Oleg Borodin

Through the evolution of their chemistry, calixpyrroles have emerged as very versatile molecules with a wide range of applications, spanning from molecular recognition to being lead‐compounds for the development of new anti cancer drugs. This review traces some contributions of the author to these topics, placing them in the context of the work developed by research groups worldwide without attempting to provide a comprehensive account of the field.

Since the discovery that calix[4]pyrrole can act as a ligand for anions and other species that can form multiple hydrogen bonds with its pyrrole NH units, this macrocyclic system, its derivatives, and related pyrrole‐containing anion binders, have been a topic of an ever growing interest for their potential applications in diverse fields, including molecular recognition, sensing, catalysis, self‐assembly, and the fabrication of smart materials. This review traces some contributions of the author to these topics, placing them in the context of the work developed by research groups worldwide without attempting to provide a comprehensive account of the field. As anions play a key role in the biological world, anion‐binding receptors and calixpyrroles have emerged as potential drugs due to their ability to transport anions across membranes. Both antibacterial and cytotoxic effects against cancer cell lines have been reported for some chloride transporters. However, in the case of certain calixpyrrole derivatives, we gained evidence of their ability to cause genotoxic effects and hence to exhibit a cytotoxic action that does not appear correlated to anion transport. These observations might pave the way towards new anticancer drugs.

We report an enantioselective catalyst based on a “mechanically chiral” rotaxane. Catalysis with chiral molecules is extremely important in modern chemistry because it is one of the most efficient ways to make chiral molecules for applications in many areas. Our results demonstrate, for the first time, that mechanically chiral molecules are a promising and underexplored platform for generating such catalysts. We achieve enantioselectivities for the AuI-catalyzed Ohe-Uemura cyclopropanation of benzoate esters comparable to previously reported covalent catalysts.

We report the unusual dynamic characteristics of the C=N bonds in the pyrazine ring promoted under basic aqueous conditions, which enables the successful synthesis of two-dimensional porous graphitic frameworks (PGFs) featuring fully annulated aromatic skeletons and periodic pores. The PGF displayed high electrical conductivity and remarkable performance as a cathode material for lithium-ion batteries, far outperforming the amorphous counterparts in terms of capacity and cycle stability.

Nanorobotics deals with artificially created micro- and nanomachines capable of active motion and conducting pre-programmed tasks. To become viable force in the biologic and environmental remediation, the nanorobots need to become true nanosized devices. At this dimensions, entropic forces of the surrounding environment dramatically change how one can assess and design self-propulsion. In this review, we explore a recent push of the nanorobotics into this fully nanosized region.

Nanorings : The first examples of cyclofluorenes possessing five constituting fluorene units were synthesized and studied. The electronic properties of [5]‐cyclofluorenes are drastically different from those of [4]‐cyclofluorenes, highlighting the key role played by the ring size in the cyclofluorene family.

Abstract

For the last ten years, ring‐shaped π‐conjugated macrocycles possessing radially directed π‐orbitals have been subject to intense research. The electronic properties of these rings are deeply dependent on their size. However, most studies involve the flagship family of nanorings: the cyclo‐para ‐phenylenes. We report herein the synthesis and study of the first examples of cyclofluorenes possessing five constituting fluorene units. The structural, optical and electrochemical properties were elucidated by X‐ray crystallography, UV‐vis absorption and fluorescence spectroscopy, and cyclic voltammetry. By comparison with a shorter analogue, we show how the electronic properties of [5]‐cyclofluorenes are drastically different from those of [4]‐cyclofluorenes, highlighting the key role played by the ring size in the cyclofluorene family.

Nature, Published online: 06 April 2020; doi:10.1038/d41586-020-01044-x

Trust, testing, treatment and a quirk of fate have kept Germany’s death rate an order of magnitude lower than those in nearby nations. Plus: step through the coronavirus’s full genome and rediscover the polio epidemic that invented intensive care.

Three tri(thio)ureido‐hexahomotrioxacalix[3]arene‐based receptors were synthesised. Phenylurea 4a, obtained in the cone conformation, displayed very high association constants for AcO^– and BzO^– anions. Compound 4a was also tested as a ditopic receptor and showed an outstanding affinity for biogenic amine hydrochlorides. The studies were done by ¹H NMR spectroscopy and corroborated by DFT calculations.

