1
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Rodgers LVH, Nguyen ST, Cox JH, Zervas K, Yuan Z, Sangtawesin S, Stacey A, Jaye C, Weiland C, Pershin A, Gali A, Thomsen L, Meynell SA, Hughes LB, Jayich ACB, Gui X, Cava RJ, Knowles RR, de Leon NP. Diamond surface functionalization via visible light-driven C-H activation for nanoscale quantum sensing. Proc Natl Acad Sci U S A 2024; 121:e2316032121. [PMID: 38451945 PMCID: PMC10945787 DOI: 10.1073/pnas.2316032121] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2023] [Accepted: 01/08/2024] [Indexed: 03/09/2024] Open
Abstract
Nitrogen-vacancy (NV) centers in diamond are a promising platform for nanoscale NMR sensing. Despite significant progress toward using NV centers to detect and localize nuclear spins down to the single spin level, NV-based spectroscopy of individual, intact, arbitrary target molecules remains elusive. Such sensing requires that target molecules are immobilized within nanometers of NV centers with long spin coherence. The inert nature of diamond typically requires harsh functionalization techniques such as thermal annealing or plasma processing, limiting the scope of functional groups that can be attached to the surface. Solution-phase chemical methods can be readily generalized to install diverse functional groups, but they have not been widely explored for single-crystal diamond surfaces. Moreover, realizing shallow NV centers with long spin coherence times requires highly ordered single-crystal surfaces, and solution-phase functionalization has not yet been shown with such demanding conditions. In this work, we report a versatile strategy to directly functionalize C-H bonds on single-crystal diamond surfaces under ambient conditions using visible light, forming C-F, C-Cl, C-S, and C-N bonds at the surface. This method is compatible with NV centers within 10 nm of the surface with spin coherence times comparable to the state of the art. As a proof-of-principle demonstration, we use shallow ensembles of NV centers to detect nuclear spins from surface-bound functional groups. Our approach to surface functionalization opens the door to deploying NV centers as a tool for chemical sensing and single-molecule spectroscopy.
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Affiliation(s)
- Lila V. H. Rodgers
- Department of Electrical and Computer Engineering, Princeton University, Princeton, NJ08540
| | - Suong T. Nguyen
- Department of Chemistry, Princeton University, Princeton, NJ08540
| | - James H. Cox
- Department of Chemistry, Princeton University, Princeton, NJ08540
| | - Kalliope Zervas
- Department of Electrical and Computer Engineering, Princeton University, Princeton, NJ08540
| | - Zhiyang Yuan
- Department of Electrical and Computer Engineering, Princeton University, Princeton, NJ08540
| | - Sorawis Sangtawesin
- School of Physics, Suranaree University of Technology, Nakhon Ratchasima30000, Thailand
- Center of Excellence in Advanced Functional Materials, Suranaree University of Technology, Nakhon Ratchasima30000, Thailand
| | - Alastair Stacey
- School of Physics, University of Melbourne, Parkville, VIC3010, Australia
- School of Science, RMIT University, Melbourne, VIC3000, Australia
| | - Cherno Jaye
- Materials Measurement Science Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD20899
| | - Conan Weiland
- Materials Measurement Science Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD20899
| | - Anton Pershin
- HUN-REN Wigner Research Centre for Physics, Institute for Solid State Physics and Optics, BudapestH-1525, Hungary
- MTA-WFK Lendület “Momentum” Semiconductor Nanostructures Research Group, BudapestH-1525, Hungary
| | - Adam Gali
- HUN-REN Wigner Research Centre for Physics, Institute for Solid State Physics and Optics, BudapestH-1525, Hungary
- MTA-WFK Lendület “Momentum” Semiconductor Nanostructures Research Group, BudapestH-1525, Hungary
- Department of Atomic Physics, Institute of Physics, Budapest University of Technology and Economics, BudapestH-1111, Hungary
| | - Lars Thomsen
- Australian Synchrotron, Australian Nuclear Science and Technology Organisation, Clayton, VIC3168, Australia
| | - Simon A. Meynell
- Physics Department, University of California, Santa Barbara, CA93106
| | - Lillian B. Hughes
- Materials Department, University of California, Santa Barbara, CA93106
| | | | - Xin Gui
- Department of Chemistry, Princeton University, Princeton, NJ08540
| | - Robert J. Cava
- Department of Chemistry, Princeton University, Princeton, NJ08540
| | | | - Nathalie P. de Leon
- Department of Electrical and Computer Engineering, Princeton University, Princeton, NJ08540
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2
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Shin NY, Tsui E, Reinhold A, Scholes GD, Bird MJ, Knowles RR. Correction to "Radicals as Exceptional Electron-Withdrawing Groups: Nucleophilic Aromatic Substitution of Halophenols Via Homolysis-Enabled Electronic Activation". J Am Chem Soc 2024; 146:2286-2287. [PMID: 38190440 DOI: 10.1021/jacs.3c13005] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2024]
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3
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Geunes EP, Meinhardt JM, Wu EJ, Knowles RR. Photocatalytic Anti-Markovnikov Hydroamination of Alkenes with Primary Heteroaryl Amines. J Am Chem Soc 2023; 145:21738-21744. [PMID: 37787499 PMCID: PMC10589911 DOI: 10.1021/jacs.3c08428] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/04/2023]
Abstract
We report a light-driven method for the intermolecular anti-Markovnikov hydroamination of alkenes with primary heteroaryl amines. In this protocol, electron transfer between an amine substrate and an excited-state iridium photocatalyst affords an aminium radical cation (ARC) intermediate that undergoes C-N bond formation with a nucleophilic alkene. Integral to reaction success is the electronic character of the amine, wherein increasingly electron-deficient heteroaryl amines generate increasingly reactive ARCs. Counteranion-dependent reactivity is observed, and iridium triflate photocatalysts are employed in place of conventional iridium hexafluorophosphate complexes. This method exhibits broad functional group tolerance across 55 examples of N-alkylated products derived from pharmaceutically relevant heteroaryl amines.
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Affiliation(s)
- Eric P Geunes
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Jonathan M Meinhardt
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Emily J Wu
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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4
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Hejna BG, Ganley JM, Shao H, Tian H, Ellefsen JD, Fastuca NJ, Houk KN, Miller SJ, Knowles RR. Catalytic Asymmetric Hydrogen Atom Transfer: Enantioselective Hydroamination of Alkenes. J Am Chem Soc 2023; 145:16118-16129. [PMID: 37432783 PMCID: PMC10544660 DOI: 10.1021/jacs.3c04591] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 07/13/2023]
Abstract
We report a highly enantioselective radical-based hydroamination of enol esters with sulfonamides jointly catalyzed by an Ir photocatalyst, Brønsted base, and tetrapeptide thiol. This method is demonstrated for the formation of 23 protected β-amino-alcohol products, achieving selectivities up to 97:3 er. The stereochemistry of the product is set through selective hydrogen atom transfer from the chiral thiol catalyst to a prochiral C-centered radical. Structure-selectivity relationships derived from structural variation of both the peptide catalyst and olefin substrate provide key insights into the development of an optimal catalyst. Experimental and computational mechanistic studies indicate that hydrogen-bonding, π-π stacking, and London dispersion interactions are contributing factors for substrate recognition and enantioinduction. These findings further the development of radical-based asymmetric catalysis and contribute to the understanding of the noncovalent interactions relevant to such transformations.
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Affiliation(s)
- Benjamin G. Hejna
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Jacob M. Ganley
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Huiling Shao
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
| | - Haowen Tian
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
| | - Jonathan D. Ellefsen
- Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
| | - Nicholas J. Fastuca
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - K. N. Houk
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
| | - Scott J. Miller
- Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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5
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Baek Y, Reinhold A, Tian L, Jeffrey PD, Scholes GD, Knowles RR. Singly Reduced Iridium Chromophores: Synthesis, Characterization, and Photochemistry. J Am Chem Soc 2023. [PMID: 37260100 DOI: 10.1021/jacs.2c13249] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
One-electron reduced photosensitizers have been invoked as crucial intermediates in photoredox catalysis, including multiphoton excitation and electrophotocatalytic processes. However, such reduced chromophores have been less investigated, limiting mechanistic studies of their associated electron transfer processes. Here, we report a total of 11 different examples of isolable singly reduced iridium chromophores. Chemical reduction of a cyclometalated iridium complex with potassium graphite affords a 19-electron species. Structural and spectroscopic characterizations reveal a ligand-centered reduction product. The reduced chromophore absorbs a wide range of light from ultraviolet to near-infrared and exhibits photoinduced bimolecular electron transfer reactivity. These studies shed light on elusive reduced iridium chromophores in both ground and excited states, providing opportunities to investigate a commonly invoked intermediate in photoredox catalysis.
