Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

In 1979, the seminal work of Wenkert set the standards for the utilization of aryl and vinyl ethers as coupling partners via C–O bond-cleavage. Although the topic remained dormant for almost three decades, the last few years have witnessed a renaissance in this area of expertise, experiencing an exponential growth and becoming a significant discipline within the cross-coupling arena. The means to utilize readily accessible aryl or vinyl ethers as counterparts does not only represent a practical, powerful and straightforward alternative to organic halides, but also constitutes an excellent opportunity to improve our chemical knowledge about a relatively unexplored area of expertise. This review summarizes the most significant developments in the area of C–O bond-cleavage when employing aryl or vinyl ethers, providing a detailed overview of the current state of the art and including future aspects, when applicable.

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Metal-catalyzed activation of ethers via C–O bond cleavage: a new strategy for molecular diversity

Thermal C-C bond cleavage reactions allow the construction of structurally diverse molecular skeletons via predictable and efficient bond reorganizations. Visible light photoredox-catalyzed radical-mediated C-C bond cleavage reactions have recently emerged as a powerful alternative method for overcoming the thermodynamic and kinetic barrier of C-C bond cleavage in diverse molecular scaffolds. In recent years, a plethora of elegant and useful reactions have been invented, and the products are sometimes otherwise inaccessible by classic thermal reactions. Considering the great influence and synthetic potential of these reactions, we provide a summary of the state of art visible light-driven radical-mediated C-C bond cleavage/functionalization strategies with a specific emphasis on the working models. We hoped that this review will be useful for medicinal and synthetic organic chemists and will inspire further reaction development in this interesting area.

Visible Light-Driven Radical-Mediated C-C Bond Cleavage/Functionalization in Organic Synthesis

Photokatalyse mit sichtbarem Licht: Welche Bedeutung hat sie für die organische Synthese?

Transition metal-catalysed cross-coupling is one of the most powerful synthetic methods and has led to vast improvements in the synthesis of pharmaceuticals, agrochemicals and precursors for materials chemistry. A major advance in cross-coupling over the past 20 years is the utilization of well-defined, bench-stable Pd and Ni precatalysts that do not require the addition of free ancillary ligand, which can hinder catalysis by occupying open coordination sites on the metal. The development of precatalysts has resulted in new reactions and expanded substrate scopes, enabling transformations under milder conditions and with lower catalyst loadings. This Review highlights recent advances in the development of Pd and Ni precatalysts for cross-coupling, and provides a critical comparison between the state of the art in Pd- and Ni-based systems. Elaborate molecules can be assembled in a relatively mild manner through organic cross-coupling reactions, with those mediated by group 10 metal complexes being particularly useful. This Review covers the development of nickel and palladium precatalysts, comparing state-of-the-art systems and offering design principles for future technologies.

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Well-defined nickel and palladium precatalysts for cross-coupling

Enantioenriched methylenecyclopentanes are synthesized by stereospecific, nickel-catalyzed Heck cyclizations of secondary benzylic ethers. The reaction proceeds in high yield and enantiospecificity for benzylic ethers of both π-extended and simple arenes. Ethers with pendant 1,2-disubstituted olefins form trisubstituted olefins with control of both absolute configuration and alkene geometry. Diastereoselective synthesis of a polycyclic furan is demonstrated.

https://escholarship.org/content/qt9nz863bt/qt9nz863bt.pdf?t=olty3u

Enantiospecific Intramolecular Heck Reactions of Secondary Benzylic Ethers

Nickel-catalyzed cross-electrophile coupling reactions of benzylic esters and aryl halides have been developed. Both inter- and intramolecular variants proceed under mild reaction conditions. A range of heterocycles and functional groups are tolerated under the reaction conditions. Additionally, the first example of a stereospecific cross-electrophile coupling of a secondary benzylic ester is described.

Intra- and Intermolecular Nickel-Catalyzed Reductive Cross-Electrophile Coupling Reactions of Benzylic Esters with Aryl Halides.

Mixed plastics waste represents an abundant and largely untapped feedstock for the production of valuable products. The chemical diversity and complexity of these materials, however, present major barriers to realizing this opportunity. In this work, we show that metal-catalyzed autoxidation depolymerizes comingled polymers into a mixture of oxygenated small molecules that are advantaged substrates for biological conversion. We engineer a robust soil bacterium, Pseudomonas putida, to funnel these oxygenated compounds into a single exemplary chemical product, either β-ketoadipate or polyhydroxyalkanoates. This hybrid process establishes a strategy for the selective conversion of mixed plastics waste into useful chemical products. Description Funneling mixed waste with microbes Current plastic recycling methods require sorting by chemical composition, a method that is expensive and results in products that are of lower quality and value than the starting plastic. If plastic waste could instead be converted to valuable chemical intermediates, then economical use of mixed waste as a feedstock might be feasible. Sullivan et al. developed a two-stage oxidation and biological funneling approach that can break down and reform mixtures of common consumer plastics (see the Perspective by Yan). The end products can be adjusted by metabolic engineering of the microbes in the second step, which should enable tailored conversion into various platform or specialty chemicals. —MAF Autoxidation and biological funneling converts mixed plastics to a single product.

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Mixed plastics waste valorization through tandem chemical oxidation and biological funneling

Alkyl with alkyl: A significant development in alkyl-alkyl cross-coupling reactions, namely the nickel-catalyzed decarboxylative Negishi coupling of N-hydroxyphthalimide esters, was recently reported by Baran and co-workers. This method enables the synthesis of various highly functionalized compounds, including natural product derivatives.

Decarboxylative Alkyl-Alkyl Cross-Coupling Reactions.

There is an urgent need for new technologies to enable circularity for synthetic polymers, spurred by the accumulation of waste plastics in landfills and the environment and the contributions of plastics manufacturing to climate change. Chemical recycling is a promising means to convert waste plastics into molecular intermediates that can be remanufactured into new products. Given the growing interest in the development of new chemical recycling approaches, it is critical to evaluate the economics, energy use, greenhouse gas emissions, and other life cycle inventory metrics for emerging processes, relative to the incumbent, linear manufacturing practices employed today. Here we offer specific definitions for classes of chemical recycling and upcycling and describe general process concepts for the chemical recycling of mixed plastics waste. We present a framework for techno-economic analysis and life cycle assessment for both closed- and open-loop chemical recycling. Rigorous application of these process analysis tools will be required to enable impactful solutions for the plastics waste problem. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Mikhail O. Konev

Papers

Enantiospecific Intramolecular Heck Reactions of Secondary Benzylic Ethers

Intra- and Intermolecular Nickel-Catalyzed Reductive Cross-Electrophile Coupling Reactions of Benzylic Esters with Aryl Halides.

Mixed plastics waste valorization through tandem chemical oxidation and biological funneling

Decarboxylative Alkyl-Alkyl Cross-Coupling Reactions.

The Critical Role of Process Analysis in Chemical Recycling and Upcycling of Waste Plastics.