Photoinduced Decarboxylative Borylation of N-Hydroxyphthalimide Esters with Hypoboric Acid

We developed a visible-light-driven photochemical transformation in which activated primary, secondary, and tertiary alkylcarboxylic acids were converted into the corresponding boronic esters in the absence of catechol and any added photocatalyst. The procedure relies on the utilization of hypoboric acid and redox-active esters of alkylcarboxylic acids to ensure a simple and economic procedure. Quantum chemical calculations and mechanistic considerations provide deeper insights into the mechanism of photochemical borylation reactions.

Prediction of Redox Power for Photocatalysts: Synergistic Combination of DFT and Machine Learning

The accurate prediction of excited state properties is a key element of rational photocatalyst design. This involves the prediction of ground and excited state redox potentials, for which an accurate description of electronic structures is needed. Even with highly sophisticated computational approaches, however, a number of difficulties arise from the complexity of excited state redox potentials, as they require the calculation of the corresponding ground state redox potentials and the estimation of the 0-0 transition energies (E_0,0). In this study, we have systematically evaluated the performance of DFT methods for these quantities on a set of 37 organic photocatalysts representing 9 different chromophore scaffolds. We have found that the ground state redox potentials can be predicted with reasonable accuracy that can be further improved by rationally minimizing the systematic underestimations. The challenging part is to obtain E_0,0, as calculating it directly is highly demanding and its accuracy depends strongly on the DFT functional employed. We have found that approximating E_0,0 with appropriately scaled vertical absorption energies offers the best compromise between accuracy and computational effort. An even more accurate and cost-effective approach, however, is to predict E_0,0 with machine learning and avoid the use of DFT for excited state calculations. Indeed, the best excited state redox potential predictions are achieved with the combination of M062X for ground state redox potentials and machine learning (ML) for E_0,0. With this protocol, the excited state redox potential windows of the photocatalyst frameworks could be adequately predicted. This shows the potential of combining DFT with ML in the computational design of photocatalysts with preferred photochemical properties.

Synthesis of Hydrofluoroolefin‐Based Iodonium Reagent via Dyotropic Rearrangement and Its Utilization in Fluoroalkylation

[1,2]‐shift of atoms in alkyl fragments belongs to the class of dyotropic rearrangements. Various atoms, including halogens can be involved in the migration, however participation of iodine is unprecedented. Herein, we report our experimental and DFT studies on the oxidation triggered dyotropic rearrangement of iodo and chloro functions via butterfly‐type transition state to demonstrate the migrating ability of λ³‐iodane centre. With the exploitation of dyotropic rearrangement we designed and synthesized a novel fluoroalkyl iodonium reagent from industrial feedstock gas HFO‐1234yf. We demonstrated that the hypervalent reagent serves as an excellent fluoroalkylation agent for various amines and nitrogen heterocycles.

Multiscale Modeling of Electronic Spectra Including Nuclear Quantum Effects

Theoretical prediction of electronic absorption spectra without input from experiments is no easy feat, as it requires addressing all of the factors that affect line shapes. In practice, however, the methodologies are limited to treat these ingredients only to a certain extent. Here, we present a multiscale protocol that addresses the temperature, solvent, and nuclear quantum effects as well as anharmonicity and the reconstruction of the final spectra from individual transitions. First, quantum mechanics/molecular mechanics (QM/MM) molecular dynamics is conducted to obtain trajectories of solute-solvent configurations, from which the corresponding quantum-corrected ensembles are generated through the generalized smoothed trajectory analysis (GSTA). The optical spectra of the ensembles are then produced by calculating vertical transitions using time-dependent density-functional theory (TDDFT) with implicit solvation. To obtain the final spectral shapes, the stick spectra from TDDFT are convoluted with Gaussian kernels where the half-widths are determined by a statistically motivated strategy. We have tested our method by calculating the UV-vis spectra of a recently discovered acridine photocatalyst in two redox states. Vibronic progressions and broadenings due to the finite lifetime of the excited states are not included in the methodology yet. Nuclear quantization affects the relative peak intensities and widths, which is necessary to reproduce the experimental spectrum. We have also found that using only the optimized geometry of each molecule works surprisingly well if a proper empirical broadening factor is applied. This is explained by the rigidity of the conjugated chromophore moieties of the selected molecules, which are mainly responsible for the excitations in the spectra. In contrast, we have also shown that other parts of the molecules are flexible enough to feature anharmonicities that impair the use of other techniques such as Wigner sampling.

