1
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Li H, Fan M, Liu Q. Unveiling the Unique Reactivity of Anionic Mn(I) Complexes via Metal-Ligand Cooperation: Nucleophilic Attack on C(sp 3)-X Bonds. J Am Chem Soc 2024; 146:26649-26656. [PMID: 39295280 DOI: 10.1021/jacs.4c01683] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/21/2024]
Abstract
Metal-ligand cooperation (MLC) has emerged as a pivotal strategy for the catalytic activation of small molecules within both synthetic and biological arenas. Leveraging this approach, a suite of potent catalytic reactions─encompassing hydrogenation, hydroelementation, and dehydrogenative processes─have been realized, with notable advances in manganese catalysis in recent years. However, the activation of alkyl halides by Mn complexes, which typically requires strong reductants to form Mn(-I) complexes that are incompatible with standard cross-coupling conditions, remains a significant challenge. This limitation underscores the urgent need to investigate alternative methods for activating C(sp3)-X bonds using higher valence state Mn complexes. In response to this challenge, we present the synthesis, characterization, and reactivity of a new anionic Mn(I) complex featuring a redox-active dianionic ligand that induces multiple MLC functionalities. We have discovered an innovative mechanism of MLC, characterized by a single ligand transferring two electrons to the metal center. This novel process facilitates an orbital-symmetry-allowed nucleophilic attack on C(sp3)-X bonds, preserving manganese's oxidative state at +1. To the best of our knowledge, this is the first instance where the MLC strategy via a two-electron transfer process has been utilized to execute an SN2 nucleophilic attack at a C(sp3)-X bond by a relatively electron-deficient metal center like Mn(I). Additionally, the dianionic ligand of the anionic Mn(I) complex exhibits ambident nucleophilicity by reacting with different electrophiles, further highlighting its versatile reactivity.
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Affiliation(s)
- Hengxu Li
- Center of Basic Molecular Science, Department of Chemistry, Tsinghua University, Beijing 100084, China
| | - Mingjie Fan
- Center of Basic Molecular Science, Department of Chemistry, Tsinghua University, Beijing 100084, China
| | - Qiang Liu
- Center of Basic Molecular Science, Department of Chemistry, Tsinghua University, Beijing 100084, China
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2
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Cagan D, Bím D, Kazmierczak NP, Hadt RG. Mechanisms of Photoredox Catalysis Featuring Nickel-Bipyridine Complexes. ACS Catal 2024; 14:9055-9076. [PMID: 38868098 PMCID: PMC11165457 DOI: 10.1021/acscatal.4c02036] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2024] [Revised: 05/07/2024] [Accepted: 05/16/2024] [Indexed: 06/14/2024]
Abstract
Metallaphotoredox catalysis can unlock useful pathways for transforming organic reactants into desirable products, largely due to the conversion of photon energy into chemical potential to drive redox and bond transformation processes. Despite the importance of these processes for cross-coupling reactions and other transformations, their mechanistic details are only superficially understood. In this review, we have provided a detailed summary of various photoredox mechanisms that have been proposed to date for Ni-bipyridine (bpy) complexes, focusing separately on photosensitized and direct excitation reaction processes. By highlighting multiple bond transformation pathways and key findings, we depict how photoredox reaction mechanisms, which ultimately define substrate scope, are themselves defined by the ground- and excited-state geometric and electronic structures of key Ni-based intermediates. We further identify knowledge gaps to motivate future mechanistic studies and the development of synergistic research approaches spanning the physical, organic, and inorganic chemistry communities.
