25 Jun 2012

DOI:10.1039/C2SC20658G

We report a method for the crossed [2 + 2] cycloaddition of styrenes using visible light photocatalysis. Few methods for the synthesis of unsymmetrically substituted cyclobutanes by photochemical [2 + 2] cycloaddition are known. We show that careful tuning of the electrochemical properties of a ruthenium photocatalyst enable the efficient crossed [2 + 2] cycloaddition of styrenes upon irradiation with visible light. We outline the logic that enables high crossed chemoselectivity, and we also demonstrate that this reaction is remarkably efficient; gram-scale reactions can be conducted with as little as 0.025 mol% of the photocatalyst.

Endoperoxide Synthesis by Photocatalytic Aerobic [2 + 2 + 2] Cycloadditions

Co-reporter:Jonathan D. Parrish, Michael A. Ischay, Zhan Lu, Song Guo, Noël R. Peters, and Tehshik P. Yoon

Organic Letters 2012 Volume 14(Issue 6) pp:1640-1643

Publication Date(Web):February 28, 2012

DOI:10.1021/ol300428q

Structurally novel endoperoxides can be sythesized by the photocatalytic cyclotrimerization of bis(styrene) substrates with molecular oxygen. The optimal catalyst for this process is Ru(bpz)32+, which is a markedly more efficient catalyst for these photooxygention reactions than conventional organic photosensitizers. The 1,2-dioxolane products are amenable to synthetic manipulation and can be easily processed to 1,4-diols and γ-hydroxyketones. An initial screen of the biological activity of these compounds reveals promising inhibition of cancer cell growth.

Photocatalytic [2 + 2] Cycloadditions of Enones with Cleavable Redox Auxiliaries

Co-reporter:Elizabeth L. Tyson, Elliot P. Farney, and Tehshik P. Yoon

Organic Letters 2012 Volume 14(Issue 4) pp:1110-1113

Publication Date(Web):February 9, 2012

DOI:10.1021/ol3000298

α,β-Unsaturated 2-imidazolyl ketones undergo [2 + 2] cycloaddition with a variety of Michael acceptors upon irradiation with visible light in the presence of Ru(bpy)32+. Cleavage of the imidazolyl auxiliary from the cycloadducts affords cyclobutane carboxamides, esters, thioesters, and acids that would not be accessible from direct cycloaddition of the corresponding unsaturated carbonyl compounds.

Accessing the Synthetic Chemistry of Radical Ions

Co-reporter:Michael A. Ischay

European Journal of Organic Chemistry 2012 Volume 2012( Issue 18) pp:3359-3372

Publication Date(Web):

DOI:10.1002/ejoc.201101071

Abstract

Organic reactions involving radical cation and radical anion intermediates are synthetically powerful umpolung processes that enable electronically mismatched couplings between pairs of electron-rich or pairs of electron-poor organic fragments. Nevertheless, the adoption of these reactions as synthetic methods has been relatively slow in comparison with that of reactions involving more conventional reactive intermediates such as carbanions, carbocations, and neutral radicals. This Microreview provides a brief survey of radical ion chemistry and highlights the use of transition metal photocatalysis as a convenient means to investigate radical-ion-mediated transformations.

Visible light photocatalysis of intramolecular radical cation Diels–Alder cycloadditions

Co-reporter:Shishi Lin, Christian E. Padilla, Michael A. Ischay, Tehshik P. Yoon

Tetrahedron Letters 2012 Volume 53(Issue 24) pp:3073-3076

Publication Date(Web):13 June 2012

DOI:10.1016/j.tetlet.2012.04.021

Intramolecular radical cation Diels–Alder reactions can be conducted under photocatalytic conditions using visible light irradiation. The photocatalyst system involves the use of a Ru(bpy)32+ chromophore and methyl viologen as a co-oxidant. These reactions enable the cycloaddition of substrates whose thermal Diels–Alder cycloadditions are electronically mismatched and thus require forcing conditions. Nevertheless, the radical cation cycloadditions can be conducted on gram scale using ambient sunlight as the only source of irradiation.

