LongLarf

Nature, Published online: 12 August 2020; doi:10.1038/s41586-020-2565-5

A mono‐coordinated silicon cation was prepared by halide abstraction from a iodosilylene. It is shown to react with an amine to form three bonds at the silicon atom in one reaction which conforms with the notion of a “supersilylene”.

Abstract

Mono‐coordinated silicon(II) cations are predicted to be reactive ambiphiles, combining the typically high Lewis acidity of silicon cations with nucleophilicity due to the presence of an electron pair at the same atomic centre. Here, a carbazole‐derived scaffold was used to isolate salts with a mono‐coordinated silicon(II) cation, [RSi]⁺ (R=bulky carbazolyl substituent), by halide abstraction from a base‐free halosilylene, RSiI, with Ag[Al(O ^t Bu^F)₄]. Despite the bulk of the carbazolyl moiety, the silylenylium cation [RSi]⁺ retains high reactivity. It was shown to react with an amine to form three bonds at the silicon atom in one reaction which conforms with the notion of a “supersilylene”. The resulting silylium cation [RSi(H)NR′₂]⁺ (in the formal oxidation state Si^IV) obtained by oxidative addition of an NH bond at [RSi]⁺ is even more acidic than the silylenylium cation (Si^II) due to the absence of a lone pair of electrons the silicon atom.

A general cascade synthesis of pyrroles from nitroarenes is reported. The process is catalyzed by a heterogeneous cobalt catalyst using either dihydrogen, formic acid, or a CO/H₂O mixture as reducing agents. This strategy was applied to the synthesis of biologically active compounds, including (+)‐Isamoltane.

Abstract

A bifunctional 3d‐metal catalyst for the cascade synthesis of diverse pyrroles from nitroarenes is presented. The optimal catalytic system Co/NGr‐C@SiO₂‐L is obtained by pyrolysis of a cobalt‐impregnated composite followed by subsequent selective leaching. In the presence of this material, (transfer) hydrogenation of easily available nitroarenes and subsequent Paal–Knorr/Clauson‐Kass condensation provides >40 pyrroles in good to high yields using dihydrogen, formic acid, or a CO/H₂O mixture (WGSR conditions) as reductant. In addition to the favorable step economy, this straightforward domino process does not require any solvents or external co‐catalysts. The general synthetic utility of this methodology was demonstrated on a variety of functionalized substrates including the preparation of biologically active and pharmaceutically relevant compounds, for example, (+)‐Isamoltane.

A new series of 2,4,6‐triaryl‐λ⁵‐phosphinines were synthesized and characterized. Supported by DFT calculations, the structure–property relationships of these unsaturated phosphorus heterocycles were investigated in detail. For the first time, an organic light‐emitting device was developed based on a blue phosphinine emitter.

Abstract

A new series of 2,4,6‐triaryl‐λ⁵‐phosphinines have been synthesized that contain different substituents both on the carbon backbone and the phosphorus atom of the six‐membered heterocycle. Their optical and redox properties were studied in detail, supported by in‐depth theoretical calculations. The modularity of the synthetic strategy allowed the establishment of structure–property relationships for this class of compounds and an OLED based on a blue phosphinine emitter could be developed for the first time.

Nature, Published online: 05 August 2020; doi:10.1038/s41586-020-2539-7

Roytman and Singleton argue that our proposed electrophilic mechanism for the sulfonation of methane in superacid conditions is "not plausible." We clarify certain terms that might have caused misinterpretation of our proposed mechanism and supplement the discussion. We reaffirm that an electrophilic mechanism may be operative under our reaction conditions.

Abstract

The valorization of lignin and its model compounds represents a key technology applied for the production of lignin‐monomers, which constitute as renewable feedstocks and intermediates for the synthesis of value‐added chemicals and fuels as well as molecules related to life science applications. Hence, in addition to lignin‐depolymerization, the effective utilization of lignin‐derived feedstocks such as phenols, ethers, alcohols, ketones and aldehydes is also gaining significant importance. In this regard, number of methodologies has been developed on the utility of these feedstocks for the production of various functional chemicals. In this review, we summarize the valorization of lignin‐derived feedstocks by amination, hydrogenation, dehydrogenation oxidation, carbonylation, and trifluoromethylation, hydrolysis reactions for the synthesis of amines, cyclohexanes, cyclohexanols, cyclohexanones, esters, ketones, cyclic ethers, trifluoromethylated compounds and heterocycles as well as pharmaceuticals and biomolecules.

Targeting makes efforts more efficient: Lignin's structure determines its bioconversion efficiency. In‐depth understanding of ideal lignin substrates for microbial metabolism could guide modifying lignin chemistry directionally. This Minireview summarizes advances in pretreatment, fractionation, and fermentation strategies to depolymerize and modify lignin for bioconversion.

