LongLarf

Nature Communications, Published online: 25 September 2019; doi:10.1038/s41467-019-12414-z

For the demonstration of physical forces in biology, a general concept in supramolecular chemistry that focuses on chalcogen bonds is introduced to access the desired stable, evolvable mechanochemistry tools. The source of inspiration of their rational design is illustrated in freeze–thaw cycles in DMSO.

Abstract

Planarizable push–pull probes have been introduced to demonstrate physical forces in biology. However, the donors and acceptors needed to polarize mechanically planarized probes are incompatible with their twisted resting state. The objective of this study was to overcome this “flipper dilemma” with chalcogen‐bonding cascade switches that turn on donors and acceptors only in response to mechanical planarization of the probe. This concept is explored by molecular dynamics simulations as well as chemical double‐mutant cycle analysis. Cascade switched flipper probes turn out to excel with chemical stability, red shifts adding up to high significance, and focused mechanosensitivity. Most important, however, is the introduction of a new, general and fundamental concept that operates with non‐trivial supramolecular chemistry, solves an important practical problem and opens a wide chemical space.

Carbon allotropes built from rings of two-coordinate atoms, known as cyclo[n]carbons, have fascinated chemists for many years, but until now they could not be isolated or structurally characterized because of their high reactivity. We generated cyclo[18]carbon (C₁₈) using atom manipulation on bilayer NaCl on Cu(111) at 5 kelvin by eliminating carbon monoxide from a cyclocarbon oxide molecule, C₂₄O₆. Characterization of cyclo[18]carbon by high-resolution atomic force microscopy revealed a polyynic structure with defined positions of alternating triple and single bonds. The high reactivity of cyclocarbon and cyclocarbon oxides allows covalent coupling between molecules to be induced by atom manipulation, opening an avenue for the synthesis of other carbon allotropes and carbon-rich materials from the coalescence of cyclocarbon molecules.

Overcoming limitations: Addressing practical difficulties in the downstream‐processing of biotransformations in biphasic reaction media, in particular extremely difficult phase separations due to emulsification and precipitation, a simple flow set‐up being capable to minimise such work‐up limitations and, thus, to overcome these challenges in process development is reported exemplified for two different enzymatic ketone reductions.

Abstract

Biphasic biocatalytic reactions have gained much attention in the field of enzyme‐catalysed synthesis. As most components being of relevance for the pharmaceutical industry are hydrophobic, often biphasic reaction media turned out to be the solvent system of choice. However, in spite of successful reaction courses practical difficulties in the downstream‐processing, in particular extremely difficult phase separations due to emulsification and precipitation, represent a challenge to overcome in process development. In this work, we report our studies on the benefits of a simple flow set‐up being capable to minimise such work‐up limitations. In detail, a segmented flow system based on a biphasic MTBE/buffer mixture was successfully applied for two types of enzymatic reductions of a hydrophobic ketone in the presence of an alcohol dehydrogenase (ADH) as an enzyme class being known for their excellent enantioselectivity and successful utilization in the synthesis of a range of active pharmaceutical ingredients. The applicability of this flow system was demonstrated with two different enzymes as well as different substrates. Besides an ADH from Lactobacillus brevis, an ADH from Ogatea minuta was utilized for the reduction of acetophenone and 2,2,2‐trifluoroacetophenone, respectively.

A small frog and a big bond: This minireview surveys advances in the selective homogeneous and heterogeneous oxyfunctionalization of non‐activated C−H bonds in hydrocarbons of natural and non‐natural targets by using isolated dioxirane or, more generally, by using ketones (i.e., the dioxirane precursors) as remarkable organocatalysts.

Abstract

The successful isolation and characterization of a dioxirane species in 1988 opened up one of the most attractive methods for the efficient oxidation of simple and/or structurally complex molecules. Dioxirane today rank among the most powerful tools in organic chemistry, with numerous applications in commercially important processes. They were quickly recognized as efficient oxygen transfer agents, especially for epoxidations and for a wide range of O‐insertion reactions into C−H bonds. Dioxirane possess catalytic activity and appear as highly (chemo‐, regio‐, and stereo‐) selective oxidants, despite their reactivity under mild and strictly neutral conditions being controlled by a combination of steric and electronic factors. In this review, we discuss some of the most recent and significant developments in the selective homogeneous and heterogeneous oxyfunctionalization of non‐activated C−H bonds in hydrocarbons of natural and non‐natural targets by using isolated dioxirane or, more generally, by using the ketones (i.e., the dioxirane precursors) as organocatalysts.