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Affiliation(s)
- David
A. Cagan
- Division
of Chemistry and Chemical Engineering, Arthur Amos Noyes Laboratory
of Chemical Physics, California Institute
of Technology, Pasadena, California 91125, United States
| | - Daniel Bím
- Institute
of Organic Chemistry and Biochemistry, The
Czech Academy of Sciences, Flemingovo nám. 2, Prague 6 166 10, Czech Republic
| | - Nathanael P. Kazmierczak
- Division
of Chemistry and Chemical Engineering, Arthur Amos Noyes Laboratory
of Chemical Physics, California Institute
of Technology, Pasadena, California 91125, United States
| | - Ryan G. Hadt
- Division
of Chemistry and Chemical Engineering, Arthur Amos Noyes Laboratory
of Chemical Physics, California Institute
of Technology, Pasadena, California 91125, United States
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3
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Sutcliffe E, Cagan DA, Hadt RG. Ultrafast Photophysics of Ni(I)-Bipyridine Halide Complexes: Spanning the Marcus Normal and Inverted Regimes. J Am Chem Soc 2024; 146:15506-15514. [PMID: 38776490 PMCID: PMC11157544 DOI: 10.1021/jacs.4c04091] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2024] [Revised: 05/09/2024] [Accepted: 05/13/2024] [Indexed: 05/25/2024]
Abstract
Owing to their light-harvesting properties, nickel-bipyridine (bpy) complexes have found wide use in metallaphotoredox cross-coupling reactions. Key to these transformations are Ni(I)-bpy halide intermediates that absorb a significant fraction of light at relevant cross-coupling reaction irradiation wavelengths. Herein, we report ultrafast transient absorption (TA) spectroscopy on a library of eight Ni(I)-bpy halide complexes, the first such characterization of any Ni(I) species. The TA data reveal the formation and decay of Ni(I)-to-bpy metal-to-ligand charge transfer (MLCT) excited states (10-30 ps) whose relaxation dynamics are well described by vibronic Marcus theory, spanning the normal and inverted regions as a result of simple changes to the bpy substituents. While these lifetimes are relatively long for MLCT excited states in first-row transition metal complexes, their duration precludes excited-state bimolecular reactivity in photoredox reactions. We also present a one-step method to generate an isolable, solid-state Ni(I)-bpy halide species, which decouples light-initiated reactivity from dark, thermal cycles in catalysis.
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Affiliation(s)
| | | | - Ryan G. Hadt
- Division of Chemistry and
Chemical Engineering, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, United States
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4
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Huang H, Alvarez-Hernandez JL, Hazari N, Mercado BQ, Uehling MR. Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling. ACS Catal 2024; 14:6897-6914. [PMID: 38737398 PMCID: PMC11087080 DOI: 10.1021/acscatal.4c00827] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/14/2024]
Abstract
A family of 4,4'-tBu2-2,2'-bipyridine (tBubpy) ligands with substituents in either the 6-position, 4,4'-tBu2-6-Me-bpy (tBubpyMe), or 6 and 6'-positions, 4,4'-tBu2-6,6'-R2-bpy (tBubpyR2; R = Me, iPr, sBu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the tBubpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of tBubpy better stabilized (tBubpyR2)NiICl species and resulted in cleaner reduction from (tBubpyR2)NiIICl2. However, bulkier substituents hindered or prevented coordination of tBubpyR2 ligands to Ni0(cod)2. In addition, by using complexes of the type (tBubpyMe)NiCl2 and (tBubpyR2)NiCl2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while (tBubpyMe)NiIICl2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing tBubpyR2 ligands. A detailed study on the catalytic intermediates (tBubpy)Ni(Ar)I and (tBubpyMe2)Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type (tBubpy)Ni(Ar)I are low spin and relatively stable, complexes of the type (tBubpyMe2)Ni(Ar)I are high-spin and less stable. Further, (tBubpyMe2)Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than (tBubpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.