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Affiliation(s)
- Yunjung Baek
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Adam Reinhold
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Lei Tian
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Philip D Jeffrey
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Gregory D Scholes
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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6
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Qiu G, Ni CL, Knowles RR. Isotopic Fractionation as a Mechanistic Probe in Light-Driven C-H Bond Exchange Reactions. J Am Chem Soc 2023; 145:11537-11543. [PMID: 37192535 PMCID: PMC10510749 DOI: 10.1021/jacs.2c11212] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/18/2023]
Abstract
Here, we report a diagnostic framework for elucidating the mechanisms of photoredox-based hydrogen isotope exchange (HIE) reactions based on hydrogen/deuterium (H/D) fractionation. Traditional thermal HIE methods generally proceed by reversible bond cleavage and bond reformation steps that share a common transition state. However, bond cleavage and bond reformation in light-driven HIE reactions can proceed via multiple, non-degenerate sets of elementary steps, complicating both mechanistic analysis and attendant optimization efforts. Building on classical treatments of equilibrium isotope effects, the fractionation method presented here extracts information regarding the nature of the key bond-forming and bond-breaking steps by comparing the extent of deuterium incorporation into an exchangeable C-H bond in the substrate relative to the H/D isotopic ratio of a solvent reservoir. We show that the extent of fractionation is sensitive to the mechanism of the exchange process and provides a means to distinguish between degenerate and non-degenerate mechanisms for isotopic exchange. In model systems, the mechanisms implied by the fractionation method align with those predicted by thermochemical considerations. We then employed the method to study HIE reactions whose mechanisms are ambiguous on thermodynamic grounds.
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Affiliation(s)
| | | | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, NJ 08544, USA
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7
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Nguyen ST, Fries LR, Cox JH, Ma Y, Fors BP, Knowles RR. Chemical Recycling of Thiol Epoxy Thermosets via Light-Driven C-C Bond Cleavage. J Am Chem Soc 2023; 145:11151-11160. [PMID: 37167410 DOI: 10.1021/jacs.3c00958] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/13/2023]
Abstract
Epoxy thermosets are high-volume materials that play a central role in a wide range of engineering applications; however, technologies to recycle these polymers remain rare. Here, we present a catalytic, light-driven method that enables chemical recycling of industrially relevant thiol epoxy thermosets to their original monomer at ambient temperature. This strategy relies on the proton-coupled electron transfer (PCET) activation of hydroxy groups within the polymer network to generate key alkoxy radicals that promote the fragmentation of the polymer through C-C bond β-scission. The method fully depolymerizes insoluble thiol epoxy thermosets into well-defined mixtures of small-molecule products, which can collectively be converted into the original monomer via a one-step dealkylation process. Notably, this process is selective and efficient even in the presence of other commodity plastics and additives commonly found in commercial applications. These results constitute an important step toward making epoxy thermosets recyclable and more generally exemplify the potential of PCET to offer a more sustainable end-of-life for a diverse array of commercial plastics.
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Affiliation(s)
- Suong T Nguyen
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Lydia R Fries
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - James H Cox
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Yuting Ma
- Department of Chemistry, Cornell University, Ithaca, New York 14853, United States
| | - Brett P Fors
- Department of Chemistry, Cornell University, Ithaca, New York 14853, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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8
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Shin NY, Tsui E, Reinhold A, Scholes GD, Bird MJ, Knowles RR. Radicals as Exceptional Electron-Withdrawing Groups: Nucleophilic Aromatic Substitution of Halophenols Via Homolysis-Enabled Electronic Activation. J Am Chem Soc 2022; 144:21783-21790. [PMID: 36395367 PMCID: PMC10512454 DOI: 10.1021/jacs.2c10296] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
While heteroatom-centered radicals are understood to be highly electrophilic, their ability to serve as transient electron-withdrawing groups and facilitate polar reactions at distal sites has not been extensively developed. Here, we report a new strategy for the electronic activation of halophenols, wherein generation of a phenoxyl radical via formal homolysis of the aryl O-H bond enables direct nucleophilic aromatic substitution of the halide with carboxylate nucleophiles under mild conditions. Pulse radiolysis and transient absorption studies reveal that the neutral oxygen radical (O•) is indeed an extraordinarily strong electron-withdrawing group [σp-(O•) = 2.79 vs σp-(NO2) = 1.27]. Additional mechanistic and computational studies indicate that the key phenoxyl intermediate serves as an open-shell electron-withdrawing group in these reactions, lowering the barrier for nucleophilic substitution by more than 20 kcal/mol relative to the closed-shell phenol form of the substrate. By using radicals as transient activating groups, this homolysis-enabled electronic activation strategy provides a powerful platform to expand the scope of nucleophile-electrophile couplings and enable previously challenging transformations.
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Affiliation(s)
- Nick Y. Shin
- Department of Chemistry, Princeton University, Princeton NJ 08544 (USA)
| | - Elaine Tsui
- Department of Chemistry, Princeton University, Princeton NJ 08544 (USA)
| | - Adam Reinhold
- Department of Chemistry, Princeton University, Princeton NJ 08544 (USA)
| | | | - Matthew J. Bird
- Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973 (USA)
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton NJ 08544 (USA)
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9
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Murray PRD, Leibler INM, Hell SM, Villalona E, Doyle AG, Knowles RR. Radical Redox Annulations: A General Light-Driven Method for the Synthesis of Saturated Heterocycles. ACS Catal 2022; 12:13732-13740. [DOI: 10.1021/acscatal.2c04316] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2022] [Revised: 10/14/2022] [Indexed: 11/29/2022]
Affiliation(s)
- Philip R. D. Murray
- Department of Chemistry, Princeton University, Princeton, New Jersey08544, United States
| | | | - Sandrine M. Hell
- Department of Chemistry, Princeton University, Princeton, New Jersey08544, United States
| | - Eris Villalona
- Department of Chemistry, Princeton University, Princeton, New Jersey08544, United States
| | - Abigail G. Doyle
- Department of Chemistry and Biochemistry, University of California Los Angeles, Los Angeles, California90095, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey08544, United States
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10
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Xu EY, Werth J, Roos CB, Bendelsmith AJ, Sigman MS, Knowles RR. Noncovalent Stabilization of Radical Intermediates in the Enantioselective Hydroamination of Alkenes with Sulfonamides. J Am Chem Soc 2022; 144:18948-18958. [PMID: 36197450 PMCID: PMC9668373 DOI: 10.1021/jacs.2c07099] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
Noncovalent interactions (NCIs) are critical elements of molecular recognition in a wide variety of chemical contexts. While NCIs have been studied extensively for closed-shell molecules and ions, very little is understood about the structures and properties of NCIs involving free radical intermediates. In this report, we describe a detailed mechanistic study of the enantioselective radical hydroamination of alkenes with sulfonamides and present evidence suggesting that the basis for asymmetric induction in this process arises from attractive NCIs between a neutral sulfonamidyl radical intermediate and a chiral phosphoric acid (CPA). We describe experimental, computational, and data science-based evidence that identifies the specific radical NCIs that form the basis for the enantioselectivity. Kinetic studies support that C-N bond formation determines the enantioselectivity. Density functional theory investigations revealed the importance of both strong H-bonding between the CPA and the N-centered radical and a network of aryl-based NCIs that serve to stabilize the favored diastereomeric transition state. The contributions of these specific aryl-based NCIs to the selectivity were further confirmed through multivariate linear regression analysis by comparing the measured enantioselectivity to computed descriptors. These results highlight the power of NCIs to enable high levels of enantioselectivity in reactions involving uncharged open-shell intermediates and expand our understanding of radical-molecule interactions.
