H-Bond Mediated Phase-Transfer Catalysis: Enantioselective Generating of Quaternary Stereogenic Centers in β-Keto Esters

In this work, we would like to present the development of a highly optimized method for generating the quaternary stereogenic centers in β-keto esters. This enantioselective phase-transfer alkylation catalyzed by hybrid Cinchona catalysts allows for the efficient generation of the optically active products with excellent enantioselectivity, using only 1 mol% of the catalyst. The vast majority of phase-transfer catalysts in asymmetric synthesis work by creating ionic pairs with the nucleophile-attacking anionic substrate. Therefore, it is a sensible approach to search for new methodologies capable of introducing functional groups into the precursor’s structure, maintaining high yields and enantiomeric purity.     

Recognition of Chiral Carboxylates by Synthetic Receptors

Recognition of anionic species plays a fundamental role in many essential chemical, biological, and environmental processes. Numerous monographs and review papers on molecular recognition of anions by synthetic receptors reflect the continuing and growing interest in this area of supramolecular chemistry. However, despite the enormous progress made over the last 20 years in the design of these molecules, the design of receptors for chiral anions is much less developed. Chiral recognition is one of the most subtle types of selectivity, and it requires very precise spatial organization of the receptor framework. At the same time, this phenomenon commonly occurs in many processes present in nature, often being their fundamental step. For these reasons, research directed toward understanding the chiral anion recognition phenomenon may lead to the identification of structural patterns that enable increasingly efficient receptor design. In this review, we present the recent progress made in the area of synthetic receptors for biologically relevant chiral carboxylates.     

Pruning of recurrent neural models: an optimal brain damage approach

Nonlinear Dynamics - This paper considers the problem of pruning recurrent neural models of perceptron type with one hidden layer which may be used for modelling of dynamic system. In order to...     

The applications of infrared thermography in surgical removal of retained teeth effects assessment

Journal of Thermal Analysis and Calorimetry - Coal has played a significant role in daily life. However, the highly effective combustion and reducing combustion-caused hazards of the coal still...     

Magnesium tetraorganyl derivatives of group 13 metals as intermediate products in the synthesis of group 13 metal alkyls and aryls

Reactions of organomagnesium halides with group 13 metal halides lead to the formation of R₃M type compounds (R = alkyl, aryl; M = Al, Ga, In) and are considered as the simplest methods of R₃M compound syntheses. These seemingly simple reactions reveal a much more complex chemistry involving mixed magnesium-group 13 metal compounds. To elucidate the reaction course of reactions of organomagnesium halides with group 13 metal halides, we have studied reactions of R₃M with organomagnesium halides. The interaction of Et₃M with R¹MgX led to the formation of following products being mixtures of crystalline ionic complexes with the general composition of [Et_4-nR¹_nM]⁻[XMg (thf)₅]⁺·(thf): [Et_2.2Al(CH=CH₂)_1.8]⁻[BrMg (thf)₅]⁺·(thf) ( 1 ), [Et₃Ga(CH=CH₂)]⁻[BrMg (thf)₅]⁺·(thf) ( 2 ), [Et₄Al]⁻[BrMg (thf)₅]⁺·(thf) ( 3 ), [Et₄Ga]⁻[BrMg (thf)₅]⁺·(thf) ( 4 ), [Et_2.9Al(C₆H₅)_1.1]⁻[BrMg (thf)₅]⁺·(thf) ( 5 ), [Et_2.9Ga(C₆H₅)_1.1]⁻[BrMg (thf)₅]⁺·(thf) ( 6 ), [Et_3.4GaMe_0.6]⁻[IMg (thf)₅]⁺·(thf) ( 7 ) and [Et₄In]⁻[BrMg (thf)₅]⁺·(thf) ( 8 ). A comparison of the production course of group 13 metal trialkyls R₃M with a thermal decomposition of 1–8 products showed that reactions of MX₃ with RMgX (X = Br, I; R = alkyl, aryl) yield initially intermediate ionic compounds, which must then be thermally decomposed to obtain pure R₃M compounds. If group 13 metal bromides and iodides, and alkyl (aryl)magnesium bromides and iodides in thf are used, only intermediate products with the [R₄M]⁻[XMg (thf)₅]⁺·(thf) structure are formed.     

The 41Σ+ electronic state of LiCs molecule

The 4¹Σ⁺ state of LiCs molecule is observed experimentally for the first time. The inverted perturbation approach (IPA) method is used to derive the potential energy curve of the state from the measured spectra. The experiment is accompanied by theoretical calculations of adiabatic potentials for excited states in LiCs including 4¹Σ⁺, performed with the MOLPRO program package. The irregular shape of the 4¹Σ⁺ state potential predicted by theory is confirmed in the experiment.     