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Affiliation(s)
- Haotian Huang
- Department of Chemistry, Yale University, P. O. Box 208107, New Haven, Connecticut, 06520, USA
| | | | - Nilay Hazari
- Department of Chemistry, Yale University, P. O. Box 208107, New Haven, Connecticut, 06520, USA
| | - Brandon Q Mercado
- Department of Chemistry, Yale University, P. O. Box 208107, New Haven, Connecticut, 06520, USA
| | - Mycah R Uehling
- Merck & Co., Inc., Discovery Chemistry, HTE and Lead Discovery Capabilities, Rahway, New Jersey, 07065, USA
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5
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Khrizanforova VV, Fayzullin RR, Kartashov SV, Morozov VI, Khrizanforov MN, Gerasimova TP, Budnikova YH. Carbon Dioxide Electroreduction and Formic Acid Oxidation by Formal Nickel(I) Complexes of Di-isopropylphenyl Bis-iminoacenaphthene. Chemistry 2024; 30:e202400168. [PMID: 38380792 DOI: 10.1002/chem.202400168] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2024] [Revised: 02/20/2024] [Accepted: 02/20/2024] [Indexed: 02/22/2024]
Abstract
Processing CO2 into value-added chemicals and fuels stands as one of the most crucial tasks in addressing the global challenge of the greenhouse effect. In this study, we focused on the complex (dpp-bian)NiBr2 (where dpp-bian is di-isopropylphenyl bis-iminoacenaphthene) as a precatalyst for the electrochemical reduction of CO2 into CH4 as the sole product. Cyclic voltammetry results indicate that the realization of a catalytically effective pattern requires the three-electron reduction of (dpp-bian)NiBr2. The chemically reduced complexes [K(THF)6]+[(dpp-bian)Ni(COD)]- and [K(THF)6]+[(dpp-bian)2Ni]- were synthesized and structurally characterized. Analyzing the data from the electron paramagnetic resonance study of the complexes in solutions, along with quantum-chemical calculations, reveals that the spin density is predominantly localized at their metal centers. The superposition of trajectory maps of the electron density gradient vector field∇ ρ r ${\nabla \rho \left({\bf r}\right)}$ and the electrostatic force density fieldF e s r ${{{\bf F}}_{{\rm e}{\rm s}}\left({\bf r}\right)}$ per electron, as well as the atomic charges, discloses that, within the first coordination sphere, the interatomic charge transfer occurs from the metal atom to the ligand atoms and that the complex anions can thus be formally described by the general formulae (dpp-bian)2-Ni+(COD) and (dpp-bian)2 -Ni+. It was also shown that the reduced nickel complexes can be oxidized by formic acid; resulting from this reaction, the two-electron and two-proton addition product dpp-bian-2H is formed.
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Affiliation(s)
- Vera V Khrizanforova
- Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences, 8 Arbuzov Street, Kazan, 420088, Russian Federation
| | - Robert R Fayzullin
- Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences, 8 Arbuzov Street, Kazan, 420088, Russian Federation
| | - Sergey V Kartashov
- Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences, 8 Arbuzov Street, Kazan, 420088, Russian Federation
| | - Vladimir I Morozov
- Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences, 8 Arbuzov Street, Kazan, 420088, Russian Federation
| | - Mikhail N Khrizanforov
- Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences, 8 Arbuzov Street, Kazan, 420088, Russian Federation
| | - Tatiana P Gerasimova
- Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences, 8 Arbuzov Street, Kazan, 420088, Russian Federation
| | - Yulia H Budnikova
- Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences, 8 Arbuzov Street, Kazan, 420088, Russian Federation
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6
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Morris RH. Reactivity umpolung (reversal) of ligands in transition metal complexes. Chem Soc Rev 2024; 53:2808-2827. [PMID: 38353155 DOI: 10.1039/d3cs00979c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/19/2024]
Abstract
The success and power of homogeneous catalysis derives in large part from the wide choice of transition metal ions and their ligands. This tutorial review introduces examples where the reactivity of a ligand is completely reversed (umpolung) from Lewis basic/nucleophilic to acidic/electrophilic or vice versa on changing the metal and co-ligands. Understanding this phenomenon will assist in the rational design of catalysts and the understanding of metalloenzyme mechanisms. Labelling a metal and ligand with Seebach donor and acceptor labels helps to identify whether a reaction involving the intermolecular attack on the ligand is displaying native reactivity or reactivity umpolung. This has been done for complexes of nitriles, carbonyls, isonitriles, dinitrogen, Fischer carbenes, alkenes, alkynes, hydrides, methyls, methylidenes and alkylidenes, silylenes, oxides, imides/nitrenes, alkylidynes, methylidynes, and nitrides. The electronic influence of the metal and co-ligands is discussed in terms of the energy of (HOMO) d electrons. The energy can be related to the pKLACa (LAC is ligand acidity constant) of the theoretical hydride complexes [H-[M]-L]+ formed by the protonation of pair of valence d electrons on the metal in the [M-L] complex. Preliminary findings indicate that a negative pKLACa indicates that nucleophilic attack by a carbanion or amine on the ligand will likely occur while a positive pKLACa indicates that electrophilic attack by strong acids on the ligand will usually occur when the ligand is nitrile, carbonyl, isonitrile, alkene and η6-arene.
