Articles in 2020

High-throughput biocatalytic screening and metagenomics have been used to discover over 300 imine reductases (IREDs) and subsequently produce a sequence-diverse panel of IREDs suitable for optimizing the synthesis of chiral amines. Additional characterization identified biocatalysts that accommodate structurally demanding amines and ketones for enzymatic reductive aminations.

Methods for producing organic molecules rich in sp³-hybridized carbon centres can be particularly useful for drug development. Now, it has been shown that the enantioselective cross-coupling of non-activated alkyl halides with alkenyl boronates enables the synthesis of chiral alkyl boronates. The reaction proceeds via nickel hydride insertion into an internal alkene followed by nickel-catalysed alkyl–alkyl cross-coupling.

Energy scrambling in intermediate complexes—which form in many chemical reactions—presents a major challenge to state-to-state control. However, nuclear spin tends to remain unchanged throughout reactions and now, by manipulating the reactants’ nuclear spins using an external magnetic field, control over the product state distribution of a bimolecular reaction has been demonstrated.

Fluorinated polyacetylene has typically proven to be inaccessible using traditional polymer synthesis, but there is much interest in its predicted properties. Now, a mechanochemical unzipping strategy has succeeded in the synthesis of a gold-coloured, semiconducting fluorinated polyacetylene with improved stability in air compared to polyacetylene.

Polyether ionophores are natural products that display antibacterial activity—but they also show activity against mammalian cells, which has limited their development as clinical antibiotics. Now, a semisynthesis principle of recycling substructures from highly abundant natural polyether ionophores has been used to prepare analogues with enhanced selectivity towards bacterial cells.

Automated synthesis technologies are often highly specialized, focusing only on a narrow set of reaction classes. Now, solid-phase peptide synthesis, iterative Suzuki–Miyaura cross-coupling and diazirine chemistry have all been automated using the same universal platform architecture. A convergent 12-step synthesis demonstrates the utility of the reported Chemputer system.

Metallocenes are attractive mechanophores because they are stable in the absence of force, yet reactive under tension. Now, covalently bridging the two cyclopentadienyl (Cp) ligands of ferrocenes embedded in a polymer has been shown to alter their mechanochemical reactivity, leading to a faster dissociation of the Fe–Cp bond, which occurs through a peeling mechanism rather than a shearing one.

A method to label membrane proteins with a DNA tag has been developed that enables the selection of DNA-encoded chemical libraries against endogenous membrane proteins on live cells. As a demonstration, a 30-million-compound DNA-encoded chemical library is screened against folate receptor, carbonic anhydrase 12 and epidermal growth factor receptor on live cells.

Many biosynthetic cyclizations are catalysed by iron oxygenases and appear to involve long-lived radical species. Now, mimicking these biosynthetic transformations, the total synthesis of highly oxidized lignan natural products has been reported using redox-neutral photocatalysis to enable late-stage radical cyclizations that install challenging 5- and 11-membered rings.

A combination of metal- and anion-template synthesis directs the weaving of molecular weft and warp strands in the assembly of a 3 × 3 interwoven grid. Connection of the ligand strands by alkene metathesis produces the topology of a seven-crossing endless knot, an important cultural and religious symbol.

Unlike ferrocene and its cationic counterpart ferrocenium, the ferrocene monoanion is an unusual species that has been observed through low-temperature electrochemical studies. Now, a family of isostructural 3d metallocenates has been isolated that consists of a manganocene, a cobaltocene and a high-spin ferrocene anion stabilized by cyclopentadienyl ligands bearing bulky aliphatic groups.

¹³⁴Ce and ¹³⁴La have great potential as companion diagnostic isotopes for radiotherapeutics labelled with α-emitting ²²⁵Ac and ²²⁷Th. Now, by controlling the Ce^III/Ce^IV redox couple, the large-scale production, purification and characterization of ¹³⁴Ce- and ¹³⁴La-based radiolabels has been achieved and their use for in vivo positron emission tomography is demonstrated.

Methods for the dehydrosilylation of alkenes typically suffer from low selectivity and narrow scope. Now, the highly selective manganese-catalysed redox-neutral dehydrosilylation and hydrosilylation of alkenes has been achieved through ligand-tuned metalloradical activity. Whereas an electron-rich bidentate ligand promotes dehydrosilylation, hydrosilylation is the favoured pathway when using an electron-deficient phosphine ligand. DFT calculations have been carried out to understand the observed selectivity.

A wide variety of organic and inorganic compounds show π-aromaticity, yet for all-metal systems it has remained restricted to compounds with three to five atoms. Now, the anionic cluster [Th@Bi₁₂]⁴⁻ has been shown to exhibit π-aromaticity, with a significant ring current despite relying on the delocalization of only two π-electrons.

Cyanide-bridged CoFe coordination networks exhibit photomagnetism because of coupled charge-transfer and spin transition. Now, femtosecond X-ray and optical absorption spectroscopies have enabled the electronic and structural dynamics of this light-induced process to be disentangled and show that it is the spin transition on the cobalt atom, occurring within ~50 fs, that induces the Fe-to-Co charge-transfer within ~200 fs.

Moving towards renewable energy sources requires pathways for efficiently converting electricity to chemicals, and gas-fed CO₂ electrolysers show promise. Now, the layer-by-layer assembly of a weak-acid cation exchange layer has been shown to affect the local pH in a bipolar-membrane-based gas-fed CO₂ electrolyser, improving the conversion efficiency of CO₂ to CO by suppressing the competing hydrogen evolution reaction.

Electronic–vibrational interplay can enable electron and energy transfer processes to be regulated. Now, coherence spectroscopy has been used to disentangle two vibrational pathways that control an electron transfer reaction. It has been shown that a fast, effectively ballistic, electron transfer along one vibrational path acts like a pulse to generate a coherent wavepacket along another vibrational pathway.

The role of the biexcitonic triplet-pair state ¹(TT) during triplet–triplet annihilation events in singlet-fission materials has been the subject of recent debate. Now, emissive ¹(TT) states have been shown to be direct products of triplet–triplet annihilation in both endothermic and exothermic singlet-fission materials.

A method for the covalent labelling of proteins by installing a biostable peptide nucleic acid (PNA) tag has now been developed. The PNA label serves as a generic landing platform that enables the recruitment of fluorescent dyes via nucleic acid hybridization and fluorophore removal by toehold-mediated strand displacement. Imaging of cell surface receptors, including internalized receptors, has been demonstrated using this approach.

Diatomic C₂ is an elusive species that has only been indirectly observed in the gas phase. It had previously been stabilized in the condensed phase using two ligands, but now a monoligated L→C₂ complex has been prepared with a bulky phosphine ligand (L) bearing two imidazolidin-2-iminato groups. Reactivity studies and theoretical quantum chemical analysis point to the C₂ moiety having a dicarbene character.