When and How Do Diaminocarbenes Dimerize?

No example of a simple uncatalyzed dimerization of a diaminocarbene has been clearly established, so it is timely to ask what factors control the thermodynamics of this reaction, and what mechanisms are responsible for the observed dimerizations? In agreement with qualitative experimental observations, the dimerizations of simple five‐ and six‐membered‐ring diaminocarbenes are calculated to be 100 kJ mol^?1 less favorable than those of acyclic counterparts. This large difference is semiquantitatively accounted for by bond and torsional angle changes around the carbene centers. Carbenes such as (Et₂N)₂C are kinetically stable in THF at 25 °C in agreement with calculated energy barriers, but they rapidly dimerize in the presence of the corresponding formamidinium ion. This proton‐catalyzed process is probably the most common mechanism for dimer formation, and involves formation of C‐protonated dimers, which can be observed in suitable cases. The possibility of alkali‐metal‐promoted dimerization is raised, and circumstantial evidence for this is presented.     

ChemInform Abstract: Are the Compounds InH3 and TlH3 Stable Gas Phase or Solid State Species?

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Are axial and radial flow chromatography different?

Radial flow chromatography can be a solution for scaling up a packed bed chromatographic process to larger processing volumes. In this study we compared axial and radial flow affinity chromatography both experimentally and theoretically. We used an axial flow column and a miniaturized radial flow column with a ratio of 1.8 between outer and inner surface area, both with a bed height of 5 cm. The columns were packed with affinity resin to adsorb BSA. The average velocity in the columns was set equal. No difference in performance between the two columns could be observed. To gain more insight into the design of a radial flow column, the velocity profile and resin distribution in the radial flow column were calculated. Using mathematical models we found that the breakthrough performance of radial flow chromatography is very similar to axial flow when the ratio between outer and inner radius of the radial flow column is around 2. When this ratio is increased, differences become more apparent, but remain small. However, the ratio does have a significant influence on the velocity profile inside the resin bed, which directly influences the pressure drop and potentially resin compression, especially at higher values for this ratio. The choice between axial and radial flow will be based on cost price, footprint and packing characteristics. For small-scale processes, axial flow chromatography is probably the best choice, for resin volumes of at least several tens of litres, radial flow chromatography may be preferable.     

Are analytical standards and reagents really reliable?

Quality assurance is one of the major challenges in analytical chemistry, whatever the scope of application. The quality of analytical standards is very seldom questioned; however, sometimes odd results are obtained, and all the other potential sources of discrepancies are eliminated. So, we investigated the reliability of three analytical standards and reagents implemented for radiochemical and chemical characterizations of nuclear waste. In particular, this work examined the purity of a source of tritiated dodecane, the trueness of a certified concentration value and the purity for a diethylenetriaminepentaacetic acid (DTPA) reagent and the trueness of a certified concentration value for a multi-anion standard used in an interlaboratory comparison exercise. It was shown that the source of tritiated dodecane contains 60 % of tritiated impurities. The trueness of the DTPA concentration certified by the supplier was questioned due to the presence of impurities in the solution. It was proven that the long-term stability of the multi-anion standard was not guaranteed for nitrite. The results clearly demonstrated that, despite the certificates delivered by the suppliers, caution has to be taken toward the reliability of the analytical standards and reagents.     

The Study on the Stability Constant and Species Distribution of Eu (III) and Tb (III) Complexes with BDBPH

Biomimetic hydrolysis of DNA or RNA is of increasing importance in biotechnology and medicine. The ability to cleave nucleic acids efficiently, in a non-degradative manner, and with high levels of selectivity for site or structure will be required by many applications for the manipulation of genes, the design of structural probes and the development of novel therapeutics1. There has been much interest in the development of lanthanide complexes as nucleic acid cleavage agents. It has been fou…     

The Study on the Stability Constant and Species Distribution of Eu （Ⅲ） and Tb （Ⅲ） Complexes with BDBPH

The stability constants and species distributions of complexes of two lanthanide ions, Eu（Ⅲ） and Tb（Ⅲ）, with a macrocyclic ligand, BDBPH, in 1：1 and 2：lsystem, were determined potentiometrically at 25.0 ℃ and I=0.100 mol L^-l. The two metal ions could form deprotonated mono- or dinuclear complexes with BDBPH after the first two protons of the ligand completely neutralized. At higher pH values, Eu（Ⅲ） could not form hydroxy complexes with BDBPH, while Tb（Ⅲ） could form hydroxy complexes in the type of M2L（OH）, M2L（OH）2, and M2L（OH）3.     