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Affiliation(s)
- Eve Y. Xu
- Department of Chemistry, Princeton University, Princeton, New Jersey, 08544, United States
| | - Jacob Werth
- Department of Chemistry, University of Utah, Salt Lake City, Utah, 84112, United States
| | - Casey B. Roos
- Department of Chemistry, Princeton University, Princeton, New Jersey, 08544, United States
| | - Andrew J. Bendelsmith
- Department of Chemistry, Princeton University, Princeton, New Jersey, 08544, United States
| | - Matthew S. Sigman
- Department of Chemistry, University of Utah, Salt Lake City, Utah, 84112, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey, 08544, United States
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11
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Abstract
A case study of catalytic carbon-carbon σ-bond homolysis is presented. The coordination of a redox-active Lewis acid catalyst reduces the bond-dissociation free energies of adjacent carbon-carbon σ-bonds, and this complexation-induced bond-weakening is used to effect reversible carbon-carbon bond homolysis. Stereochemical isomerization of 1,2-disubstituted cyclopropanes was investigated as a model reaction with a ruthenium (III/II) redox couple adopted for bond weakening. Results from our mechanistic investigation into the stereospecificity of the isomerization reaction are consistent with selective complexation-induced carbon-carbon bond homolysis. The ΔG‡ of catalyzed and uncatalyzed reactions were estimated to be 14.4 and 40.0 kcal/mol, respectively with the computational method, (U)PBE0-D3/def2-TZVPP-SMD(toluene)//(U)B3LYP-D3/def2-SVP. We report this work as the first catalytic example where the complexation-induced bond-weakening effect is quantified through transition state analysis.
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Affiliation(s)
- Suhong Kim
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Pan-Pan Chen
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
| | - K. N. Houk
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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12
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Murray PD, Cox JH, Chiappini ND, Roos CB, McLoughlin EA, Hejna BG, Nguyen ST, Ripberger HH, Ganley JM, Tsui E, Shin NY, Koronkiewicz B, Qiu G, Knowles RR. Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis. Chem Rev 2022; 122:2017-2291. [PMID: 34813277 PMCID: PMC8796287 DOI: 10.1021/acs.chemrev.1c00374] [Citation(s) in RCA: 134] [Impact Index Per Article: 67.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2021] [Indexed: 12/16/2022]
Abstract
We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.
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Affiliation(s)
- Philip
R. D. Murray
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - James H. Cox
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Nicholas D. Chiappini
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Casey B. Roos
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | | | - Benjamin G. Hejna
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Suong T. Nguyen
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Hunter H. Ripberger
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Jacob M. Ganley
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Elaine Tsui
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Nick Y. Shin
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Brian Koronkiewicz
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Guanqi Qiu
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
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13
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Abstract
A light-driven method for the contra-thermodynamic positional isomerization of olefins is described. In this work, stepwise PCET activation of a more substituted and more thermodynamically stable olefin substrate is mediated by an excited-state oxidant and a Brønsted base to afford an allylic radical that is captured by a Cr(II) cocatalyst to furnish an allylchromium(III) intermediate. In situ protodemetalation of this allylchromium complex by methanol is highly regioselective and affords an isomerized and less thermodynamically stable alkene product. The higher oxidation potential of the less substituted olefin isomer renders it inert to further oxidation by the excited-state oxidant, enabling it to accumulate in solution over the course of the reaction. A broad range of isopropylidene substrates are accommodated, including enol ethers, enamides, styrenes, 1,3-dienes, and tetrasubstituted alkyl olefins. Mechanistic investigations of the protodemetalation step are also presented.
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Affiliation(s)
- Kuo Zhao
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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14
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Zhao K, Seidler G, Knowles RR. 1,3-Alkyl Transposition in Allylic Alcohols Enabled by Proton-Coupled Electron Transfer. Angew Chem Int Ed Engl 2021; 60:20190-20195. [PMID: 34159700 DOI: 10.1002/anie.202105285] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2021] [Revised: 06/02/2021] [Indexed: 12/11/2022]
Abstract
A method is described for the isomerization of acyclic allylic alcohols into β-functionalized ketones via 1,3-alkyl transposition. This reaction proceeds via light-driven proton-coupled electron transfer (PCET) activation of the O-H bond in the allylic alcohol substrate, followed by C-C β-scission of the resulting alkoxy radical. The transient alkyl radical and enone acceptor generated in the scission event subsequently recombine via radical conjugate addition to deliver β-functionalized ketone products. A variety of allylic alcohol substrates bearing alkyl and acyl migratory groups were successfully accommodated. Insights from mechanistic studies led to a modified reaction protocol that improves reaction performance for challenging substrates.
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Affiliation(s)
- Kuo Zhao
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
| | - Gesa Seidler
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
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15
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Zhao K, Seidler G, Knowles RR. 1,3‐Alkyl Transposition in Allylic Alcohols Enabled by Proton‐Coupled Electron Transfer. Angew Chem Int Ed Engl 2021. [DOI: 10.1002/ange.202105285] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Kuo Zhao
- Department of Chemistry Princeton University Princeton NJ 08544 USA
| | - Gesa Seidler
- Department of Chemistry Princeton University Princeton NJ 08544 USA
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16
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Sayre H, Ripberger HH, Odella E, Zieleniewska A, Heredia DA, Rumbles G, Scholes GD, Moore TA, Moore AL, Knowles RR. PCET-Based Ligand Limits Charge Recombination with an Ir(III) Photoredox Catalyst. J Am Chem Soc 2021; 143:13034-13043. [PMID: 34378919 DOI: 10.1021/jacs.1c01701] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Upon photoinitiated electron transfer, charge recombination limits the quantum yield of photoredox reactions for which the rates for the forward reaction and back electron transfer are competitive. Taking inspiration from a proton-coupled electron transfer (PCET) process in Photosystem II, a benzimidazole-phenol (BIP) has been covalently attached to the 2,2'-bipyridyl ligand of [Ir(dF(CF3)ppy)2(bpy)][PF6] (dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; bpy = 2,2'-bipyridyl). Excitation of the [Ir(dF(CF3)ppy)2(BIP-bpy)][PF6] photocatalyst results in intramolecular PCET to form a charge-separated state with oxidized BIP. Subsequent reduction of methyl viologen dication (MV2+), a substrate surrogate, by the reducing moiety of the charge separated species demonstrates that the inclusion of BIP significantly slows the charge recombination rate. The effect of ∼24-fold slower charge recombination in a photocatalytic phthalimide ester reduction resulted in a greater than 2-fold increase in reaction quantum efficiency.
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Affiliation(s)
- Hannah Sayre
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Hunter H Ripberger
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Emmanuel Odella
- School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States
| | - Anna Zieleniewska
- National Renewable Energy Laboratory, Golden, Colorado 80401, United States
| | - Daniel A Heredia
- School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States
| | - Garry Rumbles
- National Renewable Energy Laboratory, Golden, Colorado 80401, United States
| | - Gregory D Scholes
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Thomas A Moore
- School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States
| | - Ana L Moore
- School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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17
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Nguyen ST, McLoughlin EA, Cox JH, Fors BP, Knowles RR. Depolymerization of Hydroxylated Polymers via Light-Driven C-C Bond Cleavage. J Am Chem Soc 2021; 143:12268-12277. [PMID: 34333967 DOI: 10.1021/jacs.1c05330] [Citation(s) in RCA: 27] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
The accumulation of persistent plastic waste in the environment is widely recognized as an ecological crisis. New chemical technologies are necessary both to recycle existing plastic waste streams into high-value chemical feedstocks and to develop next-generation materials that are degradable by design. Here, we report a catalytic methodology for the depolymerization of a commercial phenoxy resin and high molecular weight hydroxylated polyolefin derivatives upon visible light irradiation near ambient temperature. Proton-coupled electron transfer (PCET) activation of hydroxyl groups periodically spaced along the polymer backbone furnishes reactive alkoxy radicals that promote chain fragmentation through C-C bond β-scission. The depolymerization produces well-defined and isolable product mixtures that are readily diversified to polycondensation monomers. In addition to controlling depolymerization, the hydroxyl group modulates the thermomechanical properties of these polyolefin derivatives, yielding materials with diverse properties. These results demonstrate a new approach to polymer recycling based on light-driven C-C bond cleavage that has the potential to establish new links within a circular polymer economy and influence the development of new degradable-by-design polyolefin materials.
