Swadhin K. Mandal

Traditional electron‐transfer catalysis frequently requires complex procedures to separate the catalyst from the reaction mixture. In contrast, a concept of a gas‐mediated redox process may offer unique advantages, including its facile removal from the reaction medium. However, such a concept has remained unknown. Addressing this gap, we report the use of the SO ₂ gas molecule as an electron‐transfer mediator by generating the sulfur dioxide radical anion (SO ₂ ^•− ) under ambient and chemical conditions. Such SO ₂ ^•− enabled chemoselective, controlled, and scalable multi‐electron reductions (2e ⁻ /2H ⁺ , 4e ⁻ /4H ⁺ , 6e ⁻ /6H ⁺ ) of diverse nitrogen‐containing substrates. Furthermore, the method was successfully applied to the synthesis of clinically active drugs on a multi‐gram scale. Therefore, this work establishes a new paradigm in redox chemistry, demonstrating that a gas molecule can act as an electron‐transfer catalyst, capable of performing reactions that typically require transition metals.

Traditional electron‐transfer catalysis frequently requires complex procedures to separate the catalyst from the reaction mixture. In contrast, a concept of a gas‐mediated redox process may offer unique advantages, including its facile removal from the reaction medium. However, such a concept has remained unknown. Addressing this gap, we report the use of the SO ₂ gas molecule as an electron‐transfer mediator by generating the sulfur dioxide radical anion (SO ₂ ^•− ) under ambient and chemical conditions. Such SO ₂ ^•− enabled chemoselective, controlled, and scalable multi‐electron reductions (2e ⁻ /2H ⁺ , 4e ⁻ /4H ⁺ , 6e ⁻ /6H ⁺ ) of diverse nitrogen‐containing substrates. Furthermore, the method was successfully applied to the synthesis of clinically active drugs on a multi‐gram scale. Therefore, this work establishes a new paradigm in redox chemistry, demonstrating that a gas molecule can act as an electron‐transfer catalyst, capable of performing reactions that typically require transition metals.

The “Magic Methyl Effect” stands for the welcoming effect of the methyl group in enhancing the potency of drug precursors. To date, no catalytic chemical route for methylation of aryl halides that excludes organometallic species is known. In this report, we introduce an organometallic-free cross electrophile methylation of aryl halides based on the generation of a methyl anion achieved under chemical conditions. It successfully methylated several strong bonds, such as CAr–Br, CAr–Cl, and CAr–F. This catalytic protocol was further diversified by preparing methyl derivatives of various clinically used antidepressants and antipsychotic drugs. Furthermore, we have demonstrated that the methylated product of one such drug molecule has significantly superior suppressive effects on neural activity as compared to the clinically active nonmethylated parent drug.

The valorization of greenhouse gases CO₂ and N₂O into value-added chemicals has recently become a critical area of research. While the utilization of CO₂ gives quick access to important carboxylic acid building blocks, N₂O is able to act as an oxygen-transfer reagent that allows the formation of carbon-oxygen and heteroatom-oxygen bonds. No metal-free catalytic system has yet been described that is able to react with both gases. Additionally, the metal-free catalytic activation of N₂O is unprecedented in the literature. Relying on our previously established multi-PET catalysis that makes use of a BOIMPY dye, we were able to identify a single catalytic platform that allows reactions of both gases under super-reducing photochemical conditions. In this work, we report reactions between either CO₂ or N₂O and (poly)arenes, aryl halides, olefins, sulfides, and phosphines and present a thorough mechanistic investigation to understand the pathways leading to the described products. We found that N₂O is indeed catalytically activated by a nonmetal catalyst for the first time employing our system. We mention that the enclosed methodology can be utilized to react with abundant CO₂ or N₂O; however, it will not mitigate climate change.