Challenges and outlook for convection-permitting climate modelling

Climate projections at very high resolution (kilometre-scale grid spacing) are becoming affordable. These 'convection-permitting' models (CPMs), commonly used for weather forecasting, better represent land-surface characteristics and small-scale processes in the atmosphere such as convection. They provide a step change in our understanding of future changes at local scales and for extreme weather events. For short-duration precipitation extremes, this includes capturing local storm feedbacks, which may modify future increases. Despite the major advance CPMs offer, there are still key challenges and outstanding science issues. Heavy rainfall tends to be too intense; there are challenges in representing land-surface processes; sub-kilometre scale processes still need to be parametrized, with existing parametrization schemes often requiring development for use in CPMs; CPMs rely on the quality of lateral boundary forcing and typically do not include ocean-coupling; large CPM ensembles that comprehensively sample future uncertainties are costly. Significant progress is expected over the next few years: scale-aware schemes may improve the representation of unresolved convective updrafts; work is underway to improve the modelling of complex land-surface fluxes; CPM ensemble experiments are underway and methods to synthesize this information with larger coarser-resolution model ensembles will lead to local-scale predictions with more comprehensive uncertainty context for user application. Large-domain (continental or tropics-wide) CPM climate simulations, potentially with additional earth-system processes such as ocean and wave coupling and terrestrial hydrology, are an exciting prospect, allowing not just improved representation of local processes but also of remote teleconnections. This article is part of a discussion meeting issue 'Intensification of short-duration rainfall extremes and implications for flash flood risks'.

The ortho effect in directed C-H activation

The success of transition metal-catalysed ortho-directed C-H activation is often plagued by the effects of undesirable interactions between the directing group (DG) and other groups introduced into the aromatic core of the substrate. In particular, when these groups are in neighbouring positions, their interactions can affect profoundly the efficacy of the C-H activation by transition metals. In this work we introduce a simple substrate-only-based model to interpret the influence of steric hindrance of a group in ortho position to the DG in directed ortho-C-H bond activation reactions, and coined the term Ortho Effect (OE) for such situations. We consider simple descriptors such as torsion angle and torsional energy to predict and explain the reactivity of a given substrate in directed C-H activation reactions. More than 250 examples have been invoked for the model, and the nature of the ortho effect was demonstrated on a wide variety of structures. In order to guide organic chemists, we set structural and energetic criteria to evaluate a priori the efficiency of the metalation step which is usually the rate-determining event in C-H activations, i.e. we provide a simple and general protocol to estimate the reactivity of a potential substrate in C-H activation. For borderline cases these criteria help set the minimum reaction temperature to obtain reasonable reaction rates. As an example for the practical applicability of the model, we performed synthetic validations via palladium-catalysed 2,2,2-trifluoroethylation reactions in our lab. Furthermore, we give predictions for the necessary reaction conditions for several selected DGs.

Site-Selective Double and Tetracyclization Routes to Fused Polyheterocyclic Structures by Pd-Catalyzed Carbonylation Reactions

In this contribution, we report novel palladium-catalyzed carbonylative cascade approaches to highly functionalized polyheterocyclic structures. The Pd-catalyzed carbonylative process involves the regioselective insertion of one to three CO molecules and the sequential ordered formation of up to eight new bonds (one C–O, two C–C, five C–N). The exclusive formation of six-membered heterocycles is elucidated by detailed modeling studies.