Hexahomotrioxacalix[3]arene‐based receptors (phenylurea 4a, phenylthiourea 4b and tert‐butylurea 4c) were synthesised and obtained in the partial cone conformation in solution. Phenylurea 4a was also obtained in the cone conformation and its X‐ray crystal structure is reported. Their binding properties towards several anions of different geometries were studied by ¹H NMR titrations. The data showed that phenylurea cone 4a is the best anion receptor, displaying a very high affinity for the carboxylates AcO^– and BzO^– anions (log K _ass = 4.12 and 4.00, respectively). Phenylurea partial cone 4a and phenylthiourea 4b, although weaker, are still reasonably good receptors, showing the same trend as cone 4a. tert‐Butylurea 4c exhibited very low affinity for all the anions, confirming that the association constants strongly depend on the nature of the substituent (aryl/alkyl) at the urea moiety. In addition, cone and partial cone 4a derivatives were also tested in the recognition of n‐alkylammonium salts and cone 4a also as a ditopic receptor for biogenic amine hydrochlorides by ¹H NMR titrations. Cone 4a showed an outstanding affinity for the guest alkylammonium and amines, even at 298 K and in a more competitive solvent such as CDCl₃/[D₆]DMSO mixture. DFT calculations corroborated the NMR observations.

Herein, we report the synthesis of three short‐chained anthracene strapped porphyrins and their corresponding endoperoxides, formed by a cycloaddition reaction between in situ generated singlet oxygen and the anthracene moiety. We also synthesized Zn(II)complexes of the strapped porphyrins. X‐ray crystallography and UV/Vis analysis confirmed macrocyclic distortion in the strapped systems.

The syntheses of short‐chained anthracene‐strapped porphyrins and their Zn(II)complexes are reported. The key synthetic step is a [2+2] condensation between a dipyrromethane and an anthracene bisaldehyde, 2,2'‐((anthracene‐9,10‐diylbis(methylene))bis(oxy))dibenzaldehyde. Following exposure to white light, self‐sensitized singlet oxygen and the anthracene moieties underwent [4+2] cycloaddition reactions to yield the corresponding endoperoxides. ¹H NMR studies demonstrate that the endoperoxide readily formed in [D]chloroform and decayed at 85 °C. X‐ray crystallography and absorption spectroscopy were used to confirm macrocyclic distortion in the parent strapped porphyrins and endoperoxides. Additionally, X‐ray crystallography indicated that endoperoxide formation occurred exclusively on the outside face of the anthracene moiety.

Competition time: Anions, gases and solvents present in chemical reactions compete with ligands for metal coordination sites. Coordinating ability indices (CAI) rank such species according to their affinity to three families of metals: transition metals, lanthanides and s‐block metals. After a brief review of recent applications to a variety of fields (synthesis, catalysis, nanochemistry, electrochemistry.), a new extended set of CAI is presented and discussed.

Abstract

After briefly reviewing the applications of the coordination ability indices proposed earlier for anions and solvents toward transition metals and lanthanides, a new analysis of crystal structures is applied now to a much larger number of coordinating species: anions (including those that are present in ionic solvents), solvents, amino acids, gases, and a sample of neutral ligands. The coordinating ability towards s‐block elements is now also considered. The effect of several factors on the coordinating ability will be discussed: (a) the charge of an anion, (b) the chelating nature of anions and solvents, (c) the degree of protonation of oxo‐anions, carboxylates and amino carboxylates, and (d) the substitution of hydrogen atoms by methyl groups in NH₃, ethylenediamine, benzene, ethylene, pyridine and aldehydes. Hit parades of solvents and anions most commonly used in the areas of transition metal, s‐block and lanthanide chemistry are deduced from the statistics of their presence in crystal structures.

“The study combines advances in synthetic chemistry as well as characterization of materials’ properties and is the result of a collaborative effort in organic materials science in Heidelberg.” Read more about the story behind the cover in the Cover Profile and about the research itself on page 4269 ff. (DOI: https://doi.org/10.1002/chem.20190561110.1002/chem.201905611).

Abstract

Invited for the cover of this issue are Lutz H. Gade, Claudia Backes, and co‐workers at Heidelberg University. The image depicts 2‐(1,2,2‐triarylvinyl)‐pyridines, which are luminogens for aggregation‐induced emission which “light up” upon irradiation. Read the full text of the article at https://doi.org/10.1002/chem.20190561110.1002/chem.201905611.

Intrinsic advantages and uniqueness in chirality amplification are exhibited by pillararenes. By combining high affinity for guests, detectable UV absorption, and inducible chirality they can be used for detecting cryptochiral compounds.

Abstract

The contradiction between the rising demands of optical chirality sensing and the failure in chiral detection of cryptochiral compounds encourages researchers to find new methods for chirality amplification. Inspired by planar chirality and the host–guest recognition of pillararenes, we establish a new concept for amplifying CD signals of cryptochiral molecules by pillararene host–guest complexation induced chirality amplification. The planar chirality of pillararenes is induced and stabilized in the presence of the chiral guest, which makes the cryptochiral molecule detectable by CD spectroscopy. Several chiral guests are selected in these experiments and the mechanism of chiral amplification is studied with a non‐rotatable pillararene derivative and density functional theory calculations. We believe this work affords deeper understanding of chirality and provides a new perspective for chiral sensing.

