The merger of transition metal and photocatalysis
05 Jul 2017-Vol. 1, Iss: 7, pp 0052
TL;DR: The combination of Photoredox catalysis and transition metal catalysis is reviewed to show how this provides access to highly reactive oxidation states of transition metals and distinct activation modes that further enable the synthetic chemist.
Abstract: The merger of transition metal catalysis and photocatalysis, termed metallaphotocatalysis, has recently emerged as a versatile platform for the development of new, highly enabling synthetic methodologies. Photoredox catalysis provides access to reactive radical species under mild conditions from abundant, native functional groups, and, when combined with transition metal catalysis, this feature allows direct coupling of non-traditional nucleophile partners. In addition, photocatalysis can aid fundamental organometallic steps through modulation of the oxidation state of transition metal complexes or through energy-transfer-mediated excitation of intermediate catalytic species. Metallaphotocatalysis provides access to distinct activation modes, which are complementary to those traditionally used in the field of transition metal catalysis, thereby enabling reaction development through entirely new mechanistic paradigms. This Review discusses key advances in the field of metallaphotocatalysis over the past decade and demonstrates how the unique mechanistic features permit challenging, or previously elusive, transformations to be accomplished. Transition metal catalysis is well established as an enabling tool in synthetic organic chemistry. Photoredox catalysis has recently emerged as a method to effect reactions that occur through single-electron-transfer pathways. Here we review the combination of the two to show how this provides access to highly reactive oxidation states of transition metals and distinct activation modes that further enable the synthetic chemist.
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TL;DR: A comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018 is provided.
Abstract: C–H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C–H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C–H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C–H activation until summer 2018.
1,417 citations
TL;DR: This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.
Abstract: Conventional methods for carrying out carbon–hydrogen functionalization and carbon–nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon–carbon and carbon–heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon–hydrogen functionalization and carbon–nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.
626 citations
TL;DR: The discussion will use selected case studies to highlight how mechanistic investigations can be instrumental in guiding the invention and development of synthetically useful photocatalytic transformations.
Abstract: The fast-moving fields of photoredox and photocatalysis have recently provided fresh opportunities to expand the potential of synthetic organic chemistry. Advances in light-mediated processes have mainly been guided so far by empirical findings and the quest for reaction invention. The general perception, however, is that photocatalysis is entering a more mature phase where the combination of experimental and mechanistic studies will play a dominant role in sustaining further innovation. This Review outlines the key mechanistic studies to consider when developing a photochemical process, and the best techniques available for acquiring relevant information. The discussion will use selected case studies to highlight how mechanistic investigations can be instrumental in guiding the invention and development of synthetically useful photocatalytic transformations.
448 citations
TL;DR: It is found that a combination of triphenylphosphine and sodium iodide under 456-nanometer irradiation by blue light–emitting diodes can catalyze the alkylation of silyl enol ethers by decarboxylative coupling with redox-active esters in the absence of transition metals.
Abstract: Most photoredox catalysts in current use are precious metal complexes or synthetically elaborate organic dyes, the cost of which can impede their application for large-scale industrial processes. We found that a combination of triphenylphosphine and sodium iodide under 456-nanometer irradiation by blue light–emitting diodes can catalyze the alkylation of silyl enol ethers by decarboxylative coupling with redox-active esters in the absence of transition metals. Deaminative alkylation using Katritzky’s N-alkylpyridinium salts and trifluoromethylation using Togni’s reagent are also demonstrated. Moreover, the phosphine/iodide-based photoredox system catalyzes Minisci-type alkylation of N-heterocycles and can operate in tandem with chiral phosphoric acids to achieve high enantioselectivity in this reaction.
435 citations
TL;DR: The merger of photoredox catalysis with transition metal catalysis has become a mainstay in synthetic methodology over the past decade as discussed by the authors, which has combined the unparalleled capacity of transition metal catalysts for bond formation with the broad utility of photoinduced electron and energy-transfer processes.
Abstract: The merger of photoredox catalysis with transition metal catalysis, termed metallaphotoredox catalysis, has become a mainstay in synthetic methodology over the past decade. Metallaphotoredox catalysis has combined the unparalleled capacity of transition metal catalysis for bond formation with the broad utility of photoinduced electron- and energy-transfer processes. Photocatalytic substrate activation has allowed the engagement of simple starting materials in metal-mediated bond-forming processes. Moreover, electron or energy transfer directly with key organometallic intermediates has provided novel activation modes entirely complementary to traditional catalytic platforms. This Review details and contextualizes the advancements in molecule construction brought forth by metallaphotocatalysis.
408 citations
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TL;DR: The conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes.
Abstract: A fundamental aim in the field of catalysis is the development of new modes of small molecule activation. One approach toward the catalytic activation of organic molecules that has received much attention recently is visible light photoredox catalysis. In a general sense, this approach relies on the ability of metal complexes and organic dyes to engage in single-electron-transfer (SET) processes with organic substrates upon photoexcitation with visible light.
