LongLarf

“My greatest achievement has been the (unplanned) home birth of my youngest daughter. My worst nightmare is never publishing in Angewandte Chemie ever again …” Find out more about Robert Wolf in his Author Profile.

Excited deracemization: Visible‐light photocatalysis has been utilized to convert racemates into single enantiomers in the presence of a chiral photosensitizer or a pair of chiral catalysts. Mechanistic studies revealed that energy transfer (E_nT) or a sequence of electron transfer (ET), proton transfer (PT), and hydrogen atom transfer (HAT) is operative.

Abstract

Deracemization is an ideal but challenging strategy for the conversion of a racemic mixture into a single enantiomer. Recent studies have demonstrated that visible‐light photocatalysis could be utilized to promote selective deracemization of axially chiral allenes as well as cyclopropylquinolones and cyclic ureas with central chirality either through energy transfer or through a sequence of electron, proton, and hydrogen‐atom transfer.

Although pure hydrogen cyanide can spontaneously polymerize or even explode, which is initiated by small amounts of bases (e.g. CN – ), the reaction of liquid HCN with [WCC]CN (WCC = weakly coordinating cation = Ph 4 P, Ph 3 PNPPh 3 = PNP) was investi­gated. Depending on the cation, it was possible to extract salts from liquid HCN, containing the formal dihydrogen tricyanide [CN(HCN) 2 ] – and trihydrogen tetracyanide ions [CN(HCN) 3 ] – , when the crystal­lization was carried out fast and at low temperatures. X‐ray structure elucidation revealed hydrogen‐bridged linear [CN(HCN) 2 ] – and Y‐shaped [CN(HCN) 3 ] – molecular ions in the crystal. Both anions can be considered members of highly labile cyanide HCN‐solvates of the type [CN(HCN) n ] – ( n = 1, 2, 3 …) as well as formal polypseudohalide ions.

Canataxpropellane belongs to the medicinally important taxane diterpene family. The most prominent congener, Taxol, is one of the most commonly used anticancer agent in clinics today. Canataxpropellane exhibits a taxane skeleton with three additional transannular C–C bonds, resulting in a total of six contiguous quaternary carbons, of which four are located on a cyclobutane ring. Unfortunately, isolation of canataxpropellane from natural sources is inefficient. Here, we report a total synthesis of (–)-canataxpropellane in 26 steps and 0.5% overall yield from a known intermediate corresponding to 29 steps from commercial material. The core structure of the (–)-canataxpropellane (2) was assembled in two steps using a Diels–Alder/ortho-alkene-arene photocycloaddition sequence. Enantioselectivity was introduced by designing chiral siloxanes to serve as auxiliaries in the Diels–Alder reaction.

Abstract

An efficient biocatalytic approach for enantio‐ and regioselective ring‐opening of styrene oxides with cyanate was developed by using the halohydrin dehalogenase HheC from Agrobacterium radiobacter AD1, generating the corresponding chiral 5‐aryl‐2‐oxazolidinones in up to 47% yield and 90% ee. Additionally, the origin of enantioselectivity and regioselectivity of the HheC‐catalyzed cyanate‐mediated ring‐opening process was uncovered by single enantiomer bioconversions and molecular docking study.

Cyclic α‐(ortho‐iodophenyl)‐β‐oxoesters were converted in a ring‐expanding transformation through electrochemical reduction to furnish benzannulated cycloalkanone carboxylic esters. A broad variety of starting materials could be converted: cyclic oxoesters with different ring sizes, acyclic oxoesters, and cyclic compounds substituted by diverse functional groups. Additionally, the reaction mechanism was investigated by experiments and DFT calculations. More information can be found in the Communication by T. Müller, G. Hilt, J. Christoffers et al. on https://doi.org/10.1002/chem.201905570page 3222.

Tales of the unexpected: The first solid‐state structures of tetraiododiborane(4) (B₂I₄) are presented, a compound that was long believed to exist as discrete molecules in all phases like the lighter tetrahalodiboranes(4) B₂F₄, B₂Cl₄, and B₂Br₄. In fact, B₂I₄ exists as two different polymeric forms in the solid state.