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Affiliation(s)
- Robert H Morris
- Department of Chemistry, University of Toronto, 80 Saint George St., Toronto, Ontario, Canada, M5S3H6.
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7
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Baldinelli L, Belanzoni P, Bistoni G. Mechanism of Nitrous Oxide Activation in C(sp 2)-O Bond Formation Reactions Catalyzed by Nickel Complexes. J Am Chem Soc 2024; 146:6016-6024. [PMID: 38377396 DOI: 10.1021/jacs.3c12922] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/22/2024]
Abstract
Recent groundbreaking experimental reports demonstrated that Ni complexes bearing a bidentate- or tridentate-bipyridine-based ligand can be used to activate N2O for use as an O-transfer agent in C(sp2)-O bond formation reactions under mild experimental conditions. In this work, quantum chemical calculations are used to shed light on the mechanism through which such metal complexes catalytically activate nitrous oxide, providing new fundamental insights into the development of novel catalysts for N2O revalorization. As a case study, we consider the recent work by Cornella and co-workers (Nature, 2022, 604, 677) concerning the synthesis of phenols from aryl halides at room temperature, which requires the use of an external reducing agent. Our results suggest that the metal center remains in its Ni(II) oxidation state throughout the whole catalytic cycle, despite the presence of various redox steps in the mechanism and the Ni ability to maneuver between a number of oxidation states. This counterintuitive behavior is made possible by the ligand redox activity in the catalytic process, which involves accepting electrons from the reducing agent. Several possible pathways are systematically investigated, each associated with distinct activation modes, kinetics, and reaction outcomes. The governing factors in dictating the preferred path lie in the electronic nature of the ligand (strong vs weak field) and its geometric structure (specifically, the number of coordinating arms). These characteristics play a pivotal role in determining whether the process follows a catalytic or stoichiometric route and can be in principle modulated for the design of new metal complexes with tailored redox properties and reactivity.
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Affiliation(s)
- Lorenzo Baldinelli
- Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy
| | - Paola Belanzoni
- Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy
| | - Giovanni Bistoni
- Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy
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8
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Chen LM, Reisman SE. Enantioselective C(sp 2)-C(sp 3) Bond Construction by Ni Catalysis. Acc Chem Res 2024; 57:751-762. [PMID: 38346006 PMCID: PMC10918837 DOI: 10.1021/acs.accounts.3c00775] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2023] [Revised: 01/10/2024] [Accepted: 01/16/2024] [Indexed: 03/06/2024]
Abstract
ConspectusAfter decades of palladium dominating the realm of transition-metal-catalyzed cross-coupling, recent years have witnessed exciting advances in the development of new nickel-catalyzed cross-coupling reactions to form C(sp3) centers. Nickel possesses distinct properties compared with palladium, such as facile single-electron transfer to C(sp3) electrophiles and rapid C-C reductive elimination from NiIII. These properties, among others, make nickel particularly well-suited for reductive cross-coupling (RCC) in which two electrophiles are coupled and an exogenous reductant is used to turn over the metal catalyst. Ni-catalyzed RCCs use readily available and stable electrophiles as starting materials and exhibit good functional group tolerance, which makes them appealing for applications in the synthesis of complex