LongLarf

Z-Olefins are challenging synthetic targets owing to their relative thermodynamic instability. Transition metal–catalyzed asymmetric allylic substitution reactions are well known for installing stereocenters adjacent to branched or E-linear olefins. However, analogous reactions for the synthesis of optically active Z-olefin products are rare. Here we report iridium-catalyzed asymmetric allylic substitution reactions that retain Z-olefin geometries while establishing an adjacent quaternary stereocenter. The formation of transient anti--allyl-iridium intermediates and their capture by external nucleophiles before isomerization to the thermodynamically more stable syn--allyl-iridium counterparts have been observed. These results provide a promising method for preparing chiral Z-olefinic compounds.

We report the supramolecular assembly of artificial metalloenzymes (ArMs), based on the Lactococcal multidrug resistance regulator (LmrR) and an exogeneous copper(II)‐phenanthroline complex, in the cytoplasm of E. coli cells. A combination of catalysis, cell‐fractionation and inhibitor experiments, supplemented with in‐cell solid‐state NMR, confirmed the in‐cell assembly. The ArM containing whole cells were active in the catalysis of the  enantioselective Friedel‐Crafts alkylation of indoles and the Diels‐Alder reaction of azachalcone with cyclopentadiene. Directed evolution resulted in two different improved mutants for both reactions, LmrR_A92E_M8D and LmrR_A92E_V15A, respectively. The whole‐cell ArM system requires no engineering of the microbial host, the protein scaffold or the cofactor to achieve ArM assembly and catalysis.  We consider this a key step towards integrating abiological catalysis in biosynthesis, achieving a hybrid metabolism.

The utilization of carbon dioxide as a building block in organic synthesis has become a highly sought‐after research area. In this minireview, an overview of recent advances on carboxylations using carbon dioxide via visible light photoredox and transition‐metal dual catalysis is given. The merging of these methods provides new avenues to construct carboxylates.

Nature Chemistry, Published online: 25 January 2021; doi:10.1038/s41557-020-00614-w

Iron-catalysed [2+2] cycloaddition/oligomerization of neat butadiene affords (1,n′-divinyl)oligocyclobutane—a telechelic, crystalline material consisting of 1,3-enchained cyclobutyl units. This oligocyclobutane can be chemically recycled to pure butadiene using the same iron catalyst employed in its synthesis, demonstrating design principles for next-generation plastic materials that can be returned to pristine monomer.

Abstract

A visible‐light‐promoted synthesis of quinoxalin‐2(1H)‐ones has been developed using 9‐mesityl‐10‐methylacridinium perchlorate as an organo‐photocatalyst. The atmosphere‐controlled method (Ar/air) enabled the selective synthesis of hydroxyl‐ and acyl‐containing quinoxalin‐2(1H)‐ones under mild reaction conditions without the use of any metal catalysts or toxic reagents. A fluorescent labelling experiment showed that hydroxyl‐containing quinoxalin‐2(1H)‐ones may have utility in various biological applications as potent fluorophores.

This work reports the development of a synergistic nanostructured MnOx/TiO2 catalyst, with highly dispersed MnOx nanoparticles (4.5 ± 1 nm) on shape‐controlled TiO2 nanotubes (8‐11 nm width and 120‐280 nm length), for selective synthesis of valuable aromatic imines at industrially important conditions. Pristine TiO2 nanotubes exhibited 97% imine selectivity at a 38.3% benzylamine conversion, whereas very low imine selectivity was obtained over commercial TiO2 materials, indicating the catalytic significance of shape‐controlled TiO2 nanotubes. The MnOx nanoparticle/TiO2 nanotube (10 wt% Mn) catalyst calcined at 400 oC showed the best activity with 95.6% benzylamine conversion and 99.9% imine selectivity. This catalyst exhibited good recyclability for four times and is effective for converting numerous benzylamines into higher yields of imines. The high catalytic performance of MnOx/TiO2 nanotubes was attributed to more number of redox sites (Mn3+), high dispersion of Mn species, and shape‐controlled structure of TiO2, indicating that this catalyst could be a promising candidate for selective oxidation reactions.

The Cover Feature shows the remarkable progress of the venerable MSR reaction during the last decade. Like superb fireworks, sparkling results were obtained in many directions as presented in this review article. It includes the discovery of novel and very efficient catalysts, or multicatalytic systems, and the use of MSR for innovative cascade processes to prepare many carbo‐ and heterocycles. Moreover, elegant applications of MSR have been reported for the preparation of complex natural products. Finally, the first examples of the aza‐variant of MSR were published also in the recent years. More information can be found in the Review by René Grée et al.