Cytochrome P450 119 Compounds I Formed by Chemical Oxidation and Photooxidation Are the Same Species

Compound I from cytochrome P450 119 prepared by the photooxidation method involving peroxynitrite oxidation of the resting enzyme to Compound II followed by photooxidation to Compound I was compared to Compound I generated by m-chloroperoxybenzoic acid (MCPBA) oxidation of the resting enzyme. The two methods gave the same UV/Visible spectra, the same products from oxidations of lauric acid and palmitic acid and their (ω-2,ω-2,ω-3,ω-3)-tetradeuterated analogues, and the same kinetics for oxidations of lauric acid and caprylic acid. The experimental identities between the transients produced by the two methods leave no doubt that the same Compound I species is formed by the two methods.     

Are dopant-stabilized visible light-responsive photocatalysts efficient and stable?

Nitrogen and carbon codopants-stabilized hierarchical porous ZnS microspheres undergo an unexpected dynamic transformation into hollow microspheres when nitrogen and carbon are removed from the former. Thus, such a transformation is evidence for the unprecedented stability of non-metal doped visible light-responsive photocatalysts.     

Paradigms and Paradoxes Are Acyl Azides Resonance Stabilized?

By means of the generic isodesmic reaction C₆H₅COX + RCH₃ C₆H₅COCH₃ + RX, the phenyl/vinyl thermochemical equivalence, and an experimental determination of the enthalpy of the Curtius rearrangement of acryloyl azide, the resonance energy of acyl azides was ascertained to be ca. 29 kJ mol^–1.     

Are peroxyformic acid and dioxirane electrophilic or nucleophilic oxidants?

The electronic character of peroxyformic acid and dioxirane has been clarified by the analysis of donor-acceptor interactions in 16 transition states (TS) for the epoxidation of olefins. Is has been shown that the olefins are attacked by peroxyformic acid (PFA) in an electrophilic way. A relation of the electronic character to reactivity has been found: the more electrophilic the attack on the C=C bond is, the faster the reaction. In contrast, dioxirane (DO) has been identified as both an electrophilic and nucleophilic oxidant, depending on the substituents at the C=C double bond. The substrates with electron-withdrawing groups are attacked by DO in a nucleophilic way. These reactions have comparably low activation barriers. For instance, the acrylonitrile epoxidation with dioxirane is significantly faster than the corresponding reaction with PFA and proceeds via a transition state with a smaller extent of reaction and a larger extent of asymmetry.     

Heterogeneous or homogeneous catalysis? Mechanistic studies of the rhodium-catalyzed dehydrocoupling of amine-borane and phosphine-borane adducts

In depth, comparative studies on the catalytic dehydrocoupling of the amine-borane adduct Me(2)NH.BH(3) (to form [Me(2)N-BH(2)](2)) and the phosphine-borane adduct Ph(2)PH.BH(3) (to form Ph(2)PH-BH(2)-PPh(2)-BH(3)) with a variety of Rh (pre)catalysts such as [[Rh(1,5-cod)(micro-Cl)](2)], Rh/Al(2)O(3), Rh(colloid)/[Oct(4)N]Cl, and [Rh(1,5-cod)(2)]OTf have been performed in order to determine whether the dehydrocoupling proceeds by a homogeneous or heterogeneous mechanism. The results obtained suggest that the catalytic dehydrocoupling of Me(2)NH.BH(3) is heterogeneous in nature involving Rh(0) colloids, while that of Ph(2)PH.BH(3) proceeds by a homogeneous mechanism even when starting with Rh(0) precursors such as Rh/Al(2)O(3). The catalytic dehydrocoupling reactions are thought to proceed by different mechanisms due to a combination of factors such as (i) the greater reducing strength of amine-borane adducts, (ii) the increased ease of dissociation of phosphine-borane adducts, and (iii) phosphine ligation and/or poisoning of active catalytic sites on metal colloids.     

Are 1,2-dihydrodiazetes aromatic? An experimental and computational investigation

A thorough experimental and computational investigation of the aromaticity of the 1,2-dihydrodiazete ring system was carried out. The X-ray crystal structure of 1,2-dihydrodiazete 6 is reported, and the alkene-like reactivity of compound 6 is described. The compound's structure and reactivity suggest that 6 is not aromatic. This conclusion is corroborated by computational results on 6 and related compounds including homodesmotic reactions to test for aromatic stabilization, NICS calculations, and NBO calculations. Compound 6, and 1,2-dihydrodiazetes in general, are concluded to be strained heterocycles with no indication for aromatic stabilization.     