molecules. Building upon the foundational work in Ni-catalyzed RCCs by the groups of Kumada, Durandetti, Weix, and others, as well as the advancements in Ni-catalyzed enantioselective redox-neutral cross-couplings led by Fu and co-workers, we initiated a program to explore the feasibility of developing highly enantioselective Ni-catalyzed RCCs. Our research has also been driven by a keen interest in unraveling the factors contributing to enantioinduction and electrophile activation as we seek new avenues for advancing our understanding and further developing these reactions.In the first part of this Account, we organize our reported methods on the basis of the identity of the C(sp3) electrophiles, including benzylic chlorides, N-hydroxyphthalimide (NHP) esters, and α-chloro esters and nitriles. We highlight how the selection of specific chiral ligands plays a pivotal role in achieving high cross-selectivity and enantioselectivity. In addition, we show that reduction can be accomplished not only with heterogeneous reductants, such as Mn0, but also with the soluble organic reductant tetrakis(dimethylamino)ethylene (TDAE), as well as electrochemically. The use of homogeneous reductants, such as TDAE, is well suited for studying the mechanism of the transformation. Although this Account primarily focuses on RCCs, we also highlight our work using trifluoroborate (BF3K) salts as radical precursors for enantioselective dual-Ni/photoredox systems.At the end of this Account, we summarize the relevant mechanistic studies of two closely related asymmetric reductive alkenylation reactions developed in our laboratory and provide a context between our work and related mechanistic studies by others. We discuss how the ligand properties influence the rates and mechanisms of electrophile activation and how understanding the mode of C(sp3) radical generation can be used to optimize the yield of an RCC. Our research endeavors to offer insights on the intricate mechanisms at play in asymmetric Ni-catalyzed RCCs with the goal of using the rate of electrophile activation to improve the substrate scope of enantioselective RCCs. We anticipate that the insights we share in this Account can provide guidance for the development of new methods in this field.
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Affiliation(s)
- Li-Ming Chen
- The
Warren and Katharine Schlinger Laboratory for Chemistry and Chemical
Engineering, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
| | - Sarah E. Reisman
- The
Warren and Katharine Schlinger Laboratory for Chemistry and Chemical
Engineering, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
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9
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Pahar S, Sharma V, Raj KV, Sangole MP, George CP, Singh K, Vanka K, Gonnade RG, Sen SS. Tridentate NacNac Tames T-Shaped Nickel(I) Radical. Chemistry 2024; 30:e202303957. [PMID: 38051591 DOI: 10.1002/chem.202303957] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2023] [Revised: 12/05/2023] [Accepted: 12/05/2023] [Indexed: 12/07/2023]
Abstract
The reaction of a nickel(II) chloride complex containing a tridentate β-diketiminato ligand with a picolyl group [2,6-iPr2 -C6 H3 NC(Me)CHC(Me)NH(CH2 py)]Ni(II)Cl (1)] with KSi(SiMe3 )3 conveniently afforded a nickel(I) radical with a T-shaped geometry (2). The compound's metalloradical nature was confirmed through electron paramagnetic resonance (EPR) studies and its reaction with TEMPO, resulting in the formation of a highly unusual three-membered nickeloxaziridine complex (3). When reacted with disulfide and diselenide, the S-S and Se-Se bonds were cleaved, and a coupled product was formed through carbon atom of the pyridine-imine group. The nickel(I) radical activates dihydrogen at room temperature and atmospheric pressure to give the monomeric nickel hydride.