Nature Communications, Published online: 22 January 2021; doi:10.1038/s41467-020-20531-3

Indole C-H silylation preferentially occurs at the C3 and C2 positions, while functionalization of the benzene core (C4-C7 sites) remains challenging. Here, the authors report a regioselective C7-H silylation of indole derivatives assisted by strong coordination of the palladium catalyst with a phosphorus directing group.

Metal‐catalyzed hydrogenation is an effective method to transform readily available arenes into saturated motifs, however, current hydrogenation strategies are limited to the formation of C–H and N–H bonds. The stepwise addition of hydrogen yields reactive unsaturated intermediates that are rapidly reduced. In contrast, the interruption of complete hydrogenation by further functionalization of unsaturated intermediates offers great potential for increasing chemical complexity in a single reaction step. Overcoming the tenet of full reduction in arene hydrogenation has been seldom demonstrated. In this work we report the synthesis of sought‐after, enantioenriched δ‐lactams from oxazolidinone‐substiuted pyridines and water by an interrupted hydrogenation mechanism.

Nature Catalysis, Published online: 18 January 2021; doi:10.1038/s41929-020-00564-z

The coupling of aryl halides and arylboronic acids is generally performed by metal-catalysed Suzuki–Miyaura reactions while metal-free approaches remain elusive. Here an organocatalytic approach based on amine catalysts is introduced for the preparation of commercially relevant asymmetric biaryls.

Nature Communications, Published online: 18 January 2021; doi:10.1038/s41467-020-20725-9

The gem-difluoroalkene functionality is relevant to drug design as it is a bioisostere of a carbonyl group. Here, the authors report the synthesis of 2,2-difluorovinyl benzoates as versatile building blocks for modular synthesis of gem-difluoroenol ethers and gem-difluoroalkenes by nickel-catalyzed cross coupling.

The use of functionalized aldimines has been demonstrated as newly structural 1,4‐dipole precursors under carbene catalysis. More importantly, enantiodivergent organocatalysis has been successfully developed using carbene catalysts with the same absolute configuration, leading to both ( R )‐ and ( S )‐ enantiomers of six‐membered heterocycles with quaternary carbon centers. This strategy features a broad substrate scope, mild reaction conditions, and good enantiomeric ratio. DFT calculation results indicated that hydrogen bond C‐H…F interactions between the catalyst and substrate are the key factors for controlling and even switching the enantioselectivity. These new 1,4‐dipoles can also react with isatin and its imines under carbene catalysis, allowing for access to the spiro oxindoles with excellent enantiomeric ratios.

The dicarbonylation of 1,3‐butadiene to adipic acid derivatives offers the potential for a more cost‐efficient and environmentally benign industrial process. However, the complex reaction network of regioisomeric carbonylation and isomerization pathways, make a selective and direct transformation particularly difficult. Here, we report surprising solvent effects on this palladium‐catalysed process in the presence of 1,2‐bis‐di‐tert‐butylphosphin‐oxylene (dtbpx) ligands, which allow adipate diester formation from 1,3‐butadiene, carbon monoxide, and methanol with 97% selectivity and 100% atom‐economy under scalable conditions. Under optimal conditions a variety of di‐ and triesters from 1,2‐ and 1,3‐dienes can be obtained in good to excellent yields.

Metal‐catalyzed C–N cross‐coupling generally forms C–N bonds by reductive elimination from metal complexes bearing covalent C‐ and N‐ligands. We have identified a Cu‐mediated C–N cross‐coupling that uses a dative N‐ligand in the bond forming event, which, in contrast to conventional methods, generates reactive cationic products. Mechanistic studies suggest the process operates via transmetalation of an aryl organoboron to a Cu(II) complex bearing neutral N‐ligands, such as nitriles or N‐heterocycles. Subsequent generation of a putative Cu(III) complex enables the oxidative C–N coupling to take place, delivering nitrilium intermediates and pyridinium products. The reaction is general for a range of N(sp) and N(sp 2 ) precursors and can be applied to drug synthesis and late‐stage N‐arylation, and the limitations in the methodology are mechanistically evidenced.

Abstract

A visible‐light‐promoted synthesis of quinoxalin‐2(1H)‐ones has been developed using 9‐mesityl‐10‐methylacridinium perchlorate as an organo‐photocatalyst. The atmosphere‐controlled method (Ar/air) enabled the selective synthesis of hydroxyl‐ and acyl‐containing quinoxalin‐2(1H)‐ones under mild reaction conditions without the use of any metal catalysts or toxic reagents. A fluorescent labelling experiment showed that hydroxyl‐containing quinoxalin‐2(1H)‐ones may have utility in various biological applications as potent fluorophores.