Visible Light Photocatalysis of [2+2] Styrene Cycloadditions by Energy Transfer

Co-reporter:Dr. Zhan Lu ; Tehshik P. Yoon

Angewandte Chemie International Edition 2012 Volume 51( Issue 41) pp:10329-10332

Publication Date(Web):

DOI:10.1002/anie.201204835

Visible Light Photocatalysis of [2+2] Styrene Cycloadditions by Energy Transfer

Co-reporter:Dr. Zhan Lu ; Tehshik P. Yoon

Angewandte Chemie 2012 Volume 124( Issue 41) pp:10475-10478

Publication Date(Web):

DOI:10.1002/ange.201204835

[3+2] Cycloadditions of Aryl Cyclopropyl Ketones by Visible Light Photocatalysis

Co-reporter:Zhan Lu ; Meihua Shen

Journal of the American Chemical Society 2011 Volume 133(Issue 5) pp:1162-1164

Publication Date(Web):January 7, 2011

DOI:10.1021/ja107849y

We report a new method for the formal [3+2] reaction of aryl cyclopropyl ketones with olefins to generate highly substituted cyclopentane ring systems. The key initiation step in this process is the one-electron reduction of the ketone to the corresponding radical anion, which is accomplished using a photocatalytic system comprising Ru(bpy)32+, La(OTf)3, and TMEDA.

Radical Cation Diels–Alder Cycloadditions by Visible Light Photocatalysis

Co-reporter:Shishi Lin ; Michael A. Ischay ; Charles G. Fry

Journal of the American Chemical Society 2011 Volume 133(Issue 48) pp:19350-19353

Publication Date(Web):October 27, 2011

DOI:10.1021/ja2093579

Ruthenium(II) polypyridyl complexes promote the efficient radical cation Diels–Alder cycloaddition of electron-rich dienophiles upon irradiation with visible light. These reactions enable facile [4 + 2] cycloadditions that would be electronically mismatched under thermal conditions. Key to the success of this methodology is the availability of ligand-modified ruthenium complexes that enable rational tuning of the electrochemical properties of the catalyst without significantly perturbing the overall photophysical properties of the system.

Photocatalytic reductive cyclizations of enones: Divergent reactivity of photogenerated radical and radical anion intermediates

Co-reporter:Juana Du, Laura Ruiz Espelt, Ilia A. Guzei and Tehshik P. Yoon

Chemical Science 2011 vol. 2(Issue 11) pp:2115-2119

Publication Date(Web):23 Aug 2011

DOI:10.1039/C1SC00357G

Photocatalytic reactions of enones using metal polypyridyl complexes proceed by very different reaction manifolds in the presence of either Lewis or Brønsted acid additives. Previous work from our lab demonstrated that photocatalytic [2 + 2] cycloadditions of enones required the presence of a Lewis acidic co-catalyst, presumably to activate the enone and stabilize the key radical anion intermediate. On the other hand, Brønsted acid activators alter this reactivity and instead promote reductive cyclization reactions of a variety of aryl and aliphatic enonesvia a neutral radical intermediate. These two distinct reactive intermediates give rise to transformations differing in the connectivity, stereochemistry, and oxidation state of their products. In addition, this reductive coupling method introduces a novel approach to the tin-free generation of β-ketoradicals that react with high diastereoselectivity and with the high functional group compatibility typical of radical cyclization reactions.

Visible light photocatalysis of radical anion hetero-Diels–Alder cycloadditions

Co-reporter:Anna E. Hurtley, Megan A. Cismesia, Michael A. Ischay, Tehshik P. Yoon

Tetrahedron 2011 67(24) pp: 4442-4448

Publication Date(Web):

DOI:10.1016/j.tet.2011.02.066

[2+2] Cycloadditions by Oxidative Visible Light Photocatalysis

Co-reporter:Michael A. Ischay ; Zhan Lu

Journal of the American Chemical Society 2010 Volume 132(Issue 25) pp:8572-8574

Publication Date(Web):June 8, 2010

DOI:10.1021/ja103934y

Photochemical reactions are remarkable for their ability to easily assemble cyclobutanes and other strained ring systems that are difficult to construct using other conventional synthetic methods. We have previously shown that Ru(bpy)32+ is an efficient photocatalyst that promotes the [2+2] cycloadditions of electron-deficient olefins with visible light. Here, we show that Ru(bpy)32+ is also an effective photocatalyst for the [2+2] cycloaddition of electron-rich olefins. This transformation is enabled by the versatile photoelectrochemical properties of Ru(bpy)32+, which enables either one-electron reduction or one-electron oxidation of interesting organic substrates under appropriate conditions.