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Affiliation(s)
- Sanjukta Pahar
- Inorganic Chemistry and Catalysis Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India
| | - Vishal Sharma
- Inorganic Chemistry and Catalysis Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India
| | - K Vipin Raj
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India
- Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India
| | - Mayur P Sangole
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India
- Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India
| | - Christy P George
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India
- Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India
| | - Kirandeep Singh
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India
- Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India
| | - Kumar Vanka
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India
- Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India
| | - Rajesh G Gonnade
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India
- Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India
| | - Sakya S Sen
- Inorganic Chemistry and Catalysis Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India
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10
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Dawson G, Spielvogel EH, Diao T. Nickel-Catalyzed Radical Mechanisms: Informing Cross-Coupling for Synthesizing Non-Canonical Biomolecules. Acc Chem Res 2023; 56:3640-3653. [PMID: 38033206 PMCID: PMC10734253 DOI: 10.1021/acs.accounts.3c00588] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2023] [Revised: 10/30/2023] [Accepted: 11/03/2023] [Indexed: 12/02/2023]
Abstract
Nickel excels at facilitating selective radical chemistry, playing a pivotal role in metalloenzyme catalysis and modern cross-coupling reactions. Radicals, being nonpolar and neutral, exhibit orthogonal reactivity to nucleophilic and basic functional groups commonly present in biomolecules. Harnessing this compatibility, we delve into the application of nickel-catalyzed radical pathways in the synthesis of noncanonical peptides and carbohydrates, critical for chemical biology studies and drug discovery.We previously characterized a sequential reduction mechanism that accounts for chemoselectivity in cross-electrophile coupling reactions. This catalytic cycle begins with nickel(I)-mediated radical generation from alkyl halides, followed by carbon radical capture by nickel(II) complexes, and concludes with reductive elimination. These steps resonate with mechanistic proposals in nickel-catalyzed cross-coupling, photoredox, and electrocatalytic reactions. Herein, we present our insights into each step involving radicals, including initiation, propagation, termination, and the nuances of kinetics, origins of selectivity, and ligand effects.Radical generation from C(sp3) electrophiles via one-electron oxidative addition with low-valent nickel radical intermediates provides the basis for stereoconvergent and cross-electrophile couplings. Our electroanalytical studies elucidate a concerted halogen atom abstraction mechanism, where electron transfer is coupled with halide dissociation. Using this pathway, we have developed a nickel-catalyzed stereoselective radical addition to dehydroalanine, facilitating the synthesis of noncanonical peptides. In this application, chiral ligands modulate the stereochemical outcome through the asymmetric protonation of a nickel-enolate intermediate.The capture of the alkyl radical by nickel(II) expands the scope of cross-coupling, promotes reductive elimination through the formation of high-valent nickel(III) species, and governs chemo- and stereoselectivity. We discovered that nickel(II)-aryl efficiently traps radicals with a barrier ranging from 7 to 9 kcal/mol, followed by fast reductive elimination. In contrast, nickel(II)-alkyl captures radicals to form a nickel(III) species, which was characterized by EPR spectroscopy. However, the subsequent slow reductive elimination resulted in minimal product formation. The observed high diastereoselectivity of radical capture inspired investigations into C-aryl and C-acyl glycosylation reactions. We developed a redox auxiliary that readily couples with natural carbohydrates and produces glycosyl radicals upon photoredox activation. Nickel-catalyzed cross-coupling of the glycosyl radical with bromoarenes and carboxylic acids leads to diverse non-natural glycosides that can facilitate drug discovery.Stoichiometric studies on well-defined d8-nickel complexes have showcased means to promote reductive elimination, including ligand association, oxidation, and oxidative addition.In the final section, we address the influence of auxiliary ligands on the electronic structure and redox activity of organonickel intermediates. Synthesis of a series of low-valent nickel radical complexes and characterization of their electronic structures led us to a postulate that ligand redox activity correlates with coordination geometry. Our data reveal that a change in ligand redox activity can shift the redox potentials of reaction intermediates, potentially altering the mechanism of catalytic reactions. Moreover, coordinating additives and solvents may stabilize nickel radicals during catalysis by adjusting ligand redox activity, which is consistent with known catalytic conditions.
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Affiliation(s)
- Gregory
A. Dawson
- Department
of Chemistry, New York University, 100 Washington Square East, New York, New York 10003, United States
| | - Ethan H. Spielvogel
- Department
of Chemistry, New York University, 100 Washington Square East, New York, New York 10003, United States
| | - Tianning Diao
- Department
of Chemistry, New York University, 100 Washington Square East, New York, New York 10003, United States
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