The approach of combining enzymatic and transition‐metal catalysis has been focused almost exclusively on using purified, isolated enzymes. The use of whole‐cell biocatalysis, instead of isolated enzymes, with transition‐metal catalysis, however, has been investigated only sparsely, to date. Herein we present the development of two transition‐metal catalyzed reactions used to derivatize styrene obtained from whole‐cell biosynthesis. Using a biocompatible ruthenium cross‐metathesis catalyst up to 1.5 mM stilbene could be obtained in the presence of E. coli , which simultaneously produced styrene. Using palladium catalysts and arylboronic acids, titers of up to 1 mM of several stilbene derivatives were obtained. These two transition‐metal catalyzed reactions are valuable additions to the toolbox of combined whole‐cell biocatalysis and transition‐metal catalysis, offering the possibility to supplement biosynthetic pathways with the chemical versatility of abiological transition metal catalysis.

Switchable TADF: New phenothiazine and quinoline derivatives exhibit outstanding phase‐dependent reversibility between ON/OFF states (low and high emission intensity, drastic shifting of emission colors, short‐ and long‐lived fluorescence) in systematic grinding/fuming cycles, required for multichannel memory devices based on optical information multicoding. TADF: thermally activated delayed fluorescence.

Abstract

Three new quinoline and di‐tert‐butyl phenothiazine isomeric derivatives were synthesized and characterized towards applications for oxygen sensing and optical information multicoding. The compounds with phenylene linker showed outstanding phase‐dependent reversibility between ON/OFF states (low and high emission intensity, drastic shifting of emission colors, short‐ and long‐lived fluorescence) in systematic grinding/fuming cycles, as required for multichannel memory devices based on optical information multicoding. The conformational diversity of the phenothiazine unit resulted in dual emission of the doped films implemented by the different luminescence mechanisms with peaks located at 414/530, 416/540, and 440/582 nm. The presence of a phenylene linker and thus two rotational degrees of freedom resulted in quenching of the delayed fluorescence of quasi‐equatorial conformers in the solid state. The compound containing no phenylene bridge was characterized by two different driving photoluminescence mechanisms of the doped films: short fluorescence of the quasi‐axial conformer and thermally activated delayed fluorescence of the quasi‐equatorial form. This compound showed oxygen sensitivity with a Stern–Volmer constant of 7.5×10⁻⁴ ppm⁻¹.

No more trial and error! This Minireview outlines how a careful physicochemical properties analysis is instrumental towards the rational selection of a photocatalyst while delineating future applications and structural evolution.

Abstract

Organic photocatalysts are emerging as viable and more sustainable tools than metal complexes. Recently, the field of organo‐photocatalysis has experienced an explosion in terms of applications, redesign of well‐established systems, and identification of novel scaffolds. A rational approach to the structural modification of the different photocatalysts is key to accessing unprecedented reactivity, while improving their catalytic performances. We herein discuss the concepts underpinning the scaffold modification of some of the most recently used photocatalysts and analyze how specific structural changes alter their physicochemical and redox properties.

An I(I)/I(III) catalysis strategy to construct an enantioenriched hybrid fluorinated analogue of the iPr and CF(CF3)2 groups is reported. The difluorination of readily accessible α‐CF3‐styrenes is enabled by the in situ generation of a chiral ArIF2 species to forge a stereocentre with the substituents F, CH2F and CF3 (up to 95%, >20:1 vicinal:geminal difluorination). The replacement of the metabolically labile benzylic proton results in a highly preorganised scaffold as was determined by X‐ray crystallography (π→ s* and stereoelectronic gauche s→ s* interactions). A process of catalyst editing is disclosed in which preliminary validation of enantioselectivity is  placed on a structural foundation.

Formic acid is receiving attention as a promising hydrogen source as well as a valuable chemical for the industries of textiles, pharmaceuticals etc. This minireview is focused on the recent advancement in its formation from biomass, using oxidative pathways and different catalysts such as polyoxometalate, vanadium oxide, hydrothermal, and photocatalysis, developed over the past ten years.

Abstract

Formic acid (FA) is well‐known as a promising hydrogen source as well as a valuable chemical for the industries of textiles, pharmaceuticals etc. In fact, 1.137 million metric tons of FA is required worldwide per year to meet the current demand. Therefore, there is a strong interest for the generation of FA in a sustainable way to meet the future demand. Inspired by this information, several strategies have been considered for the sustainable synthesis of FA. Herein, we summarise the recent articles for the oxidative transformation of biomass into FA.