Iron-Catalyzed Aminohydroxylation of Olefins

Co-reporter:Kevin S. Williamson

Journal of the American Chemical Society 2010 Volume 132(Issue 13) pp:4570-4571

Publication Date(Web):March 16, 2010

DOI:10.1021/ja1013536

We have discovered that N-sulfonyl oxaziridines react with a broad range of olefins in the presence of iron salts to afford 1,3-oxazolidines. This process provides access to 1,2-aminoalcohols with the opposite sense of regioselectivity produced from the copper-catalyzed oxyamination previously reported by our laboratories. Thus, either regioisomeric form of 1,2-aminoalcohols can easily be obtained from the reaction of oxaziridines with olefins, and the sense of regioselectivity can be controlled by the appropriate choice of inexpensive, nontoxic, first-row transition-metal catalyst.

N-Nosyl oxaziridines as terminal oxidants in copper(II)-catalyzed olefin oxyaminations

Co-reporter:Sandra M. DePorter, Ashley C. Jacobsen, Katherine M. Partridge, Kevin S. Williamson, Tehshik P. Yoon

Tetrahedron Letters 2010 Volume 51(Issue 40) pp:5223-5225

Publication Date(Web):6 October 2010

DOI:10.1016/j.tetlet.2010.08.015

We report that N-4-nosyl-3-phenyloxaziridine is an effective terminal oxidant for copper(II)-catalyzed oxyamination recently developed in our labs. This oxaziridine can be prepared on multi-gram scale and is easily purified by recrystallization. The products of oxyamination using this oxaziridine bear protecting groups that can be readily removed in high yields under mild conditions.

Oxaziridine-Mediated Oxyamination of Indoles: An Approach to 3-Aminoindoles and Enantiomerically Enriched 3-Aminopyrroloindolines

Co-reporter:Tamas Benkovics;Dr. Ilia A. Guzei ; Tehshik P. Yoon

Angewandte Chemie 2010 Volume 122( Issue 48) pp:9339-9343

Publication Date(Web):

DOI:10.1002/ange.201004635

Oxaziridine-Mediated Oxyamination of Indoles: An Approach to 3-Aminoindoles and Enantiomerically Enriched 3-Aminopyrroloindolines

Co-reporter:Tamas Benkovics;Dr. Ilia A. Guzei ; Tehshik P. Yoon

Angewandte Chemie International Edition 2010 Volume 49( Issue 48) pp:9153-9157

Publication Date(Web):

DOI:10.1002/anie.201004635

Carbonyl Imines from Oxaziridines: Generation and Cycloaddition of NOC Dipoles

Co-reporter:KatherineM. Partridge;IliaA. Guzei ;TehshikP. Yoon

Angewandte Chemie International Edition 2010 Volume 49( Issue 5) pp:930-934

Publication Date(Web):

DOI:10.1002/anie.200905801

Carbonyl Imines from Oxaziridines: Generation and Cycloaddition of NOC Dipoles

Co-reporter:KatherineM. Partridge;IliaA. Guzei ;TehshikP. Yoon

Angewandte Chemie 2010 Volume 122( Issue 5) pp:942-946

Publication Date(Web):

DOI:10.1002/ange.200905801

Oxaziridine-Mediated Intramolecular Amination of sp3-Hybridized C−H Bonds

Co-reporter:Charles P. Allen ; Tamas Benkovics ; Amanda K. Turek

Journal of the American Chemical Society 2009 Volume 131(Issue 35) pp:12560-12561

Publication Date(Web):August 17, 2009

DOI:10.1021/ja906183g

We describe a new oxaziridine-mediated approach to the amination of sp3-hybridized C−H bonds. In the presence of a copper(II) catalyst, N-sulfonyl oxaziridines participate in efficient intramolecular cyclization reactions to afford a variety of piperidine and tetrahydroisoquinoline structures. The aminal intermediates provide a convenient functional handle for further elaboration of these structures, demonstrating the utility of this new methodology for the rapid construction of structurally complex nitrogen-containing heterocycles.

