SciCombinator

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Concept: Nitro compound

143

Phenylcarbamate derivatives of amylose and β-cyclodextrin show excellent chiral recognition when used as chiral stationary phases (CSPs) for high-performance liquid chromatography. To open up new possibilities of carbohydrate-based materials, we developed chiral fluorescent sensors based on amylose and β-cyclodextrin (Am-1b and CyD-1b, respectively) by attaching fluorescent π-conjugated units on their side chains. Their recognition abilities toward chiral analytes containing a nitrophenyl unit were evaluated by measuring the enantioselective fluorescence quenching behavior. Both sensors showed the same degree of enantioselective fluorescence response for various aromatic nitro compounds. However, in some cases, their enantioselectivities were different depending on the analytes. The difference in the chiral recognition abilities between Am-1b and CyD-1b seems to be based on the structural difference of their inherent backbones, that is, the one-handed helical structure and cyclic structure, respectively. The study on the resolution ability of the Am-1b-based CSP revealed that the terthienyl-based pendant of Am-1b provides not only a fluorescent functionality but also a different chiral recognition site from that of amylose tris(phenylcarbamate).

Concepts: Fluorescence, Chromatography, High performance liquid chromatography, Units of measurement, Nitro compound, Quenching, Reduction of nitro compounds, Pendant

28

Nitroolefin is a common and versatile reagent, synthesis of which from olefin is generally limited by the formation of mixture of cis- and trans- compounds. Here we report that silver nitrite (AgNO2) along with TEMPO can promote the regio- and stereoselective nitration of a broad range of olefins. This work discloses a new and efficient approach wherein starting from olefin, nitroalkane radical formation and subsequent transformations lead to the desired nitroolefin in a stereoselective manner.

Concepts: Amine, Functional group, Alkene, Functional groups, Silver, Aldehyde, Nitro compound, Ozone

28

N-Nitroso compounds are a versatile class of organic structures that allow expedient access to a diversity of synthetically useful architectures through demonstrated reactivities. We report herein the development of a Rh(III)-catalyzed N-nitroso-directed methodology for the ortho-olefination of arenes. The heightened reactivity endowed by the N-nitroso group translates to mild reaction conditions, high reaction yields, and synthetic compatibility of otherwise elusive substrates (e.g., an unactivated olefin, 1-octene). Comprehensive mechanistic studies on the electronic effect, deuterium exchange, kinetic isotope effect, kinetic profile, and numerous Rh(III) complexes have established [RhCp*]2+ as the catalyst resting state, electrophilic C-H activation as the turnover-limiting step, and a five-membered rhodacycle as a catalytically competent intermediate. The ability to elaborate the N-nitroso moiety to an amine functionality provides a seminal example of the innumerable synthetic possibilities offered by this transformable directing group.

Concepts: Functional group, Alkene, Isotope, Chemical kinetics, Deuterium, Kinetic isotope effect, Nitro compound, Physical organic chemistry

27

1-Methyl-2,4,5-trinitro imidazole (MTNI) is a well-known melt cast explosive possessing good thermal stability and impact insensitivity. MTNI has been synthesized from multi-step nitration followed by methylation of imidazole exhibiting low yield. It is desirable to screen the process thermodynamically for evaluating feasibility of the process. In the present investigations, B3LYP method in combination with 3-21G** basis set has been chosen to evaluate the enthalpy of formation for reaction species by designing reasonable isodesmic reactions. Thermodynamic feasibility of the processes has been worked out assuming free energies of reactions as derived from standard enthalpy and entropy of the reaction species. All possible synthesis routes for the target molecule, MTNI has been conceptualized from different precursors/intermediates viz. imidazole, 2-nitroimidazole, 4-nitroimidazole, 1-methyl imidazole and 2,4,5-triiodoimidazole. Various nitrating agents have been employed and their effect studied for deciding the feasibility of the reaction. It has been found that reaction entropy and enthalpy are favorable on usage of NO2BF4 as nitrating agent. The charge on nitro group (-QNO2) has been used for better understanding of the reactivity of substrates/intermediates. Overall, the study helped in screening several possible routes for MTNI synthesis and identified the thermodynamically feasible process by using NO2BF4 as nitrating agent.

Concepts: Energy, Temperature, Thermodynamics, Entropy, Gibbs free energy, Internal energy, Enthalpy, Nitro compound

26

Recent years have witnessed quite a number of worldwide efforts for fabricating CdSe/TiO2 nanotube arrays (CdSe/TNTAs) nanocomposites; however, the construction of a well-defined CdSe/TNTAs binary nanostructure for versatile photocatalytic and photoelectrochemical applications still poses a big challenge. In this work, a hierarchically ordered CdSe/nanoporous TiO2 nanotube arrays (CdSe/NP-TNTAs) hybrid nanostructure was fabricated through a facile electrochemical deposition strategy. The combined structural and morphological characterizations show that the CdSe ingredients, consisting of clusters of quantum dots, were uniformly assembled on the inner and outer surfaces of the NP-TNTAs framework. It was demonstrated that the CdSe/NP-TNTAs heterostructure could be utilized as an efficient photoanode for photoelectrochemical water splitting; moreover, it could be used as a multifunctional photocatalyst for photoredox applications, including the photocatalytic oxidation of organic dye pollutants and the selective reduction of aromatic nitro compounds under visible light irradiation. Furthermore, photoelectrochemical and photocatalytic mechanisms over the CdSe/NP-TNTAs heterostructure were elucidated. In addition, the predominant active species during the photocatalytic process were systematically explored and unequivocally determined. It is hoped that this work could promote further interest in the fabrication of various one dimensional NP-TNTAs-based composite materials and their applications to photoelectrochemical water splitting and photocatalytic selective redox applications.

