Competitive Intramolecular Hydrogen Bonding: Offering Molecules a Choice

The conformational preferences of N-((6-methylpyridin-2-yl)carbamothioyl)benzamide were studied in solution, the gas phase and the solid state via a combination of NMR, density functional theory (DFT) and single crystal X-ray techniques. This acyl thiourea derivative can adopt two classes of low energy conformation, each stabilized by a different 6-membered intramolecular hydrogen bond (IHB) pseudoring. Analysis in different solvents revealed that the conformational preference of this molecule is polarity dependent, with increasingly polar environments yielding a higher proportion of the minor conformer containing an NH…N IHB. The calculated barrier to interconversion is consistent with dynamic behaviour at room temperature, despite the propensity of 6-membered IHB pseudorings to be static. This work demonstrates that introducing competitive IHB pathways can render static IHBs more dynamic and that such systems could have potential as chameleons in drug design.

Prebiotically plausible chemoselective pantetheine synthesis in water

Coenzyme A (CoA) is essential to all life on Earth, and its functional subunit, pantetheine, is important in many origin-of-life scenarios, but how pantetheine emerged on the early Earth remains a mystery. Earlier attempts to selectively synthesize pantetheine failed, leading to suggestions that "simpler" thiols must have preceded pantetheine at the origin of life. In this work, we report high-yielding and selective prebiotic syntheses of pantetheine in water. Chemoselective multicomponent aldol, iminolactone, and aminonitrile reactions delivered spontaneous differentiation of pantoic acid and proteinogenic amino acid syntheses, as well as the dihydroxyl, gem-dimethyl, and β-alanine-amide moieties of pantetheine in dilute water. Our results are consistent with a role for canonical pantetheine at the outset of life on Earth.

Exploring Bismuth Coordination Complexes as Visible-Light Absorbers: Synthesis, Characterization, and Photophysical Properties

Bismuth-based coordination complexes are advantageous over other metal complexes, as bismuth is the heaviest nontoxic element with high spin-orbit coupling and potential optoelectronics applications. Herein, four bismuth halide-based coordination complexes [Bi2Cl6(phen-thio)2] (1), [Bi2Br6(phen-thio)2] (2), [Bi2I6(phen-thio)2] (3), and [Bi2I6(phen-Me)2] (4) were synthesized, characterized, and subjected to detailed photophysical studies. The complexes were characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and NMR studies. Spectroscopic analyses of 1-4 in solutions of different polarities were performed to understand the role of the organic and inorganic components in determining the ground- and excited-state properties of the complexes. The photophysical properties of the complexes were characterized by ground-state absorption, steady-state photoluminescence, microsecond time-resolved photoluminescence, and absorption spectroscopy. Periodic density functional theory (DFT) calculations were performed on the solid-state structures to understand the role of the organic and inorganic parts of the complexes. The studies showed that changing the ancillary ligand from chlorine (Cl) and bromine (Br) to iodine (I) bathochromically shifts the absorption band along with enhancing the absorption coefficient. Also, changing the halides (Cl, Br to I) affects the photoluminescent quantum yields of the ligand-centered (LC) emissive state without markedly affecting the lifetimes. The combined results confirmed that ground-state properties are strongly influenced by the inorganic part, and the lower-energy excited state is LC. This study paves the way to design novel bismuth coordination complexes for optoelectronic applications by rigorously choosing the ligands and bismuth salt.

Use of pyridazinediones for tuneable and reversible covalent cysteine modification applied to peptides, proteins and hydrogels

