Effects of Ionic Liquids on the Nucleofugality of Dimethyl Sulfide

The nucleofugality of dimethyl sulfide was measured in solvent mixtures containing ionic liquids. The first-order rate constants of the solvolysis of sulfonium salts were determined in mixtures containing different proportions of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide in ethanol, representing the first report on the solvolysis of a charged species in an ionic liquid. Temperature-dependent kinetic studies allowed determination of activation parameters and rationalization of observed solvent effects in different ionic liquid mixtures. From the solvolysis data, the nucleofugality of dimethyl sulfide in different proportions of this ionic liquid in ethanol was determined. Further, the nucleofugality of dimethyl sulfide was determined in mixtures containing high proportions of each of seven other ionic liquids in ethanol. These data allowed quantification of the effects of varying both the amount of ionic liquid present and on changing the components of the ionic liquid on the nucleofugality of dimethyl sulfide. The ionic liquid mixtures were shown to affect the nucleofugality of this nucleofuge in a different manner to the previously studied monatomic charged nucleofuges, owing to different microscopic interactions in solution. This work highlighted the necessity of considering electrofuges with an appropriate range of electrofugality values along with the importance of the nucleofuge-specific sensitivity parameter.

Optimizing ultrasound-assisted extraction of bioactive compounds from Canthium horridum blume leaves utilizing polyols: A study on skin-related activities

The focus on reducing organic solvent usage, owing to their negative environmental and health impacts, is driving a search for innovative green alternative solvents in academia and industry. Canthium horridum Blume (CH) exhibits many therapeutic activities, including antioxidant and anti-inflammatory efficacy. The objective of this study is to evaluate the optimal solvent concentration using a simplex-lattice design with an aqueous-polyols mixture and to optimize the parameters for extracting bioactive compounds and antioxidant activities from ultrasound-assisted extraction (UAE) of CH leaves through central composite design (CCD) in response surface methodology (RSM). This study examines the total phenolic content (TPC), antioxidant activities, comparison of different extraction conditions, identification of bioactive compounds, cell cytotoxicity, cellular antioxidant activity, and melanin content reduction efficacy of the extracts. According to the findings from the simplex-lattice model, the ideal solvent composition consisted of 32.57%w/w butylene glycol, 32.92%w/w glycerine, and 34.51%w/w water. Furthermore, based on the response model, optimal extraction conditions were identified as a 15-min extraction time and a solvent-to-sample ratio of 32.94:1. In comparison to alternative extraction methods, ultrasonic-assisted extraction using the aqueous-glycerine-butylene glycol (GB-UAE) extract resulted in notably elevated TPC and antioxidant responses (p < 0.05). Major antioxidant bioactive compounds included 4-(Butoxymethyl) phenol, 3-O-Caffeoyl-4-O-methylquinic acid, Quercetin 3-(2G-glucosylrutinoside), 2,4-Dihydroxybenzoic acid and other bioactive compounds. The GB-UAE extract revealed greater cell viability than UAE using ethanol (EtOH-UAE) extract in both cytotoxicity and cellular antioxidant assays at the same concentration. Additionally, it exhibited comparable melanin content reduction efficacy at a higher concentration compared to that of EtOH-UAE extract. The researcher anticipates that the current study will advance the utilization of an aqueous-polyols system for extracting bioactive compounds extending beyond CH leaves. Although the potential applications of CH leaves in cosmetics and pharmaceutical formulations have been identified, further comprehensive mechanistic and clinical studies are required to fully understand their effects.

Strong Bases and beyond: The Prominent Contribution of Neutral Push-Pull Organic Molecules towards Superbases in the Gas Phase

In this review, the principles of gas-phase proton basicity measurements and theoretical calculations are recalled as a reminder of how the basicity PA/GB scale, based on Brønsted-Lowry theory, was constructed in the gas-phase (PA-proton affinity and/or GB-gas-phase basicity in the enthalpy and Gibbs energy scale, respectively). The origins of exceptionally strong gas-phase basicity of some organic nitrogen bases containing N-sp3 (amines), N-sp2 (imines, amidines, guanidines, polyguanides, phosphazenes), and N-sp (nitriles) are rationalized. In particular, the role of push-pull nitrogen bases in the development of the gas-phase basicity in the superbasicity region is emphasized. Some reasons for the difficulties in measurements for poly-functional nitrogen bases are highlighted. Various structural phenomena being in relation with gas-phase acid-base equilibria that should be considered in quantum-chemical calculations of PA/GB parameters are discussed. The preparation methods for strong organic push-pull bases containing a N-sp2 site of protonation are briefly reviewed. Finally, recent trends in research on neutral organic superbases, leaning toward catalytic and other remarkable applications, are underlined.