Crossed Intermolecular [2+2] Cycloadditions of Acyclic Enones via Visible Light Photocatalysis

Co-reporter:Juana Du

Journal of the American Chemical Society 2009 Volume 131(Issue 41) pp:14604-14605

Publication Date(Web):May 27, 2009

DOI:10.1021/ja903732v

Efficient [2+2] heterodimerizations of dissimilar acyclic enones can be accomplished upon visible light irradiation in the presence of a ruthenium(II) photocatalyst. Similar cycloadditions under standard UV photolysis conditions are inefficient and unselective. Nevertheless, a diverse range of unsymmetrical tri- and tetrasubstituted cyclobutane structures can be produced in good yields and excellent diastereoselectivities using this new method. The reaction is promoted by any visible light source, and efficient, gram-scale cycloadditions can be conducted upon irradiating with ambient sunlight.

Oxaziridine-mediated enantioselective aminohydroxylation of styrenes catalyzed by copper(II) bis(oxazoline) complexes

Co-reporter:David J. Michaelis, Kevin S. Williamson, Tehshik P. Yoon

Tetrahedron 2009 65(26) pp: 5118-5124

Publication Date(Web):

DOI:10.1016/j.tet.2009.03.012

Iron Catalyzed Asymmetric Oxyamination of Olefins

Co-reporter:Kevin S. Williamson

Journal of the American Chemical Society () pp:

Publication Date(Web):July 13, 2012

DOI:10.1021/ja3046684

The regioselective and enantioselective oxyamination of alkenes with N-sulfonyl oxaziridines is catalyzed by a novel iron(II) bis(oxazoline) complex. This process affords oxazolidine products that can be easily manipulated to yield highly enantioenriched free amino alcohols. The regioselectivity of this process is complementary to that obtained from the analogous copper(II)-catalyzed reaction. Thus, both regioisomers of enantioenriched 1,2-aminoalcohols can be obtained using oxaziridine-mediated oxyamination reactions, and the overall sense of regiochemistry can be controlled using the appropriate choice of inexpensive first-row transition metal catalyst.

Titanium dioxide visible light photocatalysis: surface association enables photocatalysis with visible light irradiation

Co-reporter:Spencer P. Pitre, Tehshik P. Yoon and Juan C. Scaiano

Chemical Communications 2017 - vol. 53(Issue 31) pp:NaN4338-4338

Publication Date(Web):2017/03/28

DOI:10.1039/C7CC01952A

Correction: Characterizing chain processes in visible light photoredox catalysis

Co-reporter:Megan A. Cismesia and Tehshik P. Yoon

Chemical Science (2010-Present) 2015 - vol. 6(Issue 10) pp:NaN6019-6019

Publication Date(Web):2015/08/28

DOI:10.1039/C5SC90050F

Correction for ‘Characterizing chain processes in visible light photoredox catalysis’ by Megan A. Cismesia et al., Chem. Sci., 2015, DOI: 10.1039/c5sc02185e.

Crossed intermolecular [2 + 2] cycloaddition of styrenes by visible light photocatalysis

Co-reporter:Michael A. Ischay, Michael S. Ament and Tehshik P. Yoon

Chemical Science (2010-Present) 2012 - vol. 3(Issue 9) pp:NaN2811-2811

Publication Date(Web):2012/06/25

DOI:10.1039/C2SC20658G

Iron-catalyzed kinetic resolution of N-sulfonyl oxaziridines

Co-reporter:Kevin S. Williamson, James W. Sawicki and Tehshik P. Yoon

Chemical Science (2010-Present) 2014 - vol. 5(Issue 9) pp:NaN3527-3527

Publication Date(Web):2014/06/16

DOI:10.1039/C4SC01371A

Photocatalytic reductive cyclizations of enones: Divergent reactivity of photogenerated radical and radical anion intermediates

Co-reporter:Juana Du, Laura Ruiz Espelt, Ilia A. Guzei and Tehshik P. Yoon