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Affiliation(s)
- Suong T Nguyen
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Elizabeth A McLoughlin
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - James H Cox
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Brett P Fors
- Department of Chemistry, Cornell University, Ithaca, New York 14853, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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18
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Qin Y, Zhu Q, Sun R, Ganley JM, Knowles RR, Nocera DG. Mechanistic Investigation and Optimization of Photoredox Anti-Markovnikov Hydroamination. J Am Chem Soc 2021; 143:10232-10242. [PMID: 34191486 PMCID: PMC8600941 DOI: 10.1021/jacs.1c03644] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
The reaction mechanism and the origin of the selectivity for the photocatalytic intermolecular anti-Markovnikov hydroamination of unactivated alkenes with primary amines to furnish secondary amines have been revealed by time-resolved laser kinetics measurements of the key reaction intermediates. We show that back-electron transfer (BET) between the photogenerated aminium radical cation (ARC) and reduced photocatalyst complex (Ir(II)) is nearly absent due to rapid deprotonation of the ARC on the sub-100 ns time scale. The selectivity for primary amine alkylation is derived from the faster addition of the primary ARCs (as compared to secondary ARCs) to alkenes. The turnover of the photocatalyst occurs via the reaction between Ir(II) and a thiyl radical; the in situ formation of an off-cycle disulfide from thiyl radicals suppresses this turnover, diminishing the efficiency of the reaction. With these detailed mechanistic insights, the turnover of the photocatalyst has been optimized, resulting in a >10-fold improvement in the quantum yield. These improvements enabled the development of a scalable flow protocol, demonstrating a potential strategy for practical applications with improved energy efficiency and cost-effectiveness.
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Affiliation(s)
- Yangzhong Qin
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
| | - Qilei Zhu
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
| | - Rui Sun
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
| | - Jacob M Ganley
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Daniel G Nocera
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
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19
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Murray PRD, Bussink WMM, Davies GHM, van der Mei FW, Antropow AH, Edwards JT, D'Agostino LA, Ellis JM, Hamann LG, Romanov-Michailidis F, Knowles RR. Intermolecular Crossed [2 + 2] Cycloaddition Promoted by Visible-Light Triplet Photosensitization: Expedient Access to Polysubstituted 2-Oxaspiro[3.3]heptanes. J Am Chem Soc 2021; 143:4055-4063. [PMID: 33666086 DOI: 10.1021/jacs.1c01173] [Citation(s) in RCA: 31] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
This paper describes an intermolecular cross-selective [2 + 2] photocycloaddition reaction of exocyclic arylidene oxetanes, azetidines, and cyclobutanes with simple electron-deficient alkenes. The reaction takes place under mild conditions using a commercially available Ir(III) photosensitizer upon blue light irradiation. This transformation provides access to a range of polysubstituted 2-oxaspiro[3.3]heptane, 2-azaspiro[3.3]heptane, and spiro[3.3]heptane motifs, which are of prime interest in medicinal chemistry as gem-dimethyl and carbonyl bioisosteres. A variety of further transformations of the initial cycloadducts are demonstrated to highlight the versatility of the products and enable selective access to either of a syn- or an anti-diastereoisomer through kinetic or thermodynamic epimerization, respectively. Mechanistic experiments and DFT calculations suggest that this reaction proceeds through a sensitized energy transfer pathway.
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Affiliation(s)
- Philip R D Murray
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Willem M M Bussink
- Bristol Myers Squibb, 200 Cambridge Park Drive, Cambridge, Massachusetts 02140, United States
| | - Geraint H M Davies
- Bristol Myers Squibb, 200 Cambridge Park Drive, Cambridge, Massachusetts 02140, United States
| | - Farid W van der Mei
- Bristol Myers Squibb, 200 Cambridge Park Drive, Cambridge, Massachusetts 02140, United States
| | - Alyssa H Antropow
- Bristol Myers Squibb, 200 Cambridge Park Drive, Cambridge, Massachusetts 02140, United States
| | - Jacob T Edwards
- Bristol Myers Squibb, 10300 Campus Point Drive, Suite 100, San Diego, California 92121, United States
| | | | - J Michael Ellis
- Bristol Myers Squibb, 200 Cambridge Park Drive, Cambridge, Massachusetts 02140, United States
| | - Lawrence G Hamann
- Bristol Myers Squibb, 200 Cambridge Park Drive, Cambridge, Massachusetts 02140, United States
| | | | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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20
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You W, Ganley JM, Ernst BG, Peltier CR, Ko HY, DiStasio RA, Knowles RR, Coates GW. Expeditious synthesis of aromatic-free piperidinium-functionalized polyethylene as alkaline anion exchange membranes. Chem Sci 2021; 12:3898-3910. [PMID: 34163659 PMCID: PMC8179501 DOI: 10.1039/d0sc05789d] [Citation(s) in RCA: 16] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2020] [Accepted: 01/04/2021] [Indexed: 11/26/2022] Open
Abstract
Alkaline anion exchange membranes (AAEMs) with high hydroxide conductivity and good alkaline stability are essential for the development of anion exchange membrane fuel cells to generate clean energy by converting renewable fuels to electricity. Polyethylene-based AAEMs with excellent properties can be prepared via sequential ring-opening metathesis polymerization (ROMP) and hydrogenation of cyclooctene derivatives. However, one of the major limitations of this approach is the complicated multi-step synthesis of functionalized cyclooctene monomers. Herein, we report that piperidinium-functionalized cyclooctene monomers can be easily prepared via the photocatalytic hydroamination of cyclooctadiene with piperidine in a one-pot, two-step process to produce high-performance AAEMs. Possible alkaline-degradation pathways of the resultant polymers were analyzed using spectroscopic analysis and dispersion-inclusive hybrid density functional theory (DFT) calculations. Quite interestingly, our theoretical calculations indicate that local backbone morphology-which can potentially change the Hofmann elimination reaction rate constant by more than four orders of magnitude-is another important consideration in the rational design of stable high-performance AAEMs.
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Affiliation(s)
- Wei You
- Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University Ithaca NY 14853 USA
| | - Jacob M Ganley
- Department of Chemistry, Princeton University Princeton NJ 08544 USA
| | - Brian G Ernst
- Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University Ithaca NY 14853 USA
| | - Cheyenne R Peltier
- Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University Ithaca NY 14853 USA
| | - Hsin-Yu Ko
- Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University Ithaca NY 14853 USA
| | - Robert A DiStasio
- Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University Ithaca NY 14853 USA
| | - Robert R Knowles
- Department of Chemistry, Princeton University Princeton NJ 08544 USA
| | - Geoffrey W Coates
- Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University Ithaca NY 14853 USA
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21
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Tsui E, Wang H, Knowles RR. Catalytic generation of alkoxy radicals from unfunctionalized alcohols. Chem Sci 2020; 11:11124-11141. [PMID: 33384861 PMCID: PMC7747465 DOI: 10.1039/d0sc04542j] [Citation(s) in RCA: 75] [Impact Index Per Article: 18.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2020] [Accepted: 09/21/2020] [Indexed: 12/12/2022] Open
Abstract
Alkoxy radicals have long been recognized as powerful synthetic intermediates with well-established reactivity patterns. Due to the high bond dissociation free energy of aliphatic alcohol O-H bonds, these radicals are difficult to access through direct homolysis, and conventional methods have instead relied on activation of O-functionalized precursors. Over the past decade, however, numerous catalytic methods for the direct generation of alkoxy radicals from simple alcohol starting materials have emerged and created opportunities for the development of new transformations. This minireview discusses recent advances in catalytic alkoxy radical generation, with particular emphasis on progress toward the direct activation of unfunctionalized alcohols enabled by transition metal and photoredox catalysis.
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Affiliation(s)
- Elaine Tsui
- Department of Chemistry , Princeton University , Princeton , NJ 08544 , USA .
| | - Huaiju Wang
- Department of Chemistry , Princeton University , Princeton , NJ 08544 , USA .
| | - Robert R Knowles
- Department of Chemistry , Princeton University , Princeton , NJ 08544 , USA .
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22
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Abstract
Aminium radical cations have been extensively studied as electrophilic aminating species that readily participate in C─N bond forming processes with alkenes and arenes. However, their utility in synthesis has been limited, as their generation required unstable, reactive starting materials and harsh reaction conditions. Visible-light photoredox catalysis has emerged as a platform for the mild production of aminium radical cations from either unfunctionalized or N-functionalized amines. This Perspective covers recent synthetic methods that rely on the photocatalytic generation of aminium radical cations for C─N bond formation, specifically in the context of alkene hydroamination, arene C─H bond amination, and the mesolytic bond cleavage of alkoxyamines.