Effects of Ionic Liquids on the Nucleofugality of Bromide

The nucleofugality of bromide was measured in solvent mixtures containing ionic liquids. The solvolysis rate constants of the bromides of well-defined electrofuges were determined in mixtures containing different proportions of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide in ethanol. Temperature-dependent kinetic studies allowed an explanation of the observed solvent effects in different mixtures in terms of interactions in solution. Using the solvolysis data, the nucleofugality of bromide in these systems was determined. Likewise, nucleofugality data for bromide were determined in mixtures containing high proportions of seven further ionic liquids. These data allowed quantification of the effects of both varying the amount of ionic liquid and the nature of ionic liquid components on the nucleofugality of bromide. Importantly, ionic liquid mixtures were shown to affect the nucleofugality in a manner similar to chloride, providing a method for predicting the effects of ionic liquids on other electrofuges. Further, the ionic liquids were shown to move the transition state earlier along the reaction coordinate, meaning that there is less charge development in the transition state.

Entropy and Isokinetic Temperature in Fast Ion Transport

Ion transport in crystalline solids is an essential process for many electrochemical energy converters such as solid-state batteries and fuel cells. Empirical data have shown that ion transport in crystal lattices obeys the Meyer-Neldel Rule (MNR). For similar, closely related materials, when the material properties are changed by doping or by strain, the measured ionic conductivities showing different activation energies intersect on the Arrhenius plot, at an isokinetic temperature. Therefore, the isokinetic temperature is a critical parameter for improving the ionic conductivity. However, a comprehensive understanding of the fundamental mechanism of MNR in ion transport is lacking. Here the physical significance and applicability of MNR is discussed, that is, of activation entropy-enthalpy compensation, in crystalline fast ionic conductors, and the methods for determining the isokinetic temperature. Lattice vibrations provide the excitation energy for the ions to overcome the activation barrier. The multi-excitation entropy model suggests that isokinetic temperature can be tuned by modulating the excitation phonon frequency. The relationship between isokinetic temperature and isokinetic prefactor can provide information concerning conductivity mechanisms. The need to effectively determine the isokinetic temperature for accelerating the design of new fast ionic conductors with high conductivity is highlighted.

On Prototropy and Bond Length Alternation in Neutral and Ionized Pyrimidine Bases and Their Model Azines in Vacuo

In this review, the complete tautomeric equilibria are derived for disubstituted pyrimidine nucleic acid bases starting from phenol, aniline, and their model compounds-monosubstituted aromatic azines. The differences in tautomeric preferences for isolated (gaseous) neutral pyrimidine bases and their model compounds are discussed in light of different functional groups, their positions within the six-membered ring, electronic effects, and intramolecular interactions. For the discussion of tautomeric preferences and for the analysis of internal effects, recent quantum-chemical results are taken into account and compared to some experimental ones. For each possible tautomer-rotamer of the title compounds, the bond length alternation, measured by means of the harmonic oscillator model of electron delocalization (HOMED) index, is examined. Significant HOMED similarities exist for mono- and disubstituted derivatives. The lack of parallelism between the geometric (HOMED) and energetic (ΔG) parameters for all possible isomers clearly shows that aromaticity is not the main factor that dictates tautomeric preferences for pyrimidine bases, particularly for uracil and thymine. The effects of one-electron loss (positive ionization) and one-electron gain (negative ionization) on prototropy and bond length alternation are also reviewed for pyrimidine bases and their models.

Endogenous Photosensitizers in Human Skin

Endogenous photosensitizers play a critical role in both beneficial and harmful light-induced transformations in biological systems. Understanding their mode of action is essential for advancing fields such as photomedicine, photoredox catalysis, environmental science, and the development of sun care products. This review offers a comprehensive analysis of endogenous photosensitizers in human skin, investigating the connections between their electronic excitation and the subsequent activation or damage of organic biomolecules. We gather the physicochemical and photochemical properties of key endogenous photosensitizers and examine the relationships between their chemical reactivity, location within the skin, and the primary biochemical events following solar radiation exposure, along with their influence on skin physiology and pathology. An important take-home message of this review is that photosensitization allows visible light and UV-A radiation to have large effects on skin. The analysis presented here unveils potential causes for the continuous increase in global skin cancer cases and emphasizes the limitations of current sun protection approaches.

Sparteine-Free, Highly Stereoselective Construction of Complex Allylic Alcohols Using 1,2-Metallate Rearrangements

Stereotriads bearing allylic alcohols are privileged structures in natural products, and new methods accessing these in a stereoselective fashion are highly sought after. Toward this goal, we found that the use of chiral polyketide fragments allows for performing the Hoppe-Matteson-Aggarwal rearrangement in the absence of sparteine with high yields and diastereoselectivities, rendering this protocol a highly valuable alternative to the Nozaki-Hiyama-Takai-Kishi reaction. The switch of directing groups in most cases resulted in the reversed stereochemical outcome, which could be explained by conformational analysis on density functional theory level and a Felkin-like model.