Reversible cysteine modification has been found to be a useful tool for a plethora of applications such as selective enzymatic inhibition, activity-based protein profiling and/or cargo release from a protein or a material. However, only a limited number of reagents display reliable dynamic/reversible thiol modification and, in most cases, many of these reagents suffer from issues of stability, a lack of modularity and/or poor rate tunability. In this work, we demonstrate the potential of pyridazinediones as novel reversible and tuneable covalent cysteine modifiers. We show that the electrophilicity of pyridazinediones correlates to the rates of the Michael addition and retro-Michael deconjugation reactions, demonstrating that pyridazinediones provide an enticing platform for readily tuneable and reversible thiol addition/release. We explore the regioselectivity of the novel reaction and unveil the reason for the fundamental increased reactivity of aryl bearing pyridazinediones by using DFT calculations and corroborating findings with SCXRD. We also applied this fundamental discovery to making more rapid disulfide rebridging agents in related work. We finally provide the groundwork for potential applications in various areas with exemplification using readily functionalised "clickable" pyridazinediones on clinically relevant cysteine and disulfide conjugated proteins, as well as on a hydrogel material.

Aza-Cibalackrot: Turning on Singlet Fission Through Crystal Engineering

Singlet fission is a photophysical process that provides a pathway for more efficient harvesting of solar energy in photovoltaic devices. The design of singlet fission candidates is non-trivial and requires careful optimization of two key criteria: (1) correct energetic alignment and (2) appropriate intermolecular coupling. Meanwhile, this optimization must not come at the cost of molecular stability or feasibility for device applications. Cibalackrot is a historic and stable organic dye which, although it has been suggested to have ideal energetics, does not undergo singlet fission due to large interchromophore distances, as suggested by single crystal analysis. Thus, while the energetic alignment is satisfactory, the molecule does not have the desired intermolecular coupling. Herein, we improve this characteristic through molecular engineering with the first synthesis of an aza-cibalackrot and show, using ultrafast transient spectroscopy, that singlet fission is successfully "turned on."

Crystal surface defects as possible origins of cocrystal dissociation

Atomic force microscopy is used as a characterisation tool to investigate cocrystal dissociation under high relative humidity. Caffeine–glutaric acid as a model system showed possible role of crystal surface defects in the process of cocrystal dissociation.

Mechanochemical reactivity inhibited, prohibited and reversed by liquid additives: examples from crystal-form screens

We demonstrate that liquid additives can exert inhibitive or prohibitive effects on the mechanochemical formation of multi-component molecular crystals, and report that certain additives unexpectedly prompt the dismantling of such solids into physical mixtures of their constituents. Computational methods were employed in an attempt to identify possible reasons for these previously unrecognised effects of liquid additives on mechanochemical transformations.

Synthesis, structural analysis, electrochemical and magnetic properties of tetrachloroferrate ionic liquids

Eight ionic liquids have been synthesized with the tetrachloroferrate anion and varying cations, with the general formula of [RA]⁺[FeCl₄]⁻ (R = –CH₃, –CH₂C₆H₅; A = pyridine, benzimidazole, trimethylamine, triphenylphosphine).

Doubly Encapsulated Perylene Diimides: Effect of Molecular Encapsulation on Photophysical Properties

Intermolecular interactions play a fundamental role on the performance of conjugated materials in organic electronic devices, as they heavily influence their optoelectronic properties. Synthetic control over the solid state properties of organic optoelectronic materials is crucial to access real life applications. Perylene diimides (PDIs) are one of the most highly studied classes of organic fluorescent dyes. In the solid state, π-π stacking suppresses their emission, limiting their use in a variety of applications. Here, we report the synthesis of a novel PDI dye that is encapsulated by four alkylene straps. X-ray crystallography indicates that intermolecular π-π stacking is completely suppressed in the crystalline state. This is further validated by the photophysical properties of the dye in both solution and solid state and supported by theoretical calculations. However, we find that the introduction of the encapsulating "arms" results in the creation of charge-transfer states which modify the excited state properties. This article demonstrates that molecular encapsulation can be used as a powerful tool to tune intermolecular interactions and thereby gain an extra level of control over the solid state properties of organic optoelectronic materials.