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Affiliation(s)
- Jacob M Ganley
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Philip R D Murray
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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23
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Tsui E, Metrano AJ, Tsuchiya Y, Knowles RR. Catalytic Hydroetherification of Unactivated Alkenes Enabled by Proton-Coupled Electron Transfer. Angew Chem Int Ed Engl 2020; 59:11845-11849. [PMID: 32227658 PMCID: PMC7451027 DOI: 10.1002/anie.202003959] [Citation(s) in RCA: 38] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2020] [Revised: 03/28/2020] [Indexed: 12/22/2022]
Abstract
We report a catalytic, light-driven method for the intramolecular hydroetherification of unactivated alkenols to furnish cyclic ether products. These reactions occur under visible-light irradiation in the presence of an IrIII -based photoredox catalyst, a Brønsted base catalyst, and a hydrogen-atom transfer (HAT) co-catalyst. Reactive alkoxy radicals are proposed as key intermediates, generated by direct homolytic activation of alcohol O-H bonds through a proton-coupled electron-transfer mechanism. This method exhibits a broad substrate scope and high functional-group tolerance, and it accommodates a diverse range of alkene substitution patterns. Results demonstrating the extension of this catalytic system to carboetherification reactions are also presented.
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Affiliation(s)
- Elaine Tsui
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
| | - Anthony J Metrano
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
| | - Yuto Tsuchiya
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
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24
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Tsui E, Metrano AJ, Tsuchiya Y, Knowles RR. Catalytic Hydroetherification of Unactivated Alkenes Enabled by Proton‐Coupled Electron Transfer. Angew Chem Int Ed Engl 2020. [DOI: 10.1002/ange.202003959] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Affiliation(s)
- Elaine Tsui
- Department of Chemistry Princeton University Princeton NJ 08544 USA
| | | | - Yuto Tsuchiya
- Department of Chemistry Princeton University Princeton NJ 08544 USA
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25
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Shin NY, Ryss JM, Zhang X, Miller SJ, Knowles RR. Light -driven deracemization enabled by excited -state electron transfer. Science 2020; 366:364-369. [PMID: 31624212 DOI: 10.1126/science.aay2204] [Citation(s) in RCA: 127] [Impact Index Per Article: 31.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2019] [Accepted: 08/30/2019] [Indexed: 12/14/2022]
Abstract
Deracemization is an attractive strategy for asymmetric synthesis, but intrinsic energetic challenges have limited its development. Here, we report a deracemization method in which amine derivatives undergo spontaneous optical enrichment upon exposure to visible light in the presence of three distinct molecular catalysts. Initiated by an excited-state iridium chromophore, this reaction proceeds through a sequence of favorable electron, proton, and hydrogen-atom transfer steps that serve to break and reform a stereogenic C-H bond. The enantioselectivity in these reactions is jointly determined by two independent stereoselective steps that occur in sequence within the catalytic cycle, giving rise to a composite selectivity that is higher than that of either step individually. These reactions represent a distinct approach to creating out-of-equilibrium product distributions between substrate enantiomers using excited-state redox events.
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Affiliation(s)
- Nick Y Shin
- Department of Chemistry, Princeton University, Princeton, NJ 08544, USA
| | - Jonathan M Ryss
- Department of Chemistry, Yale University, New Haven, CT 06520, USA
| | - Xin Zhang
- Department of Chemistry, Princeton University, Princeton, NJ 08544, USA
| | - Scott J Miller
- Department of Chemistry, Yale University, New Haven, CT 06520, USA.
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, NJ 08544, USA.
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26
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Roos CB, Demaerel J, Graff DE, Knowles RR. Enantioselective Hydroamination of Alkenes with Sulfonamides Enabled by Proton-Coupled Electron Transfer. J Am Chem Soc 2020; 142:5974-5979. [PMID: 32182054 DOI: 10.1021/jacs.0c01332] [Citation(s) in RCA: 63] [Impact Index Per Article: 15.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
An enantioselective, radical-based method for the intramolecular hydroamination of alkenes with sulfonamides is reported. These reactions are proposed to proceed via N-centered radicals formed by proton-coupled electron transfer (PCET) activation of sulfonamide N-H bonds. Noncovalent interactions between the neutral sulfonamidyl radical and a chiral phosphoric acid generated in the PCET event are hypothesized to serve as the basis for asymmetric induction in a subsequent C-N bond forming step, achieving selectivities of up to 98:2 er. These results offer further support for the ability of noncovalent interactions to enforce stereoselectivity in reactions of transient and highly reactive open-shell intermediates.
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Affiliation(s)
- Casey B Roos
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Joachim Demaerel
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - David E Graff
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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27
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Affiliation(s)
- Suong T. Nguyen
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Philip R. D. Murray
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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28
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Miller DC, Ganley JM, Musacchio AJ, Sherwood TC, Ewing WR, Knowles RR. Anti-Markovnikov Hydroamination of Unactivated Alkenes with Primary Alkyl Amines. J Am Chem Soc 2019; 141:16590-16594. [PMID: 31603324 DOI: 10.1021/jacs.9b08746] [Citation(s) in RCA: 70] [Impact Index Per Article: 14.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We report here a photocatalytic method for the intermolecular anti-Markovnikov hydroamination of unactivated olefins with primary alkyl amines to selectively furnish secondary amine products. These reactions proceed through aminium radical cation (ARC) intermediates and occur at room temperature under visible light irradiation in the presence of an iridium photocatalyst and an aryl thiol hydrogen atom donor. Despite the presence of excess olefin, high selectivities are observed for secondary over tertiary amine products, even though the secondary amines are established substrates for ARC-based olefin amination under similar conditions.
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Affiliation(s)
- David C Miller
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Jacob M Ganley
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Andrew J Musacchio
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Trevor C Sherwood
- Discovery Chemistry , Bristol-Myers Squibb , Lawrenceville , New Jersey 08543 , United States
| | - William R Ewing
- Discovery Chemistry , Bristol-Myers Squibb , Lawrenceville , New Jersey 08543 , United States
| | - Robert R Knowles
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
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29
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Qiu G, Knowles RR. Understanding Chemoselectivity in Proton-Coupled Electron Transfer: A Kinetic Study of Amide and Thiol Activation. J Am Chem Soc 2019; 141:16574-16578. [PMID: 31573194 DOI: 10.1021/jacs.9b08398] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
While the mechanistic understanding of proton-coupled electron transfer (PCET) has advanced significantly, few reports have sought to elucidate the factors that control chemoselectivity in these reactions. Here we present a kinetic study that provides a quantitative basis for understanding the chemoselectivity in competitive PCET activations of amides and thiols relevant to catalytic olefin hydroamidation reactions. These results demonstrate how the interplay between PCET rate constants, hydrogen-bonding equilibria, and rate-driving force relationships jointly determine PCET chemoselectivity under a given set of conditions. In turn, these findings predict reactivity trends in a model hydroamidation reaction, rationalize the selective activation of amide N-H bonds in the presence of much weaker thiol S-H bonds, and deliver strategies to improve the efficiencies of PCET reactions employing thiol co-catalysts.
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Affiliation(s)
- Guanqi Qiu
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Robert R Knowles
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
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30
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Morton CM, Zhu Q, Ripberger H, Troian-Gautier L, Toa ZSD, Knowles RR, Alexanian EJ. C-H Alkylation via Multisite-Proton-Coupled Electron Transfer of an Aliphatic C-H Bond. J Am Chem Soc 2019; 141:13253-13260. [PMID: 31356059 DOI: 10.1021/jacs.9b06834] [Citation(s) in RCA: 87] [Impact Index Per Article: 17.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
The direct, site-selective alkylation of unactivated C(sp3)-H bonds in organic substrates is a long-standing goal in synthetic chemistry. General approaches to the activation of strong C-H bonds include radical-mediated processes involving highly reactive intermediates, such as heteroatom-centered radicals. Herein, we describe a catalytic, intermolecular C-H alkylation that circumvents such reactive species via a new elementary step for C-H cleavage involving multisite-proton-coupled electron transfer (multisite-PCET). Mechanistic studies indicate that the reaction is catalyzed by a noncovalent complex formed between an iridium(III) photocatalyst and a monobasic phosphate base. The C-H alkylation proceeds efficiently using diverse hydrocarbons and complex molecules as the limiting reagent and represents a new approach to the catalytic functionalization of unactivated C(sp3)-H bonds.