3H-Phosphaallenes Revisited: Facile Synthesis by Hydroalumination of Alkynylphosphines and β-Elimination,Stability and Trapping of Transient Species by Coordination to Transition Metal Atoms

A facile method for the efficient synthesis of 3H-phosphaallenes, R−P=C=C(H)−R′, is presented, which comprises treatment of dialkynylphosphines with dialkylaluminium hydrides (hydroalumination) and elimination of aluminium alkynides from intermediate alkenyl-alkynylphosphines. The stability of the phosphaallenes depends on steric shielding by the substituents at phosphorus (aryl or CH(SiMe₃)₂ groups). Only supermesityl compounds are persistent at room temperature in solution. This simple method starting with easily accessible dialkynylphosphines and commercially available aluminium hydrides (HAlEt₂, HAliBu₂) allows the generation of transient species, which were trapped by coordination to transition metals. The η¹-coordination via a P−W bond was observed for tungsten, while the side-on coordination via the P=C bond resulted with platinum. Decomposition of the mesityl derivative yielded an unprecedented product, which may be formed by 1,3-H shift to the P atom, hydrophosphination of the P=C bond of a second phosphaallene and formation of a P−P bond.     

Are isatin and isatoic anhydride antiaromatic and aromatic respectively? A combined experimental and theoretical investigation

This paper reports the results of our thermochemical/calorimetric determination of the enthalpies of combustion, phase change, and formation of isatin, isatoic anhydride, and N-methylisatin. The density functional calculations accompanied by vibrational and thermal corrections were also performed for these compounds and N-methylisatoic anhydride. Through a combination of theoretical calculations and associated isodesmic reactions, we have deduced that isatin has some antiaromatic character and isatoic anhydride enjoys some aromatic stabilization.     

Are the enolates of amides and esters stabilized by electrostatics?

The fact that amides and esters form less stable enolates than ketones might be seen as evidence that electrostatic stabilization is unimportant in these anions. However, ab initio molecular orbital calculations show that electrostatic stabilization does in fact lie beneath the competing resonance effect that causes the decrease in acidity. The electrostatic contribution is revealed by examining torsionally twisted amide and ester structures in which the pi resonance interactions are largely inhibited. These twisted amides and esters have greater enolate acidity than the corresponding ketones. Qualitatively similar behavior is observed with respect to protonation, such that twisted amides and esters are generally less basic than the reference ketones, in striking contrast to their behavior in the normal geometries.     

Structure and Bonding in First‐Row Transition‐Metal Dicarbides: Are They Related to the Stability of Met‐cars?

First-row transition-metal dicarbides MC(2) (M=Sc-Zn) have been investigated by using quantum-mechanical techniques. The competition between cyclic and linear isomers in these systems has been studied and the bonding scheme for these compounds is discussed through topological analysis of electron density. All of the systems have been found to prefer a C(2v)-symmetric arrangement, although for ZnC(2) the energy difference between this and the linear isomer is rather small. In most cases the C(2v)-symmetric structure corresponds to a T-shaped structure, with the exceptions of TiC(2), CoC(2), and NiC(2) which have been shown to be true rings. A detailed analysis of the variation of the energy of the system with geometry has been carried out. An analysis of the bonding, taking into account the main interactions between the valence orbitals of both fragments, the M atom and the C(2) molecule, has allowed the main features of these compounds to be interpreted. A clear correlation between the dissociation energies of the first-row transition-metal dicarbides and the bonding energies of the corresponding met-cars was observed.     

Are Vitamin E Supplementation Beneficial for Female Gynaecology Health and Diseases?

Vitamin E is known as an essential vitamin, and many studies had demonstrated the importance of vitamin E throughout the reproductive process, such as miscarriage, premature birth, preeclampsia, and intrauterine growth restriction, which could be caused by a lack of vitamin E during pregnancy. Its potent antioxidant properties can counteract the oxidative stress induced by oxygen free radicals and imbalance of oxidative-antioxidant levels, hence it may play a role in maintaining the normal function of the female reproductive system. Despite the fact that vitamin E is acknowledged as the substance needed for reproduction, its beneficial effects on female fertility, gynaecological health, and diseases are still poorly understood and lacking. Therefore, the goal of this paper is to provide a summary of the known roles of vitamin E supplementation in women for gynaecological health and reproductive-related diseases, as well as its future perspective.     

Updates on Dengue Vaccine and Antiviral: Where Are We Heading?

Approximately 100–400 million people from more than 100 countries in the tropical and subtropical world are affected by dengue infections. Recent scientific breakthroughs have brought new insights into novel strategies for the production of dengue antivirals and vaccines. The search for specific dengue inhibitors is expanding, and the mechanisms for evaluating the efficacy of novel drugs are currently established, allowing for expedited translation into human trials. Furthermore, in the aftermath of the only FDA-approved vaccine, Dengvaxia, a safer and more effective dengue vaccine candidate is making its way through the clinical trials. Until an effective antiviral therapy and licensed vaccine are available, disease monitoring and vector population control will be the mainstays of dengue prevention. In this article, we highlighted recent advances made in the perspectives of efforts made recently, in dengue vaccine development and dengue antiviral drug.