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Nature, Published online: 25 May 2023; doi:10.1038/d41586-023-01696-5

Nature, Published online: 03 May 2023; doi:10.1038/d41586-023-01532-w

Silver catalysis exhibits a remarkable capacity for the activation of isocyanides. In recent years, silver-catalyzed and silver-promoted organic reactions of isocyanides have attracted much attention. They offered straightforward and facile access to a wide range of highly useful linear or cyclic compounds. This Review offers a comprehensive coverage of the recent progress in this field.

Abstract

In recent years, the silver-catalyzed and silver-promoted isocyanide reactions have attracted much attention, due to its efficiency in the formation of diverse new bonds and good reaction selectivity. A series of highly useful linear or cyclic compounds have been constructed. In this review, the recent progress in this field is described in the sequence of synthesis of five-membered heterocyclic compounds, synthesis of six-membered heterocyclic compounds, synthesis of fused cyclic compounds as well as synthesis of linear nitrogen-containing compounds.

Hydrolysis of furfuryl alcohol to angelica lactones and levulinic acid is only catalyzed by strong Brønsted acid sites. NbOPO₄ is a more active catalyst due to the presence of P−OH species. 1,4-dioxane as an organic solvent in mixture with water is effective to improve selectivity to the bioproducts while dropping catalytic activity.

Abstract

Levulinic acid and angelica lactones are important building blocks obtained from hemicellulose. This process comprises multiple chemical reactions at which furfuryl alcohol is an intermediate, but only few studies have investigated its hydrolysis. This work studied the role played by the nature of acid sites and the impact of their strength and amount on furfuryl alcohol hydrolysis over Nb₂O₅ and NbOPO₄. It was disclosed that production of levulinic acid is a two-step cascade reaction at which angelica lactones are intermediates. These two hydrolysis steps were catalyzed by Brønsted acid sites with no contribution from Lewis centers. Brønsted acidity of Nb₂O₅ presented no activity while phosphate moieties on NbOPO₄ were effective. Mixtures of organic solvent and water hindered side reactions, improving selectivity to valued bioproducts. 1,4-dioxane was the most suitable organic solvent. This study shows that selectivity of 96 % of levulinic acid and angelica lactones can be accomplished over NbOPO₄.

Nature, Published online: 21 April 2023; doi:10.1038/d41586-023-01391-5

An NHC-catalyzed intermolecular dehydrogenative coupling of aldehyde with ether, amine or benzylic C(sp³)−H bonds is disclosed via metal- and light-free radical pathways.

Abstract

The development of catalyst-controlled methods for direct functionalization of two distinct C−H bonds represents an appealing approach for C−C formations in synthetic chemistry. Herein, we describe an organocatalytic approach for straightforward acylation of C(sp³)−H bonds employing readily available aldehyde as “acyl source” involving dehydrogenative coupling of aldehydes with ether, amine, or benzylic C(sp³)−H bonds. The developed method affords a broad range of ketones under mild conditions. Mechanistically, simple ortho-cyanoiodobenzene is essential in the oxidative radical N-heterocyclic carbene catalysis to give a ketyl radical and C(sp³) radical through a rarely explored intermolecular hydrogen atom transfer pathway, rendering the acylative C−C formations in high efficiency under a metal- and light-free catalytic conditions. Moreover, the prepared products show promising anti-bacterial activities that shall encourage further investigations on novel agrochemical development.

A fully renewable bio-based bicyclic lactone (TOD) was prepared and its TBD-ROP afforded high molar mass amorphous PTOD that could be selectively degraded under acidic or basic conditions. In addition, under acidic conditions, the copolymers of TOD and L–LA degraded much faster than PLA via the selective cleavage of the cyclic ketal linkages in the copolymer backbone.