One-Bond-Nucleophilicity and -Electrophilicity Parameters: An Efficient Ordering System for 1,3-Dipolar Cycloadditions

Diazoalkanes are ambiphilic 1,3-dipoles that undergo fast Huisgen cycloadditions with both electron-rich and electron-poor dipolarophiles but react slowly with alkenes of low polarity. Frontier molecular orbital (FMO) theory considering the 3-center-4-electron π-system of the propargyl fragment of diazoalkanes is commonly applied to rationalize these reactivity trends. However, we recently found that a change in the mechanism from cycloadditions to azo couplings takes place due to the existence of a previously overlooked lower-lying unoccupied molecular orbital. We now propose an alternative approach to analyze 1,3-dipolar cycloaddition reactions, which relies on the linear free energy relationship lg k₂(20 °C) = s_N(N + E) (eq 1) with two solvent-dependent parameters (N, s_N) to characterize nucleophiles and one parameter (E) for electrophiles. Rate constants for the cycloadditions of diazoalkanes with dipolarophiles were measured and compared with those calculated for the formation of zwitterions by eq 1. The difference between experimental and predicted Gibbs energies of activation is interpreted as the energy of concert, i.e., the stabilization of the transition states by the concerted formation of two new bonds. By linking the plot of lg k₂ vs N for nucleophilic dipolarophiles with that of lg k₂ vs E for electrophilic dipolarophiles, one obtains V-shaped plots which provide absolute rate constants for the stepwise reactions on the borderlines. These plots furthermore predict relative reactivities of dipolarophiles in concerted, highly asynchronous cycloadditions more precisely than the classical correlations of rate constants with FMO energies or ionization potentials. DFT calculations using the SMD solvent model confirm these interpretations.

Limited Stability of 6,13-Bis(tri(isopropyl)silylethynyl)pentacene upon One-Electron Oxidation: Electrochemically Induced (4 + 2) Cycloaddition between an Alkynyl-Substituted Acene and Its Radical Cation

6,13-Bis(tri(isopropyl)silylethynyl)pentacene, a particularly stable acene derivative important for (opto)electronic materials, turns reactive upon electrochemical one-electron oxidation. One of the typically stabilizing tri(isopropyl)silylethynyl substituents becomes involved in a (4 + 2) cycloaddition after redox umpolung. The electrosynthetic dimerization of the title compound provides easy access under mild conditions to a complex scaffold, which includes an intact pentacene, an anthracene, and a phenylene unit, all electronically separated. The product's electrochemical redox properties are explained by superimposed cyclic voltammetric features of the pentacene and the anthracene moieties. The reaction path is analyzed on the basis of electroanalytical and ESR data, and an oxidation-cycloaddition-reduction sequence is elaborated. The contribution of homogeneous electron transfers (electron transfer chain reaction) is negligible, in accordance with the relative formal redox potentials of the starting compound and the product. Quantum chemical calculations indicate that the central cycloaddition should be described as a two-step process with a distonic radical cation intermediate. We suggest an extended notation to define the contribution of the components with respect to electron count in the two-step cycloaddition, [3 + 1, 1 + 1].

Prediction of Kinetic Product Ratios: Investigation of a Dynamically Controlled Case

Of the various factors influencing kinetically controlled product ratios, the role of nonstatistical dynamics is arguably the least well understood. In this paper, reactions were chosen in which dynamics played a dominant role in product selection, by design. Specifically, the reactions studied were the ring openings of cyclopropylidene to allene and tetramethylcyclopropylidene to tetramethylallene (2,4-dimethylpenta-2,3-diene). Both reactions have intrinsic reaction coordinates that bifurcate symmetrically, leading to products that are enantiomeric once the atoms are uniquely labeled. The question addressed in the study was whether the outcomes─that is, which product well on the potential energy surface was selected─could be predicted from their initial conditions for individual trajectories in quasiclassical dynamics simulations. Hybrid potentials were developed based on cooperative interaction between molecular mechanics and artificial neural networks, trained against data from electronic structure calculations. These potentials allowed simulations of both gas-phase and condensed-phase reactions. The outcome was that, for both reactions, prediction of initial selection of product wells could be made with >95% success from initial conditions of the trajectories in the gas phase. However, when trajectories were run for longer, looking for "final" products for each trajectory, the predictability dropped off dramatically. In the gas-phase simulations, this drop off was caused by trajectories hopping between product wells on the potential energy surface. That behavior could be suppressed in condensed phases, but then new uncertainty was introduced because the intermolecular interactions between solute and bath, necessary to permit intermolecular energy transfer and cooling of the hot initial products, often led to perturbations of the initial directions of trajectories on the potential energy surface. It would consequently appear that a general ability to predict outcomes for reactions in which nonstatistical dynamics dominate remains a challenge even in the age of sophisticated machine-learning capabilities.