The mechanism of monochloramine disproportionation under acidic conditions

Monochloramine is a widely employed agent in water treatment technologies. However, its utilization has some drawbacks like the transformation of the active species into the undesired dichloramine. Although it is more pronounced in acidic solutions, the features of this reaction have still remained largely unexplored in the pH < 4 region. In this study the decomposition of monochloramine is examined under such conditions by using kinetic and computational methods. Fast kinetics measurements have convincingly showed that the disproportion into dicloramine is relatively fast and can be studied without any interference from side reactions. By varying the pH, the deprotonation constant of monochloramine has been determined by UV spectroscopy (K_a = 0.023 ± 0.005 M for I = 1.0 M NaClO₄, and T = 25.0 °C). Dichloramine formation via monochloramine disproportion was found to follow second-order kinetics. The computations have provided the reaction mechanism and its free energy profile in accord with the proposed kinetic model. This involves the reaction between the protonated and unprotonated forms of monochloramine, with a rate constant k = 335.3 ± 11.8 M^-1 s^-1, corresponding to an activation free energy barrier of 14.1 kcal mol^-1. The simulations predicted a barrier of 14.9 kcal mol^-1 and revealed a key short-lived chlorine-bridged intermediate which yields dichloroamine and ammonium ion through a deprotonation-coupled chlorine shift.

Synthesis of Multifunctional Aryl(trifloxyalkenyl)iodonium Triflate Salts

A convenient procedure for the synthesis of aryl(trifloxyalkenyl)iodonium triflate salts from commercially available (diacetoxyiodo)benzene, trimethylsilyl trifluoromethanesulfonate, and acetylenes under mild conditions was developed. The obtained multifunctional hypervalent vinyliodonium salts equipped with electrophilic and nucleophilic functions could serve as novel C2 synthons for organic transformations. The structure of the iodonium salts was identified by multidimensional NMR spectroscopy and X-ray crystallography.

Synthesis of Imidazolidin-2-ones and Imidazol-2-ones via Base-Catalyzed Intramolecular Hydroamidation of Propargylic Ureas under Ambient Conditions

The first organo-catalyzed synthesis of imidazolidin-2-ones and imidazol-2-ones via intramolecular hydroamidation of propargylic ureas is reported. The phosphazene base BEMP turned out to be the most active organo-catalyst compared with guanidine and amidine bases. Excellent chemo- and regioselectivities to five-membered cyclic ureas have been achieved under ambient conditions, with a wide substrate scope and exceptionally short reaction times (down to 1 min). A base-mediated isomerization step to an allenamide intermediate is the most feasible reaction pathway to give imidazol-2-ones, as suggested by DFT studies.

Assessment of reactivities with explicit and implicit solvent models: QM/MM and gas-phase evaluation of three different Ag-catalysed furan ring formation routes

A representative reaction illustrates cases where strong solvent–solute interactions can be sufficiently well captured by continuum solvation model rendering QM/MM unnecessary.

DFT calculations on the mechanism of copper-catalysed tandem arylation-cyclisation reactions of alkynes and diaryliodonium salts

We present a computational mechanistic study on the copper(III)-catalysed carboarylation-ring closure reactions leading to the formation of functionalised heterocycles. We have performed DFT calculations along selected routes and compared their free energy profiles. The calculations considered two viable options for the underlying mechanism which differ in the order of the oxazoline ring formation and the aryl transfer steps. In our model transformation, it was found that the reaction generally features the aryl transfer-ring closing sequence and this sequence shows very limited sensitivity to the variation of the substituent of the reactants. On the basis of the mechanism the origin of the stereoselectivity is ascribed to the interaction of the Cu ion with the oxazoline oxygen driving the ring-closure step selectively.