Abstract

Herein we present the first solid‐state structures of tetraiododiborane(4) (B₂I₄), which was long believed to exist in all phases as discrete molecules with planar, tricoordinate boron atoms, like the lighter tetrahalodiboranes(4) B₂F₄, B₂Cl₄, and B₂Br₄. Single‐crystal X‐ray diffraction, solid‐state NMR, and IR measurements indicate that B₂I₄ in fact exists as two different polymeric forms in the solid state, both of which feature boron atoms in tetrahedral environments. DFT calculations are used to simulate the IR spectra of the solution and solid‐state structures, and these are compared with the experimental spectra.

CO₂ sponges: Cerous and ceric pyrazolates have been demonstrated to insert carbon dioxide and afford the complexes [Ce^III ₄(Me₂pz⋅CO₂)₁₂] and [Ce^IV(Me₂pz⋅CO₂)₄]. The insertion is reversible and can be exploited for the catalytic formation of cyclic carbonates from epoxides. The potentially reducing behavior of tetra‐n‐butylammonium bromide (TBAB) as a co‐catalyst is also revealed.

Abstract

The homoleptic pyrazolate complexes [Ce^III ₄(Me₂pz)₁₂] and [Ce^IV(Me₂pz)₄]₂ quantitatively insert CO₂ to give [Ce^III ₄(Me₂pz⋅CO₂)₁₂] and [Ce^IV(Me₂pz⋅CO₂)₄], respectively (Me₂pz=3,5‐dimethylpyrazolato). This process is reversible for both complexes, as observed by in situ IR and NMR spectroscopy in solution and by TGA in the solid state. By adjusting the molar ratio, one molecule of CO₂ per [Ce^IV(Me₂pz)₄] complex could be inserted to give trimetallic [Ce₃(Me₂pz)₉(Me₂pz⋅CO₂)₃(thf)]. Both the cerous and ceric insertion products catalyze the formation of cyclic carbonates from epoxides and CO₂ under mild conditions. In the absence of epoxide, the ceric catalyst is prone to reduction by the co‐catalyst tetra‐n‐butylammonium bromide (TBAB).

Anionic molecular imide complexes of aluminium are accessible via a rational synthetic approach involving the reactions of organo azides with a potassium aluminyl reagent. In the case of K 2 [( NON )Al(NDipp)] 2   ( NON  = 4,5‐bis(2,6‐di iso propylanilido)‐2,7‐di‐tert‐butyl‐9,9‐dimethyl‐xanthene; Dipp = 2,6‐di iso propylphenyl) structural characterization by X‐ray crystallography reveals a short Al‐N distance, which is thought to be due primarily to the low coordinate nature of the nitrogen centre.  The Al‐N unit is highly polar, and capable of the activation of relatively inert chemical bonds, such as those found in dihydrogen and carbon monoxide. In the case of CO, uptake of two molecules of the substrate leads to C‐C coupling and C º O bond cleavage. Thermodynamically, this is driven – at least in part – by Al‐O bond formation. Mechanist‐ically, a combination of quantum chemical and experimental observations suggest that the reaction proceeds via exchange of the NR and O substituents through intermediates featuring an aluminium‐bound isocyanate fragment.

Herein we report the development of a photocatalytic strategy for divergent preparation of functionalized bicyclo[1.1.1]pentylamines. This approach exploits, for the first time, the ability of nitrogen‐radicals to undergo strain‐release reaction with [1.1.1]propellane. This reactivity is facilitated by the electrophilic nature of these open‐shell intermediates and the presence of strong polar effects in the transition‐state for C–N bond formation/ring‐opening. With the aid of a simple reductive quenching photoredox cycle, we have successfully harnessed this novel radical strain‐release amination as part of a multicomponent cascade compatible with several external trapping agents. Overall, this radical strategy enables the rapid construction of novel amino‐functionalized building blocks with potential application in medicinal chemistry programs as p ‐substituted aniline bioisosteres.

Nature Communications, Published online: 31 January 2020; doi:10.1038/s41467-020-14494-8

Heterocycles containing difluoromethyl groups are molecules with potential application in pharmaceutical, agricultural and materials science. Here, the authors show an organophotocatalytic difluoromethylation of heterocycles using O2 as green oxidant and preliminarily study the products’ bioactivity.

