Radical/Polar Crossover (RPC) chemistry is a rapidly growing subset of photoredox catalysis that is characterized by transformations featuring both radical and ionic modes of reactivity. Net‐neutral RPC is particularly interesting in that both the single‐electron oxidation and reduction steps occur through interaction with the photocatalyst, thus precluding the need for exogenous oxidants or reductants. As such, these transformations facilitate rapid incorporation of molecular complexity while maintaining mild reaction conditions. This review covers recent advances in photoredox‐mediated net‐neutral RPC synthetic methods, with a particular emphasis on C–C bond‐forming reactions.
Photoredox catalysis on unactivated substrates with strongly reducing iridium photosensitizers
Photoredox catalysis has emerged as a powerful strategy in synthetic organic chemistry, but substrates that are difficult to reduce either require complex reaction conditions or are not amenable at all to photoredox transformations. In this work, we show that strong bis-cyclometalated iridium photoreductants with electron-rich β-diketiminate (NacNac) ancillary ligands enable high-yielding photoredox transformations of challenging substrates with very simple reaction conditions that require only a single sacrificial reagent. Using blue or green visible-light activation we demonstrate a variety of reactions, which include hydrodehalogenation, cyclization, intramolecular radical addition, and prenylation via radical-mediated pathways, with optimized conditions that only require the photocatalyst and a sacrificial reductant/hydrogen atom donor. Many of these reactions involve organobromide and organochloride substrates which in the past have had limited utility in photoredox catalysis. This work paves the way for the continued expansion of the substrate scope in photoredox catalysis.
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- NSF-PAR ID:
- 10215722
- Date Published:
- Journal Name:
- Chemical Science
- ISSN:
- 2041-6520
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
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Photoredox catalysis has been prominent in many applications, including solar fuels, organic synthesis, and polymer chemistry. Photocatalytic activity directly depends on the photophysical and electrochemical properties of photocatalysts in both the ground state and excited state. Controlling those properties, therefore, is imperative to achieve the desired photocatalytic activity. Redox potential is one important factor that impacts both the thermodynamic and kinetic aspects of key elementary steps in photoredox catalysis. In many challenging reactions in organic synthesis, high redox potentials of the substrates hamper the reaction, leading to slow conversion. Thus, the development of photocatalysts with extreme redox potentials, accompanied by potent reducing or oxidizing power, is required to execute high-yielding thermodynamically demanding reactions. In this review, we will introduce strategies for accessing extreme redox potentials in photocatalytic transformations. These include molecular design strategies for preparing photosensitizers that are exceptionally strong ground-state or excited-state reductants or oxidants, highlighting both organic and metal-based photosensitizers. We also outline methodological approaches for accessing extreme redox potentials, using two-photon activation, or combined electrochemical/photochemical strategies to generate potent redox reagents from precursors that have milder potentials.more » « less
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Radical cation initiated cyclization reactions can be triggered by the one electron oxidation of an electron-rich olefin using either electrochemistry or visible light and a photoredox catalyst. In principle, the two methods can be used to give complimentary products with the electrolysis leading to products derived from a net two electron oxidation and the photoelectron transfer method being compatible with the formation of products from a redox neutral process. However, we are finding an increasing number of oxidative cyclization reactions that require the rapid removal of a second electron in order to form high yields of the desired product. In those cases, the electrochemical method can provide a superior approach to accessing the necessary two electron oxidation pathway. With that said, it is a combination of the two methods that provides the mechanistic insight needed to understand when a reaction has this requirement, and we are finding that the use of photoredox catalysis in combination with electrochemical methods is changing our understanding of even the most successful anodic cyclization reactions run to date.
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The use of photoredox catalysis for the synthesis of small organic molecules relies on harnessing and converting the energy in visible light to drive reactions. Specifically, photon energy is used to generate radical ion species that can be harnessed through subsequent reaction steps to form a desired product. Cyanoarenes are widely used as arylating agents in photoredox catalysis because of their stability as persistent radical anions. However, there are marked, unexplained variations in product yields when using different cyanoarenes. In this study, the quantum yield and product yield of an α-aminoarylation photoredox reaction between five cyanoarene coupling partners and N-phenylpyrrolidine were characterized. Significant discrepancies in cyanoarene consumption and product yield suggested a chemically irreversible, unproductive pathway in the reaction. Analysis of the side products in the reaction demonstrated the formation of species consistent with radical anion fragmentation. Electrochemical and computational methods were used to study the fragmentation of the different cyanoarenes and revealed a correlation between product yield and cyanoarene radical anion stability. Kinetic modeling of the reaction demonstrates that cross-coupling selectivity between N-phenylpyrrolidine and the cyanoarene is controlled by the same phenomenon present in the persistent radical effect.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0SC06306A