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Affiliation(s)
- Carla M Morton
- Department of Chemistry , The University of North Carolina at Chapel Hill , Chapel Hill , North Carolina 27599 , United States
| | - Qilei Zhu
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Hunter Ripberger
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Ludovic Troian-Gautier
- Department of Chemistry , The University of North Carolina at Chapel Hill , Chapel Hill , North Carolina 27599 , United States
| | - Zi S D Toa
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Robert R Knowles
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Erik J Alexanian
- Department of Chemistry , The University of North Carolina at Chapel Hill , Chapel Hill , North Carolina 27599 , United States
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31
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Zhao K, Yamashita K, Carpenter JE, Sherwood TC, Ewing WR, Cheng PTW, Knowles RR. Catalytic Ring Expansions of Cyclic Alcohols Enabled by Proton-Coupled Electron Transfer. J Am Chem Soc 2019; 141:8752-8757. [PMID: 31117664 DOI: 10.1021/jacs.9b03973] [Citation(s) in RCA: 63] [Impact Index Per Article: 12.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
We report here a catalytic method for the modular ring expansion of cyclic aliphatic alcohols. In this work, proton-coupled electron transfer activation of an allylic alcohol substrate affords an alkoxy radical intermediate that undergoes subsequent C-C bond cleavage to furnish an enone and a tethered alkyl radical. Recombination of this alkyl radical with the revealed olefin acceptor in turn produces a ring-expanded ketone product. The regioselectivity of this C-C bond-forming event can be reliably controlled via substituents on the olefin substrate, providing a means to convert a simple N-membered ring substrate to either n+1 or n+2 ring adducts in a selective fashion.
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Affiliation(s)
- Kuo Zhao
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Kenji Yamashita
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Joseph E Carpenter
- Discovery Chemistry , Bristol-Myers Squibb Co. , Princeton , New Jersey 08543 , United States
| | - Trevor C Sherwood
- Discovery Chemistry , Bristol-Myers Squibb Co. , Princeton , New Jersey 08543 , United States
| | - William R Ewing
- Discovery Chemistry , Bristol-Myers Squibb Co. , Princeton , New Jersey 08543 , United States
| | - Peter T W Cheng
- Discovery Chemistry , Bristol-Myers Squibb Co. , Princeton , New Jersey 08543 , United States
| | - Robert R Knowles
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
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32
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Abstract
Olefin aminations are important synthetic technologies for the construction of aliphatic C-N bonds. Here we report a catalytic protocol for olefin hydroamidation that proceeds through transient amidyl radical intermediates that are formed via proton-coupled electron transfer (PCET) activation of the strong N-H bonds in N-alkyl amides by an excited-state iridium photocatalyst and a dialkyl phosphate base. This method exhibits a broad substrate scope, high functional group tolerance, and amenability to use in cascade polycyclization reactions. The feasibility of this PCET protocol in enabling the intermolecular anti-Markovnikov hydroamidation reactions of unactivated olefins is also demonstrated.
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Affiliation(s)
- Suong T. Nguyen
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Qilei Zhu
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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33
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Sherwood TC, Xiao HY, Bhaskar RG, Simmons EM, Zaretsky S, Rauch MP, Knowles RR, Dhar TGM. Decarboxylative Intramolecular Arene Alkylation Using N-(Acyloxy)phthalimides, an Organic Photocatalyst, and Visible Light. J Org Chem 2019; 84:8360-8379. [DOI: 10.1021/acs.joc.9b00432] [Citation(s) in RCA: 36] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Trevor C. Sherwood
- Research and Development, Bristol-Myers Squibb Company, P.O. Box 4000, Princeton, New Jersey 08543-4000, United States
| | - Hai-Yun Xiao
- Research and Development, Bristol-Myers Squibb Company, P.O. Box 4000, Princeton, New Jersey 08543-4000, United States
| | - Roshan G. Bhaskar
- Research and Development, Bristol-Myers Squibb Company, P.O. Box 4000, Princeton, New Jersey 08543-4000, United States
| | - Eric M. Simmons
- Chemical and Synthetic Development, Bristol-Myers Squibb, 1 Squibb Drive, New Brunswick, New Jersey 08903, United States
| | - Serge Zaretsky
- Chemical and Synthetic Development, Bristol-Myers Squibb, 1 Squibb Drive, New Brunswick, New Jersey 08903, United States
| | - Martin P. Rauch
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - T. G. Murali Dhar
- Research and Development, Bristol-Myers Squibb Company, P.O. Box 4000, Princeton, New Jersey 08543-4000, United States
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34
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Wang D, Loose F, Chirik PJ, Knowles RR. N-H Bond Formation in a Manganese(V) Nitride Yields Ammonia by Light-Driven Proton-Coupled Electron Transfer. J Am Chem Soc 2019; 141:4795-4799. [PMID: 30803234 DOI: 10.1021/jacs.8b12957] [Citation(s) in RCA: 35] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
A method for the reduction of a manganese nitride to ammonia is reported, where light-driven proton-coupled electron transfer enables the formation of weak N-H bonds. Photoreduction of (saltBu)MnVN to ammonia and a Mn(II) complex has been accomplished using 9,10-dihydroacridine and a combination of an appropriately matched photoredox catalyst and weak Brønsted acid. Acid-reductant pairs with effective bond dissociation free energies between 35 and 46 kcal/mol exhibited high efficiencies. This light-driven method may provide a blueprint for new approaches to catalytic homogeneous ammonia synthesis under ambient conditions.
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Affiliation(s)
- Dian Wang
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Florian Loose
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Paul J Chirik
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Robert R Knowles
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
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35
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Abstract
Here we present a detailed kinetic study of the multisite proton-coupled electron transfer (MS-PCET) activations of aryl ketones using a variety of Brønsted acids and excited-state Ir(III)-based electron donors. A simple method is described for simultaneously extracting both the hydrogen-bonding equilibrium constants and the rate constants for the PCET event from deconvolution of the luminescence quenching data. These experiments confirm that these activations occur in a concerted fashion, wherein the proton and electron are transferred to the ketone substrate in a single elementary step. The rates constants for the PCET events were linearly correlated with their driving forces over a range of nearly 19 kcal/mol. However, the slope of the rate-driving force relationship deviated significantly from expectations based on Marcus theory. A rationalization for this observation is proposed based on the principle of non-perfect synchronization, wherein factors that serve to stabilize the product are only partially realized at the transition state. A discussion of the relevance of these findings to the applications of MS-PCET in organic synthesis is also presented.
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Affiliation(s)
- Guanqi Qiu
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Robert R Knowles
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
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36
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Ota E, Wang H, Frye NL, Knowles RR. A Redox Strategy for Light-Driven, Out-of-Equilibrium Isomerizations and Application to Catalytic C-C Bond Cleavage Reactions. J Am Chem Soc 2019; 141:1457-1462. [PMID: 30628777 DOI: 10.1021/jacs.8b12552] [Citation(s) in RCA: 125] [Impact Index Per Article: 25.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
We report a general protocol for the light-driven isomerization of cyclic aliphatic alcohols to linear carbonyl compounds. These reactions proceed via proton-coupled electron-transfer activation of alcohol O-H bonds followed by subsequent C-C β-scission of the resulting alkoxy radical intermediates. In many cases, these redox-neutral isomerizations proceed in opposition to a significant energetic gradient, yielding products that are less thermodynamically stable than the starting materials. A mechanism is presented to rationalize this out-of-equilibrium behavior that may serve as a model for the design of other contrathermodynamic transformations driven by excited-state redox events.
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Affiliation(s)
- Eisuke Ota
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Huaiju Wang
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Nils Lennart Frye
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
| | - Robert R Knowles
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States
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37
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Loose F, Wang D, Tian L, Scholes GD, Knowles RR, Chirik PJ. Evaluation of excited state bond weakening for ammonia synthesis from a manganese nitride: stepwise proton coupled electron transfer is preferred over hydrogen atom transfer. Chem Commun (Camb) 2019; 55:5595-5598. [PMID: 31025662 DOI: 10.1039/c9cc01046g] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Concepts for the thermodynamically challenging synthesis of weak N–H bonds by photoinduced proton coupled electron transfer are explored. By harvesting visible light as driving force, ammonia synthesis was achieved and mechanistically elucidated.
