Decatungstate photocatalyzed oxidation of aryl alkanols. Electron transfer or hydrogen abstraction mechanism?

[reaction: see text] Decatungstate W(10)O(32)(4-) photosensitized oxidation of a series of para-X-substituted 1-aryl-1-alkanols was investigated. The only oxidation product of the side-chain of the 1-aryl-1-alkanol was the aryl ketone. The product analysis and kinetic data of the title reaction support a hydrogen atom transfer mechanism in the rate-determining step.     

Energy transfer or electron transfer?—DFT study on the mechanism of [2+2] cycloadditions induced by visible light photocatalysts

The intramolecular [2+2] cycloaddition of 1,3-dienes under visible light irradiation investigated by Yoon and his co-workers shows remarkably high yield and stereoselective differences under different photocatalysts. The reaction was speculated to be induced by energy transfer. However, the origin for these phenomena is still unclear. In this scene, the detailed mechanism for the [2+2] cycloaddition of 1,3-dienes under visible light has been investigated using density functional theory B3LYP and TPSSTPSS methods. The result shows that the reaction not only can be induced by energy transfer between photocatalysts and reactants, but also can be induced by electron transfer between them. The [2+2] cycloaddition induced by energy transfer is carried out along the potential energy surface (PES) of triplet excited states (T₁) firstly, and then goes back to the singlet ground state (S₀) via MECPs (minimum energy crossing points) between the PESs of the S₀ and T₁ states, forming the product in the S₀ state. The [2+2] reaction induced by electron transfer proceeds along the doublet state PES of the cation radical reactant and the neutral four-membered ring product could be obtained by electron transfer from the corresponding reactant or reduced photocatalyst. The origin of stereoselectivity of the [2+2] reaction is attributed to the reaction mechanism difference under different photocatalysts.     

Which is more likely in trichodiene biosynthesis: hydride or proton transfer?

The mechanisms proposed for enzyme-catalyzed formation of the sesquiterpene natural product trichodiene consistently include a step involving a 1,4-hydride transfer. Using quantum chemical methods (B3LYP/6-31+G(d,p) and mPW1PW91/6-31+G(d,p)), we discovered two alternative pathways for transformation of the intermediate bisabolyl cation to the cuprenyl cation, one of which--a proton-transfer pathway--appears to be much more energetically favorable (by more than 10 kcal/mol) than the hydride transfer pathways usually proposed.     

Anion receptors containing -NH binding sites: hydrogen-bond formation or neat proton transfer?

When the amide-containing receptor 1(+) is in a solution of dimethyl sulfoxide (DMSO) in the presence of basic anions (CH(3)COO(-), F(-), H(2)PO(4) (-)), it undergoes deprotonation of the -NH fragment to give the corresponding zwitterion, which can be isolated as a crystalline solid. In the presence of less basic anions (Cl(-), Br(-), NO(3) (-)), 1(+) establishes true hydrogen-bond interactions of decreasing intensity. The less acidic receptor 2(+) undergoes neat proton transfer with only the more basic anions CH(3)COO(-) and F(-), and establishes hydrogen-bond interactions with H(2)PO(4) (-). An empirical criterion for discerning neutralisation and hydrogen bonding, based on UV/Vis and (1)H NMR spectra, is proposed.     

Mechanism of Mukaiyama-Michael Reaction of Ketene Silyl Acetal: Electron Transfer or Nucleophilic Addition?

Mechanism of Mukaiyama-Michael reaction of ketene silyl acetal has been discussed. The competition reaction employing various types of ketene silyl acetals reveals that those bearing more substituents at the beta-position react preferentially over less substituted ones. However, when ketene silyl acetals involve bulky siloxy and/or alkoxy group(s), less substituted compounds react preferentially. The Lewis acids play an important role in these reactions. Enhanced preference for the more sterically demanding Michael adducts is obtained with Bu(2)Sn(OTf)(2), SnCl(4), and Et(3)SiClO(4) in the former reaction while TiCl(4) gives the highest selectivity for the less sterically demanding products in the latter case. These results are interpreted in terms of alternative reaction mechanisms. The reaction of less bulky ketene silyl acetals are initiated by electron transfer from these compounds to a Lewis acid. On the other hand, bulkier ketene silyl acetals undergo a ubiquitous nucleophilic reaction. Such a mechanistic change is discussed based on a variety of experimental results as well as the semiempirical PM3 MO calculations.     

Gold-catalyzed nitrene transfer to activated alkynes: formation of α,β-unsaturated amidines

A gold-catalyzed intermolecular nitrene transfer to alkynes was developed for the first time, revealing a new mode of nitrene transfer and providing a novel access to versatile α-imino metal carbenes. Various mild nitrene-transfer reagents were examined, and iminopyridium ylides especially those based on 3,5-dichloropyridine proved be highly effective. With activated alkynes such as N-alkynyloxazolidinones as substrates, α,β-unsaturated amidines were formed in mostly good yields.     