Terminal copper-nitrenoid complexes have inspired interest in their fundamental bonding structures as well as their putative intermediacy in catalytic nitrene-transfer reactions. Here, we report that aryl azides react with a copper(I) dinitrogen complex bearing a sterically encumbered dipyrrin ligand to produce terminal copper nitrene complexes with near-linear, short copper–nitrenoid bonds [1.745(2) to 1.759(2) angstroms]. X-ray absorption spectroscopy and quantum chemistry calculations reveal a predominantly triplet nitrene adduct bound to copper(I), as opposed to copper(II) or copper(III) assignments, indicating the absence of a copper–nitrogen multiple-bond character. Employing electron-deficient aryl azides renders the copper nitrene species competent for alkane amination and alkene aziridination, lending further credence to the intermediacy of this species in proposed nitrene-transfer mechanisms.

Characterisation? NNO worries: The synthesis and comprehensive characterisation of two rhodium(I) complexes of nitrous oxide are reported. These normally elusive adducts are stable in the solid state and persist in solution at ambient temperature.

Abstract

The synthesis of two well‐defined rhodium(I) complexes of nitrous oxide (N₂O) is reported. These normally elusive adducts are stable in the solid state and persist in solution at ambient temperature, enabling comprehensive structural interrogation by ¹⁵N NMR and IR spectroscopy, and single‐crystal X‐ray diffraction. These methods evidence coordination of N₂O through the terminal nitrogen atom in a linear fashion and are supplemented by a computational energy decomposition analysis, which provides further insights into the nature of the Rh–N₂O interaction.

DFT calculations: Cyclometalated iron complexes have been suggested as potent chromophores based on theoretical investigations. Herein, the impact of cyclometalation on the ground and excited state is experimentally shown for the first time with the complex [Fe(pbpy)(tpy)]⁺ (Hpbpy=6‐phenyl‐2,2′‐bipyridine and tpy=2,2′:6′,2′′‐terpyridine; see figure) of [Fe(C^N^N)(N^N^N)]⁺ in comparison to [Fe(tpy)₂]²⁺ _.

Abstract

The complex class [Fe(N^N^C)(N^N^N)]⁺ with an Earth‐abundant metal ion has been repeatedly suggested as a chromophore and potential photosensitizer on the basis of quantum chemical calculations. Synthesis and photophysical properties of the parent complex [Fe(pbpy)(tpy)]⁺ (Hpbpy=6‐phenyl‐2,2′‐bipyridine and tpy=2,2′:6′,2′′‐terpyridine) of this new chromophore class are now reported. Ground‐state characterization by X‐ray diffraction, electrochemistry, spectroelectrochemistry, UV/Vis, and X‐ray spectroscopy in combination with DFT calculations proves the high impact of the cyclometalating ligand on the electronic structure. The photophysical properties are significantly improved compared to the prototypical [Fe(tpy)₂]²⁺ complex. In particular, the metal‐to‐ligand absorption extends into the near‐IR and the ³MLCT lifetime increases by 5.5, whereas the metal‐centered excited triplet state is very short‐lived.

Palladium(IV) tris(halido) complexes supported by aryl‐bridged bis‐mesoionic carbenes show unusual stability under ambient conditions. The synthesis, solid‐state structures, and reactivity of the complexes are presented and investigated.

Abstract

The study of palladium(IV) species has great implications for Pd^II/Pd^IV‐mediated catalysis. However, most of the Pd^IV complexes rapidly decompose under ambient conditions, which makes the isolation, characterization and further reactivity study very challenging. The reported ancillary ligand platforms to stabilize Pd^IV species are dominated by chelating N‐donors such as bipyridines. In this work, we present two Pd^IV complexes with scarcely used C‐donors as the supporting platform. The anionic aryl donor and MIC (MIC=mesoionic carbene) are combined in a [CC′C]‐type pincer framework to access a series of ambient‐stable Pd^IV tris(halido) complexes. Their synthesis, solid‐state structures, stability, and reactivity are presented. To the best of our knowledge, the work presented herein reports the first isolated Pd^IV–MIC as well as the first Pd^IV carbene‐based aryl pincer.