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Affiliation(s)
- Florian Loose
- Department of Chemistry
- Princeton University
- Frick Laboratory
- Princeton
- USA
| | - Dian Wang
- Department of Chemistry
- Princeton University
- Frick Laboratory
- Princeton
- USA
| | - Lei Tian
- Department of Chemistry
- Princeton University
- Frick Laboratory
- Princeton
- USA
| | | | - Robert R. Knowles
- Department of Chemistry
- Princeton University
- Frick Laboratory
- Princeton
- USA
| | - Paul J. Chirik
- Department of Chemistry
- Princeton University
- Frick Laboratory
- Princeton
- USA
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38
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Abstract
Triplet-triplet annihilation photon upconversion (TTA-UC) is a photophysical process in which the energy of two photons are combined into a single photon of higher energy. While this strategy has been recognized in applications ranging from bioimaging to solar energy conversion, its uses in synthetic organic chemistry have not been extensively developed. Here, we present a short tutorial on the theoretical underpinnings and the design principles of TTA-UC systems. Selected applications are then discussed to highlight key features of the TTA mechanism, along with a prospective discussion of the potential of these mechanisms to enable innovation in photocatalysis, methods development, and synthetic organic chemistry.
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Affiliation(s)
- Martin P Rauch
- Department of Chemistry Princeton University Princeton, New Jersey 08544, United States
| | - Robert R Knowles
- Department of Chemistry Princeton University Princeton, New Jersey 08544, United States;,
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39
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Gentry EC, Rono LJ, Hale ME, Matsuura R, Knowles RR. Enantioselective Synthesis of Pyrroloindolines via Noncovalent Stabilization of Indole Radical Cations and Applications to the Synthesis of Alkaloid Natural Products. J Am Chem Soc 2018; 140:3394-3402. [PMID: 29432006 PMCID: PMC5896747 DOI: 10.1021/jacs.7b13616] [Citation(s) in RCA: 143] [Impact Index Per Article: 23.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
While interest in the synthetic chemistry of radical cations continues to grow, controlling enantioselectivity in the reactions of these intermediates remains a challenge. Based on recent insights into the oxidation of tryptophan in enzymatic systems, we report a photocatalytic method for the generation of indole radical cations as hydrogen-bonded adducts with chiral phosphate anions. These noncovalent open-shell complexes can be intercepted by the stable nitroxyl radical TEMPO· to form alkoxyamine-substituted pyrroloindolines with high levels of enantioselectivity. Further elaboration of these optically enriched adducts can be achieved via a catalytic single-electron oxidation/mesolytic cleavage sequence to furnish transient carbocation intermediates that may be intercepted by a wide range of nucleophiles. Taken together, this two-step sequence provides a simple catalytic method to access a wide range of substituted pyrroloindolines in enantioenriched form via a standard experimental protocol from a common synthetic intermediate. The design, development, mechanistic study, and scope of this process are presented, as are applications of this method to the synthesis of several dimeric pyrroloindoline natural products.
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Affiliation(s)
- Emily C. Gentry
- Department of Chemistry, Princeton University, Princeton NJ 08544, USA
| | - Lydia J. Rono
- Department of Chemistry, Princeton University, Princeton NJ 08544, USA
| | - Martina E. Hale
- Department of Chemistry, Princeton University, Princeton NJ 08544, USA
| | - Rei Matsuura
- Department of Chemistry, Princeton University, Princeton NJ 08544, USA
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton NJ 08544, USA
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40
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Zhu Q, Graff DE, Knowles RR. Intermolecular Anti-Markovnikov Hydroamination of Unactivated Alkenes with Sulfonamides Enabled by Proton-Coupled Electron Transfer. J Am Chem Soc 2018; 140:741-747. [PMID: 29268020 PMCID: PMC6247111 DOI: 10.1021/jacs.7b11144] [Citation(s) in RCA: 117] [Impact Index Per Article: 19.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Here we report a catalytic method for the intermolecular anti-Markovnikov hydroamination of unactivated alkenes using primary and secondary sulfonamides. These reactions occur at room temperature under visible light irradiation and are jointly catalyzed by an iridium(III) photocatalyst, a dialkyl phosphate base, and a thiol hydrogen atom donor. Reaction outcomes are consistent with the intermediacy of an N-centered sulfonamidyl radical generated via proton-coupled electron transfer activation of the sulfonamide N-H bond. Studies outlining the synthetic scope (>60 examples) and mechanistic features of the reaction are presented.
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Affiliation(s)
- Qilei Zhu
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - David E. Graff
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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41
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Musacchio AJ, Lainhart BC, Zhang X, Naguib SG, Sherwood TC, Knowles RR. Catalytic intermolecular hydroaminations of unactivated olefins with secondary alkyl amines. Science 2017; 355:727-730. [PMID: 28209894 DOI: 10.1126/science.aal3010] [Citation(s) in RCA: 245] [Impact Index Per Article: 35.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2016] [Accepted: 01/10/2017] [Indexed: 01/14/2023]
Abstract
The intermolecular hydroamination of unactivated alkenes with simple dialkyl amines remains an unsolved problem in organic synthesis. We report a catalytic protocol for efficient additions of cyclic and acyclic secondary alkyl amines to a wide range of alkyl olefins with complete anti-Markovnikov regioselectivity. In this process, carbon-nitrogen bond formation proceeds through a key aminium radical cation intermediate that is generated via electron transfer between an excited-state iridium photocatalyst and an amine substrate. These reactions are redox-neutral and completely atom-economical, exhibit broad functional group tolerance, and occur readily at room temperature under visible light irradiation. Certain tertiary amine products generated through this method are formally endergonic relative to their constituent olefin and amine starting materials and thus are not accessible via direct coupling with conventional ground-state catalysts.
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Affiliation(s)
| | | | - Xin Zhang
- Department of Chemistry, Princeton University, Princeton, NJ 08544, USA
| | - Saeed G Naguib
- Department of Chemistry, Princeton University, Princeton, NJ 08544, USA
| | - Trevor C Sherwood
- Discovery Chemistry, Bristol-Myers Squibb Co., Princeton, NJ 08543, USA
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, NJ 08544, USA.
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42
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Yayla HG, Wang H, Tarantino KT, Orbe HS, Knowles RR. Catalytic Ring-Opening of Cyclic Alcohols Enabled by PCET Activation of Strong O-H Bonds. J Am Chem Soc 2016; 138:10794-7. [PMID: 27515494 DOI: 10.1021/jacs.6b06517] [Citation(s) in RCA: 225] [Impact Index Per Article: 28.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
We report a new photocatalytic protocol for the redox-neutral isomerization of cyclic alcohols to linear ketones via C-C bond scission. Mechanistic studies demonstrate that key alkoxy radical intermediates in this reaction are generated via the direct homolytic activation of alcohol O-H bonds in an unusual intramolecular PCET process, wherein the electron travels to a proximal radical cation in concert with proton transfer to a weak Brønsted base. Effective bond strength considerations are shown to accurately forecast the feasibility of alkoxy radical generation with a given oxidant/base pair.