Stereochemical control in the construction of carbon-carbon bonds between an alkyl electrophile and an alkyl nucleophile is a persistent challenge in organic synthesis. Classical substitution reactions via S_N1 and S_N2 pathways are limited in their ability to generate carbon-carbon bonds (inadequate scope, due to side reactions such as rearrangements and eliminations) and to control stereochemistry when beginning with readily available racemic starting materials (racemic products). Here, we report a chiral nickel catalyst that couples racemic electrophiles (propargylic halides) with racemic nucleophiles (β-zincated amides) to form carbon-carbon bonds in doubly stereoconvergent processes, affording a single stereoisomer of the product from two stereochemical mixtures of reactants.

Abstract

Manganese‐catalysed reactions have attracted great attention recently due to the high relative abundance and cheap and eco‐friendly behaviour. Applications of manganese catalysis in cross‐dehydrogenative coupling are among the hottest areas since 90% of the contributions are very recent. Dehydrogenation of alcohols using Mn‐pincer systems is highly explored nowadays for cross‐coupling to synthesise a variety of products and Mn‐catalysed C−H activation, and radical reactions are applied in dehydrogenative couplings of various synthons. This review focuses on the synthesis of synthetically as well as biologically important motifs such as carbonyl compounds, olefins, nitrogen heterocycles, amines, imines, etc. using manganese‐catalysed dehydrogenative cross‐coupling reactions.

Abstract

The purpose of this review is to present an overview of the patent landscape for catalysts used in hydrogenation reactions. Based on patent data extracted from PatBase®, we use predefined patent classifications as well as a keyword‐based search for our analyses. The results indicate that the number of patent families that protect heterogeneous catalysts grows twice as fast as that for their homogeneous counterparts. Furthermore, the data show a shift towards abundant and non‐toxic elements in heterogeneous catalysis, while the noble metals continue to dominate the patent landscape of homogeneous catalysis. A subsequent geographical analysis reveals that the high growth rates in heterogeneous catalysis, especially for nickel and iron, are driven by China. Conversely, patenting activities with regard to homogeneous catalysts mainly take place in the USA, the EU, and Japan. The subsequent keyword‐based search illustrates the continuous industrial relevance of enantioselective hydrogenation and transfer hydrogenation, as well as the rapidly increasing body of patents in hydrodeoxygenation. Setting these finding into context, we present and apply two concepts that are commonly used in patent analyses, namely the technology life cycle and the S‐curve. We conclude that hydrogenation catalysis has not reached its peak economic relevance yet and will continue to spark valuable patents and innovations in the future.

Alkylation with ruthenium: Eight new Ru pincer complexes were prepared by introduction of C,N‐bound phenyl heterocycle ligands. These complexes were applied as catalysts in the α‐alkylation of ketones with alcohols through hydrogen autotransfer. Here, superior yields in comparison to [RuHCl(CO)(HN(CH₂CH₂PPh₂)₂)] (Ru‐MACHO) were achieved and 21 different alkylated ketones were synthesized in up to 97 % yield. Moreover, investigations regarding the catalyst species present during the reaction were conducted.

Abstract

Ruthenium PNP pincer complexes bearing supplementary cyclometalated C,N‐bound ligands have been prepared and fully characterized for the first time. By replacing CO and H⁻ as ancillary ligands in such complexes, additional electronic and steric modifications of this topical class of catalysts are possible. The advantages of the new catalysts are demonstrated in the general α‐alkylation of ketones with alcohols following a hydrogen autotransfer protocol. Herein, various aliphatic and benzylic alcohols were applied as green alkylating agents for ketones bearing aromatic, heteroaromatic or aliphatic substituents as well as cyclic ones. Mechanistic investigations revealed that during catalysis, Ru carboxylate complexes are predominantly formed whereas neither the PNP nor the CN ligand are released from the catalyst in significant amounts.

Dry reforming of methane: Proper thermal pretreatment of MgO−Al₂O₃ mixed oxides and impregnation with only 2.5 wt % Ni, in some cases together with citric acid and La, provides Ni catalysts with outstanding stability against coking at severe CH₄‐rich conditions.

Abstract

Highly active and coking‐resistant Ni catalysts suited for the dry reforming of CH₄‐rich gases (70 vol %, e. g. biogas or sour natural gas) were prepared starting from a Mg‐rich Mg−Al hydrotalcite support precursor. Calcination at 1000 °C yields two phases, MgO and MgAl₂O₄ spinel. Complexation‐deposition of Ni with citric acid on the preformed support as well as lanthanum addition yields a catalyst with remarkably low carbon accumulation over 100 h on stream attributed to both high Ni dispersion and preferred interactions of Ni with MgO on MgAl₂O₄.