Electron counting and bonding analysis in triruthenium clusters containing sulfoximido ligands: true or false electron-deficient systems?

Triruthenium clusters containing a methylphenylsulfoximido cap or bridge, Ru₃(CO)₉(μ₂-H)[μ₃-NS(O)MePh] (1), Ru₃(CO)₁₀(μ₂-H)[μ₃-NS(O)MePh] (2), Ru₃(CO)₈(μ₃-η²-CPhCHBu)[μ₃-NS(O)MePh] (3), Ru₃(CO)₉(μ₃-η²-PhCCCCHPh)[μ₂-NS(O)MePh] (4), and Ru₃(CO)₇(μ₂-CO)(μ₃-η²-PhCCCCHPh)[μ₃-NS(O)MePh] (5) have been examined by EHT and DFT calculations in order to analyze the bonding present in the clusters and to establish the electron counting. They clearly show that a μ₃-sulfoximido group is not a 3e⁻ ligand as one may be led to think at first sight, but rather acts as a three-orbital/5e⁻ system, i.e. should be considered as isolobal to an N---R⁻ ligand. Because of some delocalization of its π-type orbitals on the sulfur and oxygen atoms, it is expected to bind slightly less strongly to metal atoms than classical imido ligands. Once in a μ₂ coordination mode, the sulfoximido ligand retains a lone pair on its pyramidalized N atom and becomes a two-orbital/3e⁻ ligand. It follows that clusters 1, 2, 4 and 5 are electron-precise, whereas cluster 3 is electron deficient with respect to the 18e⁻ rule but obeys the polyhedral skeletal electron pair electron-counting rules. Consistently, all the calculated clusters exhibit large HOMO–LUMO gaps and no trace of electron deficiency can be found in their electronic structures.     

Probing Molecular Dynamics with Ultrafast Electron Diffraction?

Recent progress in ultrafast lasers,ul-trafast X-rays and ultrafast electron beams has made it possible to watch the motion of atoms in real time through pump-p...     

Capillary electrophoresis/visible‐LED induced fluorescence of tryptophan: What's new?

Tryptophane (Trp) labelled by 3‐(4‐carboxybenzoyl)‐2‐quinolinecarboxaldehyde (CBQCA) is very difficult to identify using CE and fluorescence detection (480 nm). Why in this article some mass spectrometry experiments show that Trp is really labelled by CBQCA as Leucine (Leu)? If the maximum of UV absorption (λ_max) is the same between Leu‐CBQCA and Trp‐CBQCA, the molar extinction coefficient is around 2 fold higher for Trp‐CBQCA. The fluorescence of the Leu‐CBQCA derivative is 50 times more important than for Trp‐CBQCA. The addition of 7.5 mM of β‐cyclodextrin (β‐CD) was found to be a good mean to improve 2.1 fold the sensitivity of the Trp‐CBQCA fluorescence. Using a buffer containing SDS and β‐CD in CE, a LOD of 0.7 µM of L‐Trp can be reached and the ratio of the intensities between Leu, Isoleucine, Valine, Trp is 100, 21, 15, 1. Negative ESI^/MS and MS/MS of the labeled amino acids show that a loss of the carboxylate function takes place. In the presence of two enantiomers of Trp‐CBQCA, we have shown that this decarboxylation is not due to the derivatization process in the solution but rather occurs in the source of the mass spectrometer.     

Biotin synthase: enzyme or reactant?

Biotin synthase catalyzes formation of a thiophane ring through a radical mechanism that is difficult to reconstitute in vitro. Choi-Rhee and Cronan measure a turnover of 20-60 equivalents of biotin in vivo, but also find that turnover renders the protein susceptible to proteolytic destruction.     

Nanocomposites: Industrial opportunity or challenge?

Polymer nanocomposites based on organically modified layered silicates are an area of substantial scientific interest and of emerging industrial practice. Despite the proven benefits of nanocomposites such as mechanical properties, barrier properties and contribution to fire retardancy, polymer nanocomposites are used today only in niche applications. The reasons for the limited growth of nanocomposites are explained through the availability of alternative solutions, processing and dispersion challenges and inferior oxidative and photooxidative stability. Recent developments show the improved dispersion of unmodified nanoclays in polyolefins with the help of selected copolymer structures. The (photo)oxidative instability of nanocomposites is compensated with adjusted stabilizer systems.     

Green Tea Versus Traditional Korean Teas: Antibacterial/Antifungal or Both?