Design of Trifluoroalkenyl Iodonium Salts for a Hypervalency-Aided Alkenylation-Cyclization Strategy: Metal-Free Construction of Aziridine Rings

The synthesis of fluorinated compounds and their use as pharmaceutical ingredients or synthetic building blocks have been in the focus of chemical and medicinal research. However, the efficient synthesis of trifluoromethylated nitrogen heterocycles is sometimes challenging. Herein, we disclose a simple aziridination process that relies on the use of amines and novel alkenyl iodonium reagents for the synthesis of strained, trifluoromethylated heterocycles. With the utilization of a newly designed and bench-stable but highly reactive hypervalent alkenyl iodonium species, these three-membered-ring heterocyclic compounds can be efficiently constructed from simple amines under mild conditions in the absence of transition-metal catalysts. The special reactivity of the new trifluoropropenyl synthon towards nucleophilic centers could be exploited in more general cyclization and alkenylation reactions in the future.

Enantioselective synthesis of substituted α-aminophosphonates catalysed by d-glucose-based crown ethers: pursuit of the origin of stereoselectivity

Tuning the electronic properties of the catalyst can control the enantioselectivity of d-glucose-based crown ethers by a simple mechanism.

Understanding and Exploitation of Neighboring Heteroatom Effect for the Mild N-Arylation of Heterocycles with Diaryliodonium Salts under Aqueous Conditions: A Theoretical and Experimental Mechanistic Study

The mechanism of arylation of N-heterocycles with unsymmetric diaryliodonium salts is elucidated. The fast and efficient N-arylation reaction is interpreted in terms of the bifunctionality of the substrate: The consecutive actions of properly oriented Lewis base and Brønsted acid centers in sufficient proximity result in the fast and efficient N-arylation. The mechanistic picture points to a promising synthetic strategy where suitably positioned nucleophilic and acidic centers enable functionalization, and it is tested experimentally.

Rearrangements of Cycloalkenyl Aryl Ethers

Rearrangement reactions of cycloalkenyl phenol and naphthyl ethers and the acid-catalyzed cyclization of the resulting product were investigated. Claisen rearrangement afforded 2-substituted phenol and naphthol derivatives. Combined Claisen and Cope rearrangement resulted in the formation of 4-substituted phenol and naphthol derivatives. In the case of cycloocthylphenyl ether the consecutive Claisen and Cope rearrangements were followed by an alkyl migration. The mechanism of this novel rearrangement reaction is also discussed.

Prebiotic NH3 Formation: Insights from Simulations

Simulations of prebiotic NH₃ synthesis from NO₃⁻ and NO₂⁻ on pyrite surfaces under hydrothermal conditions are reported. Ab initio metadynamics calculations have successfully explored the full reaction path which explains earlier experimental observations. We have found that the reaction mechanism can be constructed from stepwise single atom transfers which are compatible with the expected reaction time scales. The roles of the hot-pressurized water and of the pyrite surfaces have been addressed. The mechanistic picture that emerged from the simulations strengthens the theory of chemoautotrophic origin of life by providing plausible reaction pathways for the formation of ammonia within the iron-sulfur-world scenario.

Synthesis of α-d-galactose-based azacrown ethers and their application as enantioselective catalysts in Michael reactions

Crown ethers derived from d-galactose generated good to excellent enantioselectivities in a few Michael reactions under solid–liquid phase transfer conditions.

Ring-shaped variation of the coeliac trunk branches

Aberrant arterial variations in the branching pattern of the coeliac trunk are of great interest to surgeons and radiologists. We report on a rare arterial variation found in a 79-year-old cadaver during educational dissection. Specifically, the coeliac axis formed a unique incomplete trunk termed the hepato-hepatic trunk. The splenic artery arose separately from the anterior aspect of the abdominal aorta. On the right side, there was a right hepatic artery giving rise to a gastroduodenal but an absence of the left hepatic. On the left side, there was a branch coursing towards the porta hepatis; the left hepatic artery, dividing into the left gastric, an accessory left gastric, and a branch to the distal oesophagus. The hepato-hepatic trunk formed a ring-shaped vascular structure around the caudate lobe of the liver. Precise mapping and observation of the extrahepatic arteries and bile duct branches is essential in a variety of hepato-biliary laparoscopic procedures of the liver and gallbladder. Other operative procedures requiring, a comprehensive kno-wledge of the varied coeliac trunk patterns are liver transplantation and arterial embolism for hepatic tumour therapy.