Sensing and Discrimination of Explosives at Variable Concentrations with a Large-Pore MOF as Part of a Luminescent Array

Metal-organic frameworks (MOFs) have shown great promise for sensing of dangerous chemicals, including environmental toxins, nerve agents, and explosives. However, challenges remain, such as the sensing of larger analytes and the discrimination between similar analytes at different concentrations. Herein, we present the synthesis and development of a new, large-pore MOF for explosives sensing and demonstrate its excellent sensitivity against a range of relevant explosive compounds including trinitrotoluene and pentaerythritol tetranitrate. We have developed an improved, thorough methodology to eliminate common sources of error in our sensing protocol. We then combine this new MOF with two others as part of a three-MOF array for luminescent sensing and discrimination of five explosives. This sensor works at part-per-million concentrations and, importantly, can discriminate explosives with high accuracy without reference to their concentration.

Divergent prebiotic synthesis of pyrimidine and 8-oxo-purine ribonucleotides

Understanding prebiotic nucleotide synthesis is a long standing challenge thought to be essential to elucidating the origins of life on Earth. Recently, remarkable progress has been made, but to date all proposed syntheses account separately for the pyrimidine and purine ribonucleotides; no divergent synthesis from common precursors has been proposed. Moreover, the prebiotic syntheses of pyrimidine and purine nucleotides that have been demonstrated operate under mutually incompatible conditions. Here, we tackle this mutual incompatibility by recognizing that the 8-oxo-purines share an underlying generational parity with the pyrimidine nucleotides. We present a divergent synthesis of pyrimidine and 8-oxo-purine nucleotides starting from a common prebiotic precursor that yields the β-ribo-stereochemistry found in the sugar phosphate backbone of biological nucleic acids. The generational relationship between pyrimidine and 8-oxo-purine nucleotides suggests that 8-oxo-purine ribonucleotides may have played a key role in primordial nucleic acids prior to the emergence of the canonical nucleotides of biology.

Synthesis of substituted benzooxaborinin-1-ols via palladium-catalysed cyclisation of alkenyl- and alkynyl-boronic acids

Two new palladium-catalysed reactions have been developed for the synthesis of stable 4-substituted benzooxaborinin-1-ols. A palladium-catalysed cyclisation of ortho-alkenylbenzene boronic acids can be used to access 4-chlorobenzooxaborinin-1-ols via a Wacker-type oxidation and chlorination. Alternatively, ortho-alkynylbenzene boronic acids undergo a palladium-catalysed oxyallylation reaction to provide 4-allylbenzooxaborinin-1-ols.

Rationalization of the Color Properties of Fluorescein in the Solid State: A Combined Computational and Experimental Study

Fluorescein is known to exist in three tautomeric forms defined as quinoid, zwitterionic, and lactoid. In the solid state, the quinoid and zwitterionic forms give rise to red and yellow materials, respectively. The lactoid form has not been crystallized pure, although its cocrystal and solvate forms exhibit colors ranging from yellow to green. An explanation for the observed colors of the crystals is found using a combination of UV/Vis spectroscopy and plane-wave DFT calculations. The role of cocrystal coformers in modifying crystal color is also established. Several new crystal structures are determined using a combination of X-ray and electron diffraction, solid-state NMR spectroscopy, and crystal structure prediction (CSP). The protocol presented herein may be used to predict color properties of materials prior to their synthesis.

On the predictability of supramolecular interactions in molecular cocrystals – the view from the bench

A series of cocrystals involving theophylline and fluorobenzoic acids highlights the difficulty of predicting supramolecular interactions in molecular crystals.

The application of design of experiments (DoE) reaction optimisation and solvent selection in the development of new synthetic chemistry

This article outlines the benefits of using 'Design of Experiments' (DoE) optimisation during the development of new synthetic methodology. A particularly important factor in the development of new chemical reactions is the choice of solvent which can often drastically alter the efficiency and selectivity of a process. Whilst solvent optimisation is usually done in a non-systematic way based upon a chemist's intuition and previous laboratory experience, we illustrate how optimisation of the solvent for a reaction can be carried out by using a 'map of solvent space' in a DoE optimisation. A new solvent map has been developed specifically for optimisation of new chemical reactions using principle component analysis (PCA) incorporating 136 solvents with a wide range of properties. The new solvent map has been used to identify safer alternatives to toxic/hazardous solvents, and also in the optimisation of an S(N)Ar reaction.