Abstract

A fully renewable bio-based bicyclic lactone containing a five-membered cyclic ketal moiety, 7-methyl-3,8,10-trioxabicyclo[5.2.1]decan-4-one (TOD), was synthesized through a two-step acid-catalyzed process from glycerol and levulinic acid. The ring-opening polymerization (ROP) of TOD at 30°C with benzyl alcohol (BnOH) as the initiator and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as the catalyst can afford high molar mass PTOD with a cis-2.4-disubstitued 2-methyl 1,3-dioxolane moiety in its repeating unit. PTOD is an amorphous polymer with a glass transition temperature (T _g) of 13°C. It can be hydrolyzed into structurally defined small molecules under acidic or basic conditions by the selective cleavage of either the cyclic ketal or the ester linkage respectively. The TBD-catalyzed copolymerization of L-lactide (L–LA) and TOD at −20°C was investigated. It was confirmed that L–LA polymerized quickly with racemization to form PLA, followed by a slow incorporation of TOD into the formed PLA chains via transesterification. By varying the feed ratios of L–LA to TOD, a series of random copolymers (PLA-co-PTOD) with different TOD incorporation ratios and tunable T _gs were obtained. Under acidic conditions, PLA-co-PTOD degrades much faster than PLA via the selective cleavage of the cyclic ketal linkages. This work provides insights for the development of more sustainable and acid-accelerated degradable alternatives to aliphatic polyesters.

Excellence in amination: Chiral 5-methyl-2-pyrrolidinone is synthesized by direct reductive amination of levulinic acid or methyl levulinate (a byproduct from the industrial production of 2,5-furandicarboxylic acid) as a renewable feedstock employing a commercially available ruthenium bisphosphine catalyst with excellent enantioselectivity (up to 96 % ee) and yield (up to 89 %).

Abstract

Direct asymmetric reductive amination of bio-based levulinic acid (LA) to the enantioenriched 5-methylpyrrolidinone is achieved by using a readily available chiral Ru/bisphosphine catalyst with excellent enantioselectivity (up to 96 % ee) and high isolated yield (up to 89 %). Methyl levulinate (ML), a byproduct from the industrial production of 2,5-furandicarboxylic acid (FDCA), can be used instead of LA with similar reactivity and selectivity. Mass spectrometry and isotope labelling studies indicate that the chiral lactam is formed via imine-enamine tautomerization/cyclization followed by asymmetric hydrogenation of the cyclic enamide.

A gold (Au) single-atom catalyst (SAC) supported on amino-functionalized graphdiyne was synthesized, which displayed a fivefold higher catalytic activity than the Au SAC supported on graphdiyne. Experimental and computational studies reveal that the amino groups in the second sphere of the Au atom serve as proton relays to facilitate the H₂ formation from an Au−H intermediate, thus substantially accelerating the rate of formic acid dehydrogenation.

Abstract

Regulating the second sphere of homogeneous molecular catalysts is a common and effective method to boost their catalytic activities, while the second sphere effects have rarely been investigated for heterogeneous single-atom catalysts primarily due to the synthetic challenge for installing functional groups in their second spheres. Benefiting from the well-defined and readily tailorable structure of graphdiyne (GDY), an Au single-atom catalyst on amino-substituted GDY is constructed, where the amino group is located in the second sphere of the Au center. The Au atoms on amino-decorated GDY displayed superior activity for formic acid dehydrogenation compared with those on unfunctionalized GDY. The experimental studies, particularly the proton inventory studies, and theoretical calculations revealed that the amino groups adjacent to an Au atom could serve as proton relays and thus facilitate the protonation of an intermediate Au−H to generate H₂. Our study paves the way to precisely constructing the functional second sphere on single-atom catalysts.

The oxidative conversion of technical Kraft lignin into vanillin is reported in high yields. Through the ex-cell electrolysis of aqueous carbonate, peroxodicarbonate is produced and serves as a “green” oxidizer for the degradation of lignin.

Abstract

Lignin, the world's largest resource of renewable aromatics, with annually roughly 50 million tons of accruing technical lignin, mainly Kraft lignin, is highly underdeveloped regarding the production of monoaromatics. We demonstrate the oxidative depolymerization of Kraft lignin at 180 °C to produce vanillin 1 in yields up to 6.2 wt % and 92 % referred to the maximum yield gained from the quantification reaction utilizing nitrobenzene. Using peroxodicarbonate (C₂O₆ ²⁻) as “green” oxidizer for the degradation, toxic and/or harmful reagents are prevented. Also, the formed waste can serve as makeup chemical in the pulping process. Na₂C₂O₆ is synthesized in an ex-cell electrolysis of aqueous Na₂CO₃ at BDD anodes, achieving a yield of Na₂C₂O₆ with 41 %. At least, the oxidation and degradation of Kraft lignin is analysis via UV/Vis and NMR spectroscopy.