Kurzzusammenfassung: Übergangsmetallverbindungen sind allgemein für ihre Fähigkeit bekannt, kleine Moleküle zu aktivieren, und eignen sich hervorragend als Katalysatoren für eine Vielzahl von Reaktionen. Aufgrund ihrer Toxizität und geringen Verfügbarkeit sind jedoch umweltfreundliche und kostengünstige Alternativen sehr wünschenswert. In den letzten Jahren hat die Erforschung von Verbindungen auf Basis von Hauptgruppenelementen, vor allem von Verbindungen, die von p‐Block‐Elementen abgeleitet sind, erheblich zugenommen, da diese eine ähnliche Reaktivität wie Übergangsmetallkomplexe aufweisen. In jüngster Zeit haben sich p‐Block Addukte von mesoionischen N‐heterocyclischen Carbenen (mNHCs), die in der exocyclischen Position Fragmente wie = CH ₂ (mNHOs), = NR (mNHIs), = PR (mNHPs) und = S (mNHTs) enthalten, aufgrund ihrer starken Nucleophilie, zu Liganden für die Koordinationschemie, zur Aktivierung kleiner Moleküle, sowie zu effizienten metallfreien Katalysatoren für verschiedene anspruchsvolle organische Transformationen entwickelt. Dieser Mini‐Review dokumentiert die jüngsten Entwicklungen bei der Synthese, Reaktivität und katalytischen Anwendung von mNHOs, mNHIs, mNHTs und mNHPs.

Transition metal compounds are widely known for their ability to activate small molecules and are excellent catalysts for a wide range of reactions. However, environmentally friendly and cost‐effective alternatives are highly desirable due to their toxicity and low abundance. In recent years, there has been a significant surge in the study of main‐group‐based species, mainly p‐block element‐derived compounds, which revealed reactivity resembling that of transition metal complexes. Recently, the mesoionic N‐heterocyclic carbenes (mNHCs) adducts of p‐block elements, containing exocyclic fragments such as ═CH ₂ (mNHOs), ═NR (mNHIs), ═PR (mNHPs), and ═S (mNHTs), have evolved as ligands for coordination chemistry, reactive species for the activation of small molecules, as well as efficient metal‐free catalysts for various challenging organic transformations, driven by their strong nucleophilicity. This minireview documents recent breakthroughs in the synthesis, reactivity, and catalytic applications of mNHOs, mNHIs, mNHTs, and mNHPs.

Phenalenyl (PLY) is the smallest, odd alternant polycyclic hydrocarbon known for its magnetic, conducting, and electronic properties. Such material properties are based on the capability of PLY to act as an electron reservoir. PLY has recently emerged as a reductive catalyst, wherein both its ground and excited states can form long-lived radical anions, driving diverse reductive transformations. However, the rich electronic properties of the π-backbone in PLY systems have never been exploited for oxidative transformations. Here, we incarcerate PLY inside a water-soluble Pd₆L₄¹²⁺ nanocavity to provide the first transient absorption signature of PLY-radical cation under photoirradiation, acting as the electron donor instead of its electron acceptor properties; demonstrating redox-umpolung behavior. Photoexciting host-guest charge transfer states, we drive a single electron transfer from PLY to the nanocavity within ∼2 ps, while a solvent-assisted deprotonation of the radical cation within ∼80 ps generates a reactive, long-lived neutral radical (>2 ns). As a proof of concept, utilizing this redox-umpolung photochemistry of PLY, we demonstrate a direct route for the synthesis of aromatic nitriles in water. We envision that our findings will pave the way for advancing water-compatible catalysis using PLY-based molecules, leveraging the unique reactivity of open-shell organic systems.

Abnormal N‐heterocyclic carbene‐stabilized phosphinidene, a class of low‐valent phosphorus compounds, has been recently stabilized by an isolated abnormal NHC. However, its coordination chemistry with various metal ions remained unexplored. Herein, we have explored the coordination chemistry of a mesoionic N‐heterocyclic phosphinidene (mNHP, 1 ), stabilized by an abnormal N‐heterocyclic carbene (aNHC) with various metals (Cu, Zn, and Sn) to afford trimetallic, bimetallic, and monometallic complexes, respectively. All these complexes were characterized by single‐crystal X‐ray diffraction studies. We have also explored the catalytic activity of one of these complexes for alkynylation of trifluoromethyl ketones and found that mNHP ( 1 )‐bound trimetallic Cu‐complex is an excellent catalyst for this reaction with up to 97% isolated yield under low catalyst loading (3 mol%).