Concepts: Hydrogen, Redox, Electrochemistry, Photoelectrochemical cell, Titanium dioxide, Photocatalysis, Nitro compound, Reduction of nitro compounds

25

A new method has been proposed to understand and predict the stability of nitro compounds. This method uses the maximum electron densities at the critical points of two N-O bonds of nitro groups (ρ max), and it is more simple and faster than the existing methods and applicable to bigger systems. The correlations between the ρ max and total energy (E), bond lengths ([Formula: see text], [Formula: see text] and [Formula: see text]), bond dissociation energy (BDE), and impact sensitivity (h 50) reveal that the molecular stability, which can be reflected by E, R, BDE and h 50, generally decreases with the increasing ρ max. The compound with the larger ρ max is less stable. For the nitrating reaction, the smaller ρ max of the product generally implies the easier and faster reaction and the higher occurrence ratio of the product. Therefore, ρ max can be applied to predict the stability of nitro compounds and the easiness of the nitrating reaction.

Concepts: Scientific method, Chemical reaction, Light, Water, Electromagnetic radiation, Chemical bond, Chemical compound, Nitro compound

25

The SNAr reaction of 2,4-dichloropyrimidines with an electron-withdrawing substituent at C-5 has excellent selectivity for substitution at C-4. Here we report that tertiary amine nucleophiles give excellent C-2 selectivity. In situ N-dealkylation of an intermediate gives the product that formally corresponds to the reaction of a secondary amine nucleophile at C-2. This reaction is practical (fast under simple reaction conditions, with good generality for tertiary amine structure and moderate to excellent yields) and significantly expands access to pyrimidine structures.

Concepts: Amino acid, Amine, Ammonia, Nucleophilic substitution, Nitro compound, Nucleophilic acyl substitution

25

Using a comprehensive set of drop weight impact test data (h50) newly compiled from literature for 308 materials, a recent approach to predict impact sensitivities of nitro compounds is generalized to most explosive substances of interest. Compared to previous ones, this procedure is more thoroughly validated and exhibits a good predictive value. Furthermore, it yields new insight into the physical mechanisms involved, explaining for instance the unexpected desensitization of some oxygen-deficient triazoles upon nitration.

Concepts: Scientific method, Object-oriented programming, Nitro compound, Nitromethane, Nitration, Nitroglycerin

25

The nitrobenzofurazan (NBD) moiety has gained tremendous popularity over the last decades due to its fluorogenic nature. Indeed, upon interaction with aliphatic amines, it generates a stable fluorescent adduct, which has been used for protein and lipid labeling. In fact the 4-amino substituted NBD belongs to the broad family of intramolecular charge transfer molecules, with the amino group acting as an electron donor upon photoexcitation, and the nitro group as an electron acceptor. Whereas the singlet excited state of 4-amino NBD derivatives has been abundantly studied, investigation of its triplet manifold is scarce and even the absence of intersystem crossing for this type of molecules has been suggested. However, intramolecular charge transfer molecules are known to undergo intersystem crossing and high phosphorescence quantum yields have been reported in non-polar solvent. In the present paper, we have investigated the photophysical and photochemical properties of N-hexyl-7-nitrobenzo[c][1,2,5]xadiazole-4-amine. We have shown the existence of a triplet state for this molecule in cyclohexane via nanosecond laser flash photolysis. Interestingly, deactivation of the triplet state leads to photoproducts formation, which are only present in the absence of oxygen. This article is protected by copyright. All rights reserved.

Concepts: Oxygen, Amine, Quantum mechanics, Functional group, Atom, Benzene, Excited state, Nitro compound

24

New all-conjugated C-C coupling products bearing both an electron-poor and an electron-rich aromatic moiety have been obtained from the reaction between sym-triaminobenzene derivatives and a series of isomeric chloro-nitrobenzofurazans. The reactions occur under mild reaction conditions, and in some cases a different behaviour depending on the presence, or not, of triethylamine was observed. From 1,3,5-tris(N-morpholinyl)benzene and 5-chloro-4-nitrobenzofurazan in the presence of triethylamine an unexpected product derived from the shift of the nitro group from C-4 to C-5 of the electrophile and bearing the nucleophile at position 4 was obtained. Moreover, from the coupling between 1,3,5-tris(N-pyrrolidinyl)benzene and 4-chloro-7-nitrobenzofurazan a highly stable Wheland intermediate was isolated.

Concepts: Amine, Chemical reaction, Functional group, Nucleophile, Nitro compound, Electrophile, Electrophilic aromatic substitution, Arenium ion