The feasibility of utilizing the antimicrobial activity of naturally available teas was studied. Eleven teas including 2 green teas and 9 other traditional Korean mixed teas were tested for their antimicrobial properties. Antibacterial and antifungal properties were assessed. The results showed that green teas possessed significant antifungal and antibacterial properties, while most of the mixed teas showed some amount of antifungal activity and almost insignificant antibacterial properties. Confocal microscopic imaging revealed mycelial damage as well as attack on sporophores rather than spores/spore germination to be the reason behind the antifungal activity. EGCG was identified as the crucial catechin for antimicrobial activity. The study confirmed that green tea had a clear edge over the traditional mixed teas when it comes to antimicrobial activity.     

The first step in layer-by-layer deposition: electrostatics and/or non-electrostatics?

A critical discussion is presented on the properties and prerequisites of adsorbed polyelectrolytes that have to function as substrates for further layer-by-layer deposition. The central theme is discriminating between the roles of electrostatic and non-electrostatic interactions. In order to emphasize this feature we refrain from discussing practical problems sometimes incurred in polyelectrolyte adsorption like freezing-in of non-equilibrium situations, patchwise attachment, unclear chemistry and only consider solid substrates. Although it is in principle ambiguous to discriminate between coulombic and non-coulombic or "chemical" interactions, it will be shown that, as a rule, non-coulombic contributions to the interactions cannot be neglected. They are responsible for the familiar overcharging. For obtaining more insight, it is recommended to consider electrometric techniques such as electrokinetics, conductometry and potentiometry, in combination with other analytical techniques applied to well-defined systems, for which various parameters can be modulated in a systematic way.     

Single electron localisation on the cystine/cysteine couple: sulphur or carbon?

Protein sulphur functions can host a single electron on sulphur atoms in redox processes linking thiols to disulphides. However, experimental results have shown that the single electron can also reside on carbon atoms leading to protein damage. We have investigated this possibility on cystine for two initial conformations. The other site of electron fixation is always the carbonyl function. When there is no carbonyl, the electron remains on the sulphur atoms. In a model of the active site of thioredoxin (cystine, the carboxylic group of aspartic acid 30 and a water molecule), only the carbonyl group of the cystine is reactive.     

Phosphonium-phosphates/thiophosphates: Ionic liquids or liquid ion pairs? NMR spectroscopic classification

Structurally unique ionic liquids phosphonium-phosphate and phosphonium-thiophosphate, having both phosphorus based counter ions, in which the anionic part is represented by di-aryl phosphate or di-aryl thiophosphate and cations been tetraalkylphosphonium groups, behave differently in terms of their NMR behaviour. While phosphonium-phosphates show significant changes in its ¹H, ¹³C and ³¹P NMR chemical shifts vis. á vis. corresponding chemical shifts for a physical mixture of tetraalkylphosphonium bromide and di-aryl phosphate, phosphonium-thiophosphates behave almost similarly in terms of NMR with their synthetic precursors, hence indicating phosphate-phosphonium interaction has a significant covalent component resembling more to a liquid ion pair while thiophosphate-phosphonium interaction is principally ionic in nature. Translational diffusion behavior studied by PFGSE-NMR experiments and ionic conductivities of these ionic liquids in chloroform solution corroborated the hypothesis. The effect of variable alkyl chain length in phosphonium cation is effectively observed in the extent of ion association. Results of this study may provide insight into the solution state behavior of these ionic liquids, would help to classify those in terms of their strength of ion association and thus potential application thereof.     

Ion/Neutral,Ion/Electron,Ion/Photon,and Ion/Ion Interactions in Tandem Mass Spectrometry: Do We Need Them All? Are They Enough?

A range of strategies and tools have been developed to facilitate the determination of primary structures of analyte molecules of interest via tandem mass spectrometry (MS/MS). The two main factors that determine the primary structural information present in an MS/MS spectrum are the type of ion generated from the analyte molecule and the dissociation method. The ion type subjected to dissociation is determined by the ionization method/conditions and ion transformation processes that might take place after initial gas-phase ion formation. Furthermore, the range of analyte-related ion types can be expanded via derivatization reactions prior to mass spectrometry. Dissociation methods include those that simply alter the population of internal states of the mass-selected ion (i.e., activation methods like collision-induced dissociation) as well as processes that rely on the transformation of the ion type prior to dissociation (e.g., electron capture dissociation). A variety of ion interactions have been studied for the purpose of ion dissociation and ion transformation, including ion/neutral, ion/photon, ion/electron, and ion/ion interactions. A wide range of phenomena have been observed, many of which have been explored/developed as means for structural analysis. The techniques arising from these phenomena are discussed within the context of the elements of structural determination in tandem mass spectrometry: ion-type definition and dissociation. Unique aspects of the various ion interactions are emphasized along with any barriers to widespread implementation.