Sustainable Synthesis of Chiral Tetrahydrofurans through the Selective Dehydration of Pentoses

L-Arabinose is an abundant resource available as a waste product of the sugar beet industry. Through use of a hydrazone-based strategy, L-arabinose was selectively dehydrated to form a chiral tetrahydrofuran on a multi-gram scale without the need for protecting groups. This approach was extended to other biomass-derived reducing sugars and the mechanism of the key cyclization investigated. This methodology was applied to the synthesis of a range of functionalized chiral tetrahydrofurans, as well as a formal synthesis of 3R-3-hydroxymuscarine.

Disappearing polymorphs revisited

Nearly twenty years ago, Dunitz and Bernstein described a selection of intriguing cases of polymorphs that disappear. The inability to obtain a crystal form that has previously been prepared is indeed a frustrating and potentially serious problem for solid-state scientists. This Review discusses recent occurrences and examples of disappearing polymorphs (as well as the emergence of elusive crystal forms) to demonstrate the enduring relevance of this troublesome, but always captivating, phenomenon in solid-state research. A number of these instances have been central issues in patent litigations. This Review, therefore, also highlights the complex relationship between crystal chemistry and the law.

Irreversible endo-selective diels-alder reactions of substituted alkoxyfurans: a general synthesis of endo-cantharimides

The [4+2] cycloaddition of 3-alkoxyfurans with N-substituted maleimides provides the first general route for preparing endo-cantharimides. Unlike the corresponding reaction with 3H furans, the reaction can tolerate a broad range of 2-substitued furans including alkyl, aromatic, and heteroaromatic groups. The cycloaddition products were converted into a range of cantharimide products with promising lead-like properties for medicinal chemistry programs. Furthermore, the electron-rich furans are shown to react with a variety of alternative dienophiles to generate 7-oxabicyclo[2.2.1]heptane derivatives under mild conditions. DFT calculations have been performed to rationalize the activation effect of the 3-alkoxy group on a furan Diels-Alder reaction.

Sonocrystallization yields monoclinic paracetamol with significantly improved compaction behavior

Ultrasound-assisted crystallization (sonocrystallization) was used to prepare a mixture of nano- and micrometer-sized crystals of the monoclinic form of paracetamol-a widely used analgesic known for its particularly problematic mechanical behavior under compression (i.e. poor tabletability). The nano- and micrometer-sized crystals yielded a powder which exhibits elastic moduli and bulk cohesions that are significantly higher than those observed in samples consisting of macrometer-sized crystals, thus leading to enhanced tabletability without the use of excipients, particle coating, salt, or cocrystal formation. Experimental compaction and finite element analysis were utilized to rationalize the significantly improved compaction behavior of the monoclinic form of paracetamol.

Solid-state photoreactivity of 9-substituted acridizinium bromide salts

A series of substituted acridizinium bromides was studied to determine how substituents affect the regioselectivity of the solid-state [4 + 4] photodimerisation.

Organic nanocrystals of the resorcinarene hexamer via sonochemistry: evidence of reversed crystal growth involving hollow morphologies

Nano- and micrometer-scale crystals of a self-assembled hexamer have been synthesized via sonochemistry. The application of ultrasonic irradiation afforded hollow rhombic-dodecahedral crystals of the C-methylcalix[4]resorcinarene hexamer. The formation of the hollow crystals is attributed to a reversed crystal growth mechanism heretofore described only in the synthesis of inorganic-based materials.

The hydrazide/hydrazone click reaction as a biomolecule labeling strategy for M(CO)3 (M = Re, (99m)Tc) radiopharmaceuticals

Facile reactivity of hydrazides and aldehydes was explored as potential coupling partners for incorporation into M(CO)(3) (M = Re, (99m)Tc) based radiopharmaceuticals. Both 'click, then chelate' and 'prelabel, then click' synthetic routes produced identical products in high yields and lacked metal-hydrazide/-hydrazone interactions, highlighting the potential of this click strategy.