Many of the most commonly employed visible light photocatalysts are polypyridyl complexes of ruthenium and iridium, and are typified by the complex tris(2,2′-bipyridine) ruthenium(II), or Ru(bpy)32+ (Figure 1). These complexes absorb light in the visible region of the electromagnetic spectrum to give stable, long-lived photoexcited states.1,2 The lifetime of the excited species is sufficiently long (1100 ns for Ru(bpy)32+) that it may engage in bimolecular electron-transfer reactions in competition with deactivation pathways.3 Although these species are poor single-electron oxidants and reductants in the ground state, excitation of an electron affords excited states that are very potent single-electron-transfer reagents. Importantly, the conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes.
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Figure 1
Ruthenium polypyridyl complexes: versatile visible light photocatalysts.
6,252 citations
TL;DR: A class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates.
Abstract: The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.
5,297 citations
TL;DR: This tutorial review provides a sampling of renowned fluorinated drugs and their mode of action with a discussion clarifying the role and impact of fluorine substitution on drug potency.
Abstract: It has become evident that fluorinated compounds have a remarkable record in medicinal chemistry and will play a continuing role in providing lead compounds for therapeutic applications. This tutorial review provides a sampling of renowned fluorinated drugs and their mode of action with a discussion clarifying the role and impact of fluorine substitution on drug potency.
4,664 citations
25 Aug 2004
TL;DR: In this paper, the authors present an approach to the formation of C-X (X = N, O, S) bonds in metal-catalyzed cross-coupling reactions.
Abstract: Preface.List of Contributors.1 Mechanistic Aspects of Metal-Catalyzed C,C- and C,X-Bond-Forming Reactions (Antonio M. Echavarren and Diego J. Cardenas).1.1 Mechanisms of Cross-Coupling Reactions.1.2 Formation of C,C-Bonds in the Palladium-Catalyzed alpha-Arylation of Carbonyl Compounds and Nitriles.1.3 Key Intermediates in the Formation of C-X (X = N, O, S) bonds in Metal-Catalyzed Reactions 251.3.1 Reductive Elimination of C-N, C-O, and C-S Bonds From Organopalladium(II) Complexes.1.4 Summary and Outlook.Abbreviations.References.2 Metal-Catalyzed Cross-Coupling Reactions of Organoboron Compounds with Organic Halides (Norio Miyaura).2.1 Introduction.2.2 Advances in the Synthesis of Organoboron Compounds.2.3 Reaction Mechanism.2.4 Reaction Conditions.2.5 Side Reactions.2.6 Reactions of B-Alkyl Compounds.2.7 Reactions of B-Alkenyl Compounds.2.8 Reactions of B-Aryl Compounds.2.9 Reactions of B-Allyl and B-Alkynyl Compounds.2.10 Reactions Giving Ketones.2.11 Dimerization of Arylboronic Acids.2.12 N-, O-, and S-Arylation.Abbreviations.References.3 Organotin Reagents in Cross-Coupling Reactions (Terence N. Mitchell).3.1 Introduction.3.2 Mechanism and Methodology.3.3 Natural Product Synthesis.3.4 Organic Synthesis.3.5 Polymer Chemistry.3.6 Inorganic Synthesis.3.7 Conclusions.3.8 Experimental Procedures.Abbreviations.References.4 Organosilicon Compounds in Cross-Coupling Reactions (Scott E. Denmark and Ramzi F. Sweis).4.1 Introduction.4.2 Modern Organosilicon-Cross-Coupling.4.3 Mechanistic Studies in Silicon-Cross-Coupling.4.4 Applications to Total Synthesis.4.5 Summary and Outlook.4.6 Experimental Procedures.Abbreviations.References.5 Cross-Coupling of Organyl Halides with Alkenes: The Heck Reaction (Stefan Brase and Armin de Meijere).5.1 Introduction.5.2 Principles.5.3 Cascade Reactions and Multiple Couplings.5.4 Related Palladium-Catalyzed Reactions.5.5 Enantioselective Heck-Type Reactions.5.6 Syntheses of Heterocycles, Natural Products and Other Biologically Active Compounds Applying Heck Reactions.5.7 Carbopalladation Reactions in Solid-Phase Syntheses.5.8 The Heck Reaction in Fine Chemicals Syntheses.5.9 Conclusions.5.10 Experimental Procedures.Acknowledgments.Abbreviations and Acronyms.References.6 Cross-Coupling Reactions to sp Carbon Atoms (Jeremiah A. Marsden and Michael M. Haley).6.1 Introduction.6.2 Alkynylcopper Reagents.6.3 Alkynyltin Reagents.6.4 Alkynylzinc Reagents.6.5 Alkynylboron Reagents.6.6 Alkynylsilicon Reagents.6.7 Alkynylmagnesium Reagents.6.8 Other Alkynylmetals.6.9 