Nature, Published online: 01 April 2020; doi:10.1038/d41586-020-00872-1

An electrically neutral radical has been found to be a potent chemical reducing agent when excited by light. Remarkably, it is produced from a positively charged precursor that has long been used as a strong excited-state oxidizing agent.

Nature Protocols, Published online: 31 March 2020; doi:10.1038/s41596-020-0307-7

Guidance for quantitative confocal microscopy

Nature Protocols, Published online: 31 March 2020; doi:10.1038/s41596-020-0313-9

This tutorial and the accompanying poster (https://doi.org/10.1038/s41596-020-0307-7) provide a guide for performing quantitative fluorescence imaging using confocal microscopy. It includes advice and troubleshooting information from sample preparation and microscope setup to data analysis and statistics.

At the flick of a switch : An electrically controllable reaction at the single‐molecule level is demonstrated. The reversible reaction can be electrically controlled to perform switching and memory functions, where the conductance values of two molecular isomers are encoded as 0 and 1. Multiple switching cycles are observed when a voltage signal is applied, and the absence of stochastic switching suggests application as a memory device.

Abstract

The exponential proliferation of data during the information age has required the continuous exploration of novel storage paradigms, materials, and devices with increasing data density. As a step toward the ultimate limits in data density, the development of an electrically controllable single‐molecule memristive element is reported. In this device, digital information is encoded through switching between two isomer states by applying a voltage signal to the molecular junction, and the information is read out by monitoring the electrical conductance of each isomer. The two states are cycled using an electrically controllable local‐heating mechanism for the forward reaction and catalyzed by a single charge‐transfer process for the reverse switching. This single‐molecule device can be modulated in situ, is fully reversible, and does not display stochastic switching. The I –V curves of this single‐molecule system also exhibit memristive character. These features suggest a new approach for the development of molecular switching systems and storage‐class memories.

Molecular muscle: The synthesis and characterization of a fluorescent, small, pi‐rich [c2]daisy chain is described. The synthesis is accomplished using a multi‐component Cu^I‐catalysed active‐template Cadiot–Chodkiewicz reaction, ultimately assembling four molecular components into a single mechanically interlocked architecture. Through variable temperature NMR, the motion of this structure is then described.

Abstract

Molecules and materials that demonstrate large amplitude responses to minor changes in their local environment play an important role in the development of new forms of nanotechnology. Molecular daisy chains are a type of a mechanically interlocked molecule that are particularly sensitive to such changes in which, in the presence of certain stimuli, the molecular linkage enables muscle‐like movement between a reduced‐length contracted form and an increased‐length expanded form. To date, all reported syntheses of molecular daisy chains are accomplished via passive‐template methods, resulting in a majority of structures being switchable only through the addition of an exogenous stimuli such as metal ions or changes in pH. Here, we describe a new approach to these structural motifs that exploits a multi‐component active‐metal template synthesis to mechanically interlock two pi‐rich nanohoop macrocycles into a molecular daisy chain that undergoes large conformational changes using thermal energy.

Supramolecular substrate orientation is a powerful tool to control selectivity in transition metal catalysis, which has been mainly demonstrated for substrates in which the supramolecular functional group is close to the reactive group. Now it has been demonstrated that this tool can also work when this group is remote from the reactive group, thereby increasing the scope of the approach.

Abstract

Regioselective catalytic transformations using supramolecular directing groups are increasingly popular as it allows for control over challenging reactions that may otherwise be impossible. In most examples the reactive group and the directing group are close to each other and/or the linker between the directing group is very rigid. Achieving control over the regioselectivity using a remote directing group with a flexible linker is significantly more challenging due to the large conformational freedom of such substrates. Herein, we report the redesign of a supramolecular Rh–bisphosphite hydroformylation catalyst containing a neutral carboxylate receptor (DIM pocket) with a larger distance between the phosphite metal binding moieties and the DIM pocket. For the first time regioselective conversion of internal and terminal alkenes containing a remote carboxylate directing group is demonstrated. For carboxylate substrates that possess an internal double bond at the Δ‐9 position regioselectivity is observed. As such, the catalyst was used to hydroformylate natural monounsaturated fatty acids (MUFAs) in a regioselective fashion, forming of an excess of the 10‐formyl product (10‐formyl/9‐formyl product ratio of 2.51), which is the first report of a regioselective hydroformylation reaction of such substrates.

Nature, Published online: 18 March 2020; doi:10.1038/d41586-020-00758-2

From papers published to carbon emissions to confirmed cases, these data reveal an unprecedented viral outbreak and its impacts around the world.