The Front Cover picture shows that “it is hard to stand just on one leg”. In their Full Paper, Reni Grauke et al. show that the combination of Electron Paramagnetic Resonance (EPR) and X‐ray absorption spectroscopy (XAS) is necessary to unravel formation, structure and function of the active Cr complex in the presence of different aluminium containing activators during ethylene tetramerization. Best catalytic performance was obtained with MMAO, being the only activator that ensures proper bidentate coordination of the PNP ligand which is essential to enable formation of the chromacyclononane intermediate. More information can be found in the Full Paper by Reni Grauke et al. on page 1025 in Issue 4, 2020 (DOI: https://doi.org/10.1002/cctc.20190144110.1002/cctc.201901441).

“ it was only successful through the collaboration of two research teams ” This and more about the story behind the research featured on the front cover can be found in this issue's Cover Profile. Read the full text of the corresponding research at https://doi.org/10.1002/cctc.201901441https://doi.org/10.1002/cctc.201901441.

Abstract

The front cover artwork for Issue 04/2020 is a collaboration between Leibniz Institute for Catalysis (Germany) and University of Paderborn (Germany). The image illustrates that it's hard to stand just on one leg when investigating a very complex reaction. A proper bidentate coordination of a PNP ligand to a Cr complex catalyst is needed to ensure high performance in homogeneous ethylene tetramerization. The synergy between in situ EPR and XAS spectroscopy is essential to derive reliable information. See the Full Paper itself at https://doi.org/10.1002/cctc.201901441.

How exciting! For aromatic substitutions and cross‐couplings, photochemical reactions provide an alternative to the classical reactions under thermal control. This Minireview summarizes the recent rapid progress of light‐enabled transition‐metal‐free aromatic reactions and discusses different reactivity modes of excited aryl electrophiles with nucleophiles from the mechanistic perspective.

Abstract

Transition‐metal‐catalyzed cross‐couplings between aromatic electrophiles and nucleophiles have revolutionized modern chemical syntheses. Nevertheless, transition‐metal‐free approaches are preferable, considering the various issues caused by metal catalysts. This Minireview summarizes the recent progress in the light‐enabled transition‐metal‐free formation of carbon–carbon and carbon–heteroatom bonds in aromatics, which opens a new avenue in aromatic reactions. From the mechanistic perspective, it classifies different reaction types of aryl electrophiles in an excited state with various nucleophiles. We believe this will provide more rationales for metal‐free aromatic substitutions and cross‐couplings with light, and guide the development of novel transformations of aromatic compounds facilitated by light.

We have developed an azanorbornadiene bromovinyl sulfone reagent that allows cysteine‐selective bioconjugation. Subsequent reaction with  dipyridyl tetrazine led to bond‐cleavage and formation of a pyrrole‐linked conjugate. The latter involves ligation of the tetrazine to the azanorbornadiene‐tagged protein through  inverse electron demand Diels–Alder cycloaddition  with subsequent double  retro ‐Diels–Alder reactions to form a stable pyrrole linkage. The sequence of site‐selective bioconjugation followed by bioorthogonal bond‐cleavage was efficiently employed for the labelling of three different proteins. This method benefits from easy preparation of these reagents, selectivity for cysteine, and stability after reaction with a commercial tetrazine, which lends it to the routine preparation of protein conjugates for chemical biology studies.

Molecular shape defines function in both biological and material settings, and chemists have developed an ever-increasing vernacular to describe these shapes. Noncanonical atropisomers—shape-defined molecules that are formally topologically trivial but are interconvertible only by complex, nonphysical multibond torsions—form a unique subset of atropisomers that differ from both canonical atropisomers (e.g., binaphthyls) and topoisomers (i.e., molecules that have identical connectivity but nonidentical molecular graphs). Small molecules, in contrast to biomacromolecules, are not expected to exhibit such ambiguous shapes. Using total synthesis, we found that the peptidic alkaloid tryptorubin A can be one of two noncanonical atropisomers. We then devised a synthetic strategy that drives the atropospecific synthesis of a noncanonical atrop-defined small molecule.