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Affiliation(s)
- Hatice G Yayla
- Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States
| | - Huaiju Wang
- Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States
| | - Kyle T Tarantino
- Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States
| | - Hudson S Orbe
- Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States
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43
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Abstract
Redox events in which an electron and proton are exchanged in a concerted elementary step are commonly referred to as proton-coupled electron transfers (PCETs). PCETs are known to operate in numerous important biological redox processes, as well as recent inorganic technologies for small molecule activation. These studies suggest that PCET catalysis might also function as a general mode of substrate activation in organic synthesis. Over the past three years, our group has worked to advance this hypothesis and to demonstrate the synthetic utility of PCET through the development of novel catalytic radical chemistries. The central aim of these efforts has been to demonstrate the ability of PCET to homolytically activate a wide variety of common organic functional groups that are energetically inaccessible using known molecular H atom transfer catalysts. To do so, we made use of a simple formalism first introduced by Mayer and co-workers that allowed us to predict the thermodynamic capacity of any oxidant/base or reductant/acid pair to formally add or remove H· from a given substrate. With this insight, we were able to rationally select catalyst combinations thermodynamically competent to homolyze the extraordinarily strong E-H σ-bonds found in many common protic functional groups (BDFEs > 100 kcal/mol) or to form unusually weak bonds to hydrogen via the reductive action of common organic π-systems (BDFEs < 35 kcal/mol). These ideas were reduced to practice through the development of new catalyst systems for reductive PCET activations of ketones and oxidative PCET activation of amide N-H bonds to directly furnish reactive ketyl and amidyl radicals, respectively. In both systems, the reaction outcomes were found to be successfully predicted using the effective bond strength formalism, suggesting that these simple thermochemical considerations can provide useful and actionable insights into PCET reaction design. The ability of PCET catalysis to control enantioselectivity in free radical processes has also been established. Specifically, multisite PCET requires the formation of a pre-equilibrium hydrogen bond between the substrate and a proton donor/acceptor prior to charge transfer. We recognized that these H-bond interfaces persist following the PCET event, resulting in the formation of noncovalent complexes of the nascent radical intermediates. When chiral proton donors/acceptors are employed, this association can provide a basis for asymmetric induction in subsequent bond-forming steps. We discuss our efforts to capitalize on this understanding via the development of a catalytic protocol for enantioselective aza-pinacol cyclizations. Lastly, we highlight an alternative PCET mechanism that exploits the ability of redox-active metals to homolytically weaken the bonds in coordinated ligands, enabling nominally strong bonds (BDFEs ∼ 100 kcal) to be abstracted by weak H atom acceptors with concomitant oxidation of the metal center. This "soft homolysis" mechanism enables the generation of metalated intermediates from protic substrates under completely neutral conditions. The first example of this form of catalysis is presented in the context of a catalytic C-N bond forming reaction jointly mediated by bulky titanocene complexes and the stable nitroxyl radical TEMPO.
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Affiliation(s)
- Emily C. Gentry
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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44
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Zhu Q, Gentry EC, Knowles RR. Catalytic Carbocation Generation Enabled by the Mesolytic Cleavage of Alkoxyamine Radical Cations. Angew Chem Int Ed Engl 2016; 55:9969-73. [PMID: 27403637 DOI: 10.1002/anie.201604619] [Citation(s) in RCA: 62] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2016] [Indexed: 01/11/2023]
Abstract
A new catalytic method is described to access carbocation intermediates via the mesolytic cleavage of alkoxyamine radical cations. In this process, electron transfer between an excited state oxidant and a TEMPO-derived alkoxyamine substrate gives rise to a radical cation with a remarkably weak C-O bond. Spontaneous scission results in the formation of the stable nitroxyl radical TEMPO(.) as well as a reactive carbocation intermediate that can be intercepted by a wide range of nucleophiles. Notably, this process occurs under neutral conditions and at comparatively mild potentials, enabling catalytic cation generation in the presence of both acid sensitive and easily oxidized nucleophilic partners.
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Affiliation(s)
- Qilei Zhu
- Department of Chemistry, Princeton University, Princeton, New Jersey, 08544, USA
| | - Emily C Gentry
- Department of Chemistry, Princeton University, Princeton, New Jersey, 08544, USA
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey, 08544, USA.
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45
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Miller DC, Tarantino KT, Knowles RR. Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities. Top Curr Chem (Cham) 2016; 374:30. [PMID: 27573270 PMCID: PMC5107260 DOI: 10.1007/s41061-016-0030-6] [Citation(s) in RCA: 78] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2015] [Accepted: 04/21/2016] [Indexed: 10/21/2022]
Abstract
Proton-coupled electron transfers (PCETs) are unconventional redox processes in which both protons and electrons are exchanged, often in a concerted elementary step. While PCET is now recognized to play a central a role in biological redox catalysis and inorganic energy conversion technologies, its applications in organic synthesis are only beginning to be explored. In this chapter, we aim to highlight the origins, development, and evolution of the PCET processes most relevant to applications in organic synthesis. Particular emphasis is given to the ability of PCET to serve as a non-classical mechanism for homolytic bond activation that is complimentary to more traditional hydrogen atom transfer processes, enabling the direct generation of valuable organic radical intermediates directly from their native functional group precursors under comparatively mild catalytic conditions. The synthetically advantageous features of PCET reactivity are described in detail, along with examples from the literature describing the PCET activation of common organic functional groups.
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Affiliation(s)
- David C Miller
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
| | - Kyle T Tarantino
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA.
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46
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Affiliation(s)
- Lucas Q. Nguyen
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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47
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Yayla HG, Peng F, Mangion IK, McLaughlin M, Campeau LC, Davies IW, DiRocco DA, Knowles RR. Discovery and mechanistic study of a photocatalytic indoline dehydrogenation for the synthesis of elbasvir. Chem Sci 2016; 7:2066-2073. [PMID: 29899932 PMCID: PMC5968518 DOI: 10.1039/c5sc03350k] [Citation(s) in RCA: 85] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2015] [Accepted: 11/27/2015] [Indexed: 12/14/2022] Open
Abstract
Elbasvir is a potent NS5A antagonist for the treatment of chronic hepatitis C. A seemingly trivial indoline oxidation en route to the target compound was complicated by epimerization of a stereogenic hemiaminal center under most standard oxidation conditions. To address this issue, a novel visible light photoredox process for indoline oxidation was developed involving an iridium photosensitizer and environmentally-benign perester oxidant. The reaction was discovered through a high-throughput experimentation campaign and the optimized process was demonstrated on 100 g scale in flow to afford a key intermediate towards the target compound. A battery of kinetic, electrochemical, and spectroscopic studies of this process indicates a radical chain mechanism of dehydrogenation involving selective HAT from the substrate by an alkoxy radicals. Notably, isotope effects were used to validate the chain mechanism when quantum yield data proved ambiguous.
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Affiliation(s)
- Hatice G Yayla
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , USA .
| | - Feng Peng
- Department of Process Chemistry , Merck & Co., Inc. Rahway , New Jersey 07065 , USA .
| | - Ian K Mangion
- Department of Process Chemistry , Merck & Co., Inc. Rahway , New Jersey 07065 , USA .
| | - Mark McLaughlin
- Department of Process Chemistry , Merck & Co., Inc. Rahway , New Jersey 07065 , USA .
| | - Louis-Charles Campeau
- Department of Process Chemistry , Merck & Co., Inc. Rahway , New Jersey 07065 , USA .
| | - Ian W Davies
- Department of Process Chemistry , Merck & Co., Inc. Rahway , New Jersey 07065 , USA .
| | - Daniel A DiRocco
- Department of Process Chemistry , Merck & Co., Inc. Rahway , New Jersey 07065 , USA .
| | - Robert R Knowles
- Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , USA .
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48
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Abstract
Here we report a ternary catalyst system for the intramolecular hydroamidation of unactivated olefins using simple N-aryl amide derivatives. Amide activation in these reactions occurs via concerted proton-coupled electron transfer (PCET) mediated by an excited state iridium complex and weak phosphate base to furnish a reactive amidyl radical that readily adds to pendant alkenes. A series of H-atom, electron, and proton transfer events with a thiophenol cocatalyst furnish the product and regenerate the active forms of the photocatalyst and base. Mechanistic studies indicate that the amide substrate can be selectively homolyzed via PCET in the presence of the thiophenol, despite a large difference in bond dissociation free energies between these functional groups.
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Affiliation(s)
- David
C. Miller
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Gilbert J. Choi
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Hudson S. Orbe
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
| | - Robert R. Knowles
- Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States
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49
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Knowles RR. Reaching Your Full (Over)Potential: A Novel Approach to Electrocatalytic Oxygen Reduction. ACS Cent Sci 2015; 1:224-225. [PMID: 27162976 PMCID: PMC4827482 DOI: 10.1021/acscentsci.5b00257] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
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50
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Abstract
Here we describe a dual catalyst system comprised of an iridium photocatalyst and weak phosphate base that is capable of both selectively homolyzing the N-H bonds of N-arylamides (bond dissociation free energies ∼ 100 kcal/mol) via concerted proton-coupled electron transfer (PCET) and mediating efficient carboamination reactions of the resulting amidyl radicals. This manner of PCET activation, which finds its basis in numerous biological redox processes, enables the formal homolysis of a stronger amide N-H bond in the presence of weaker allylic C-H bonds, a selectivity that is uncommon in conventional molecular H atom acceptors. Moreover, this transformation affords access to a broad range of structurally complex heterocycles from simple amide starting materials. The design, synthetic scope, and mechanistic evaluation of the PCET process are described.
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Affiliation(s)
- Gilbert J Choi
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Robert R Knowles
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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