Solution and Solid State Studies of Urea Derivatives of DITIPIRAM Acting as Powerful Anion Receptors

Herein, we present the synthesis and anion binding studies of a family of homologous molecular receptors 4–7 based on a DITIPIRAM (8-propyldithieno-[3,2-b:2′,3′-e]-pyridine-3,5-di-amine) platform decorated with various urea para-phenyl substituents (NO₂, F, CF₃, and Me). Solution, X-ray, and DFT studies reveal that the presented host–guest system offers a convergent array of four urea NH hydrogen bond donors to anions allowing the formation of remarkably stable complexes with carboxylates (acetate, benzoate) and chloride anions in solution, even in competitive solvent mixtures such as DMSO-d₆/H₂O 99.5/0.5 (v/v) and DMSO-d₃/MeOH-d₃ 9:1 (v/v). The most effective derivatives among the series turned out to be receptors 5 and 6 containing electron-withdrawing F- and -CF₃ para-substituents, respectively.     

Potent strategy towards strongly emissive nitroaromatics through a weakly electron-deficient core

Nitroaromatics seldom fluoresce. The importance of electron-deficient (n-type) conjugates, however, has inspired a number of strategies for suppressing the emission-quenching effects of the strongly electron-withdrawing nitro group. Here, we demonstrate how such strategies yield fluorescent nitroaryl derivatives of dipyrrolonaphthyridinedione (DPND). Nitro groups near the DPND core quench its fluorescence. Conversely, nitro groups placed farther from the core allow some of the highest fluorescence quantum yields ever recorded for nitroaromatics. This strategy of preventing the known processes that compete with photoemission, however, leads to the emergence of unprecedented alternative mechanisms for fluorescence quenching, involving transitions to dark nπ* singlet states and aborted photochemistry. Forming nπ* triplet states from ππ* singlets is a classical pathway for fluorescence quenching. In nitro-DPNDs, however, these ππ* and nπ* excited states are both singlets, and they are common for nitroaryl conjugates. Understanding the excited-state dynamics of such nitroaromatics is crucial for designing strongly fluorescent electron-deficient conjugates.

Dipyrrolonaphthyridinedione appended with para- or meta-nitrophenyl substituents exhibits strong fluorescence from a ¹ππ* S₁ state.     

Going beyond the borders: pyrrolo[3,2-b]pyrroles with deep red emission

A two-step route to strongly absorbing and efficiently orange to deep red fluorescent, doubly B/N-doped, ladder-type pyrrolo[3,2-b]pyrroles has been developed. We synthesize and study a series of derivatives of these four-coordinate boron-containing, nominally quadrupolar materials, which mostly exhibit one-photon absorption in the 500–600 nm range with the peak molar extinction coefficients reaching 150 000, and emission in the 520–670 nm range with the fluorescence quantum yields reaching 0.90. Within the family of these ultrastable dyes even small structural changes lead to significant variations of the photophysical properties, in some cases attributed to reversal of energy ordering of alternate-parity excited electronic states. Effective preservation of ground-state inversion symmetry was evidenced by very weak two-photon absorption (2PA) at excitation wavelengths corresponding to the lowest-energy, strongly one-photon allowed purely electronic transition. π-Expanded derivatives and those possessing electron-donating groups showed the most red-shifted absorption- and emission spectra, while displaying remarkably high peak 2PA cross-section (σ_2PA) values reaching ∼2400 GM at around 760 nm, corresponding to a two-photon allowed higher-energy excited state. At the same time, derivatives lacking π-expansion were found to have a relatively weak 2PA peak centered at ca. 800–900 nm with the maximum σ_2PA ∼50–250 GM. Our findings are augmented by theoretical calculations performed using TD-DFT method, which reproduce the main experimental trends, including the 2PA, in a nearly quantitative manner. Electrochemical studies revealed that the HOMO of the new dyes is located at ca. −5.35 eV making them relatively electron rich in spite of the presence of two B⁻–N⁺ dative bonds. These dyes undergo a fully reversible first oxidation, located on the diphenylpyrrolo[3,2-b]pyrrole core, directly to the di(radical cation) stage.

Ladder-type heterocycles encompassing two B⁻–N⁺ dative bonds possess intense green to red emission, large 2PA cross-sections and superb photostability.