Abstract

Biological lignin valorization represents a promising approach contributing to sustainable and economic biorefineries. The low level of valuable lignin‐derived products remains a major challenge hindering the implementation of microbial lignin conversion. Lignin's properties play a significant role in determining the efficiency of lignin bioconversion. To date, despite significant progress in the development of biomass pretreatment, lignin fractionation, and fermentation over the last few decades, little efforts have gone into identifying the ideal lignin substrates for an efficient microbial metabolism. In this Minireview, emerging and state‐of‐the‐art strategies for biomass pretreatment and lignin fractionation are summarized to elaborate their roles in modifying lignin structure for bioconversion. Fermentation strategies aimed at enhancing lignin depolymerization for microbial utilization are systematically reviewed as well. With an improved understanding of the ideal lignin structure elucidated by comprehensive metabolic pathways and/or big data analysis, modifying lignin chemistry could be more directional and effective. Ultimately, together with the progress of fermentation process optimization, biological lignin valorization will become more competitive in biorefineries.

Sticks like glue: This Review critically assesses the role of 5‐hydroxymethylfurfural (HMF) and its derivatives as renewable key reactants in adhesives. Application fields range from wood‐based materials, sand casting, and composites. The technological performance of many HMF‐based adhesives reaches the minimum requirements, but further research on economically feasible HMF‐based resins is needed.

Abstract

5‐Hydroxymethylfurfural (HMF) is a promising bio‐derived platform chemical with a broad scope of application, for example, in the production of solvents, fuels, polymers, or adhesives. The wood and foundry industries are among the largest adhesive users and currently both rely to a large extent on the use of fossil‐based binders, such as by using formaldehyde as a crosslinker in many commercial adhesive systems. The industry is thus looking for suitable alternatives to replace fossil‐based chemicals. HMF and its derivatives are considered to be key renewable reactants in adhesive systems. The core of this Review is the critical evaluation of the potential of HMF and its derivatives in adhesive systems. The technological performance was assessed in the fields of wood‐based materials, sand casting and composites. As an overall conclusion, HMF and its derivatives have a high application potential in alternative adhesives. Clearly, further research is needed to improve the performance and produce economically competitive adhesives.

The development of sigmatropic rearrangement reactions via ylide intermediates has witnessed significant advances in recent years. In this minireview, we summarize these advances that have contributed to the fundamental understanding of sigmatropic rearrangement reactions and that opened up new avenues in organic synthesis methodology beyond classic rearrangements.

Abstract

Among the available methods to increase the molecular complexity, sigmatropic rearrangements occupy a distinct position in organic synthesis. Despite being known for over a century sigmatropic rearrangement reactions of ylides via carbene transfer reaction have only recently come of age. Most of the ylide mediated rearrangement processes involve rupture of a σ‐bond and formation of a new bond between π‐bond and negatively charged atom followed by simultaneous redistribution of π‐electrons. This minireview describes the advances in this research area made in recent years, which now opens up metal‐catalyzed enantioselective sigmatropic rearrangement reactions, metal‐free photochemical rearrangement reactions and novel reaction pathways that can be accessed via ylide intermediates.

Reported is the first remote difunctionalization of unactivated alkenes with CO₂ by visible‐light photoredox catalysis. Mechanistic studies indicate that a 1,5‐hydrogen atom‐transfer process is the rate‐limiting step and reduction of radical intermediates generates the corresponding carbanions. Other electrophiles, including aldehydes, ketones, and benzylic bromides, are also applicable in this process, demonstrating a general strategy for redox‐neutral remote difunctionalization of unactivated alkenes.

Abstract

Remote difunctionalization of unactivated alkenes is challenging but a highly attractive tactic to install two functional groups across long distances. Reported herein is the first remote difunctionalization of alkenes with CO₂. This visible‐light photoredox catalysis strategy provides a facile method to synthesize a series of carboxylic acids bearing valuable fluorine‐ or phosphorus‐containing functional groups. Moreover, this versatile protocol shows mild reaction conditions, broad substrate scope, and good functional‐group tolerance. Based on DFT calculations, a radical adds to an unactivated alkene to smoothly form a new carbon radical, followed by a 1,5‐hydrogen atom‐transfer process, the rate‐limiting step, generating a more stable benzylic radical. The reduction of the benzylic radicals by an Ir^II species generates the corresponding benzylic carbanions as the key intermediates, which further undergo nucleophilic attack with CO₂ to generate carboxylates.

Many classical and emerging methodologies in organic chemistry rely on carbon dioxide (CO₂) extrusion to generate reactive intermediates for bond-forming events. Synthetic reactions that involve the microscopic reverse—the carboxylation of reactive intermediates—have conventionally been undertaken using very different conditions. We report that chemically stable C(sp³) carboxylates, such as arylacetic acids and malonate half-esters, undergo uncatalyzed reversible decarboxylation in dimethylformamide solution. Decarboxylation-carboxylation occurs with substrates resistant to protodecarboxylation by Brønsted acids under otherwise identical conditions. Isotopically labeled carboxylic acids can be prepared in high chemical and isotopic yield by simply supplying an atmosphere of ¹³CO₂ to carboxylate salts in polar aprotic solvents. An understanding of carboxylate reactivity in solution enables conditions for the trapping of aldehydes, ketones, and α,β-unsaturated esters.