Concluding Remarks.6.10 Experimental Procedures.Acknowledgments.Abbreviations and Acronyms.References.7 Carbometallation Reactions (Ilan Marek, Nicka Chinkov, and Daniella Banon-Tenne).7.1 Introduction.7.2 Carbometallation Reactions of Alkynes.7.3 Carbometallation Reactions of Alkenes.7.4 Zinc-Enolate Carbometallation Reactions.7.5 Carbometallation Reactions of Dienes and Enynes.7.6 Carbometallation Reactions of Allenes.7.7 Conclusions.7.8 Experimental Procedures.Acknowledgments.References.8 Palladium-Catalyzed 1,4-Additions to Conjugated Dienes (Jan-E. Backvall).8.1 Introduction.8.2 Palladium(0)-Catalyzed Reactions.8.3 Palladium(II)-Catalyzed Reactions.References.9 Cross-Coupling Reactions via PI-Allylmetal Intermediates (Uli Kazmaier and Matthias Pohlman)9.1 Introduction.9.2 Palladium-Catalyzed Allylic Alkylations.9.3 Allylic Alkylations with Other Transition Metals.9.4 Experimental Procedures.Abbreviations.References.10 Palladium-Catalyzed Coupling Reactions of Propargyl Compounds (Jiro Tsuji and Tadakatsu Mandai).10.1 Introduction.10.2 Classification of Pd-Catalyzed Coupling Reactions of Propargyl Compounds.10.3 Reactions with Insertion into the sp2 Carbon Bond of Allenylpalladium Intermediates (Type I).10.4 Transformations via Transmetallation of Allenylpalladium Intermediates and Related Reactions (Type II).10.5 Reactions with Attack of Soft Carbon and Oxo Nucleophiles on the sp-Carbon of Allenylpalladium Intermediates (Type III).10.6 Experimental Procedures.Abbreviations.References.11 Carbon-Carbon Bond-Forming Reactions Mediated by Organozinc Reagents (Paul Knochel, M. Isabel Calaza, and Eike Hupe).11.1 Introduction.11.2 Methods of Preparation of Zinc Organometallics.11.3 Uncatalyzed Cross-Coupling Reactions.11.4 Copper-Catalyzed Cross-Coupling Reactions.11.5 Transition Metal-Catalyzed Cross-Coupling Reactions.11.6 Conclusions.11.7 Experimental Procedures.Abbreviations.References.12 Carbon-Carbon Bond-Forming Reactions Mediated by Organomagnesium Reagents (Paul Knochel, Ioannis Sapountzis, and Nina Gommermann).12.1 Introduction.12.2 Preparation of Polyfunctionalized Organomagnesium Reagents via a Halogen-Magnesium Exchange.12.3 Conclusions.12.4 Experimental Procedures.References.13 Palladium-Catalyzed Aromatic Carbon-Nitrogen Bond Formation (Lei Jiang and Stephen L. Buchwald).13.1 Introduction.13.2 Mechanistic Studies.13.3 General Features.13.4 Palladium-Catalyzed C-N Bond Formation.13.5 Vinylation.13.6 Amination On Solid Support.13.7 Conclusion.13.8 Representative Experimental Procedures.References.14 The Directed ortho-Metallation (DoM) Cross-Coupling Nexus. Synthetic Methodology for the Formation of Aryl-Aryl and Aryl-Heteroatom-Aryl Bonds (Eric J.-G. Anctil and Victor Snieckus).14.1 Introduction.14.2 The Aim of this Chapter.14.3 Synthetic Methodology derived from the DoM-Cross-Coupling Nexus.14.4 Applications of DoM in Synthesis.14.5 Conclusions and Prognosis.14.6 Selected Experimental Procedures.Abbreviations.References and Notes.15 Palladium- or Nickel-Catalyzed Cross-Coupling with Organometals Containing Zinc, Aluminum, and Zirconium: The Negishi Coupling (Ei-ichi Negishi, Xingzhong Zeng, Ze Tan, Mingxing Qian, Qian Hu, and Zhihong Huang).15.1 Introduction and General Discussion of Changeable Parameters.15.2 Recent Developments in the Negishi Coupling and Related Pd- or Ni-Catalyzed Cross-Coupling Reactions.15.3 Summary and Conclusions.15.4 Representative Experimental Procedures.References.Index.
4,387 citations
TL;DR: An overview of the basic photophysics and electron transfer theory is presented in order to provide a comprehensive guide for employing this class of catalysts in photoredox manifolds.
Abstract: In this review, we highlight the use of organic photoredox catalysts in a myriad of synthetic transformations with a range of applications. This overview is arranged by catalyst class where the photophysics and electrochemical characteristics of each is discussed to underscore the differences and advantages to each type of single electron redox agent. We highlight both net reductive and oxidative as well as redox neutral transformations that can be accomplished using purely organic photoredox-active catalysts. An overview of the basic photophysics and electron transfer theory is presented in order to provide a comprehensive guide for employing this class of catalysts in photoredox manifolds.
3,550 citations