Pyrite in contact with supercritical water: the desolation of steam

The supercritical water and pyrite interface has been studied by DFT calculations. A surprisingly dry surface has been found which points to a new reactivity under extreme conditions which has relevance in the iron–sulfur world prebiotic chemistry of the early Earth.

Efficient direct 2,2,2-trifluoroethylation of indoles via C–H functionalization

Metal free direct trifluoroethylation of unprotected indoles at position 3 via C–H functionalization is presented: straightforward synthesis and DFT studies.

Supraclavicularis proprius muscle associated with supraclavicular nerve entrapment

Entrapment neuropathy of the supraclavicular nerve is rare and, when it occurs, is usually attributable to branching of the nerve into narrow bony clavicular canals. We describe another mechanism for entrapment of this nerve with the aberrant muscle; supraclavicularis being found during the routine dissection of an embalmed 82-year-old cadaver. Our report details a unique location for this rare muscular variation whereby the muscle fibres originated posteriorly on the medial aspect of the clavicle before forming a muscular arch over the supraclavicular nerve and passing laterally towards the trapezius and acromion. We recommend that in clinical instances of otherwise unexplained unilateral clavicular pain or tenderness, nerve compression from the supraclavicularis muscle must be borne in mind.

Challenges in modelling homogeneous catalysis: new answers from ab initio molecular dynamics to the controversy over the Wacker process

We present here a review of the mechanistic studies of the Wacker process stressing the long controversy about the key reaction steps. We give an overview of the previous experimental and theoretical studies on the topic. Then we describe the importance of the most recent Ab Initio Molecular Dynamics (AIMD) calculations in modelling organometallic reactivity in water. As a prototypical example of homogeneous catalytic reactions, the Wacker process poses serious challenges to modelling. The adequate description of the multiple role of the water solvent is very difficult by using static quantum chemical approaches including cluster and continuum solvent models. In contrast, such reaction systems are suitable for AIMD, and by combining with rare event sampling techniques, the method provides reaction mechanisms and the corresponding free energy profiles. The review also highlights how AIMD has helped to obtain a novel understanding of the mechanism and kinetics of the Wacker process.

Out of cross-conjugation: the unexpected structure of tetrazinones

Combined experimental and theoretical investigations helped to elucidate the mechanisms responsible for the unique structural and electronic properties of NHC-substituted tetrazinone compounds.

Giant cell tumour with a lipoma of the sacrum

We report the case of a 58-year old female who presented with insidious low back pain radiating to the buttock and thigh. Magnetic resonance imaging revealed a heterogenous high-signal (on T2-weighting) in the midline of the sacrum. Within this mass was a distinct area of low-signal intensity. Computed tomography revealed a lytic lesion within the sacrum. Biopsy confirmed the presence of a lipoma that contained within it a giant cell tumour. The patient proceeded to have surgical excision and bone graft augmentation.

The nature of [PdCl(2)(C(2)H(4))(H(2)O)] as an active species in the Wacker process: new insights from ab initio molecular dynamics simulations

First-principles molecular dynamics coupled with metadynamics have been used to gain a deeper insight into the reaction mechanism of the Wacker process by determining the nature of the active species. An explicit and dynamic representation of the aqueous solvent, which was essential for modeling this reaction, was efficiently included into the simulations. Prompted by our earlier results, which showed that the configuration of the catalytically active species [PdCl(2)(H(2)O)(C(2)H(4))] was crucial in the subsequent steps of the Wacker process, herein we focused on the preceding equilibria that led to the formation of both the cis and trans isomers. Starting from the initial catalyst, [PdCl(4)](2-), the free-energy barriers for the forward and backward reactions were calculated. These results confirmed the relevance of the trans intermediate in the reaction mechanism, whilst conversely, they showed that the cis configuration played no role in it. This sole participation of the trans intermediate has some very important implications; besides the mechanistic interpretation of the initial steps in the Wacker reaction mechanism, the analysis of these equilibria provides additional information about the chemical nature of these ligand-substitution processes.