Tetracoordinate phosphorus cage compounds with endocyclic P–C bonds: Synthesis and reactivity

The survey summarizes modern methods of synthesis and reactivity of tetracoordinated phosphorus cage compounds containing one or more endocyclic P–C bonds (phosphine oxides, phosphinates and phosphonates, phosphatripticenes and others). Intramolecular cyclization reactions including electrophilic aromatic substitution with unsaturated organophosphorus compounds, reactions of P–H-derivatives with carbonyl compounds as well as cycloaddition reactions involving I–IV-coordinated phosphorus derivatives are considered.     

Covalent attachment of antagonists to the α7 nicotinic acetylcholine receptor: synthesis and reactivity of substituted maleimides

The 3-methylmaleimide congeners of the natural product methyllycaconitine (MLA) and an analogue covalently attach to functional cysteine mutants of the α7 nicotinic acetylcholine receptor (nAChR).     

Electrophilic reactivities of 7-L-4-nitrobenzofurazans in σ-complexation processes: Kinetic studies and structure–reactivity relationships

The second-order rate constants (k) for reaction of 7-chloro-4-nitrobenzofurazan 1 and 7-methoxy-4-nitrobenzofurazan 2 with a series of nitroalkyl anions and several of para-substituted phenoxide anions in aqueous solution at 20 °C have been reported. On the basis of the linear novel approach recently designed by Mayr and coworkers, the electrophilicity parameters E at the C-5 position of the two nitrobenzofurazans 1 and 2 have been quantified and ranked on the comprehensive electrophilicity scale. Mayr's approach was found to correctly predict the rate constants for the addition of phenoxide anions at the C-5 position of 1 and 2 witting a factor of <2. Analysis of the kinetic measurements using Brønsted's model shows that β_nuc values remain remarkably constant for changes in the nature of the substituent and that the σ-complexation process is associated with high Marcus intrinsic barriers. In addition, satisfactory correlations between the log k_exp (k_exp values measured in this work for reactions of benzofurazans 1 and 2 with a series of phenoxide anions in aqueous solution at 20 °C) and log k_calcd (k_calcd values calculated from equation 1 using the electrophilicity parameters E of benzofurazans 1 and 2 and the previously published nucleophilicity parameters N and s_N of the phenoxide anions) with a slope very close to unity have been obtained and discussed.     

The Diels–Alder reactions of para-benzoquinone nitrogen-derivatives: an experimental and theoretical study

An experimental and theoretical study of the comparative reactivity and selectivity of the Diels–Alder reactions of para-benzoquinones and three nitrogen derivatives have been performed. The mono-oximes derivatives do not react under the tested reaction conditions, whereas the tosylated mono-oximes react slowly. However, the mono N-tosyl imines show excellent reactivity, and superior to the parent para-benzoquinones. DFT calculations support these experimental results.     

Synthesis and theoretical study of first generation CCK-functional dendrimers

A convenient synthesis and theoretical study of five CCK-functional dendrimers is described.The dendritic cores were linked with the CCK(Trp-Met-Asp(OBn)-PheNH2) fragment by forming amide bonds to give five target molecules.The structures of CCK-functional dendrimers were characterized by NMR,MS spectrum and predicted using molecular mechanics and PM3 semi- empirical molecular orbital theory.     

Kinetic study of the protonations and substitutions of different cobalt(III)–nta complexes

The behaviour of the Cobalt(III)–nta (nta = nitrilotriacetate) system in an acidic medium was investigated. The acid dissociation constant, pK _a1, of [(nta)(H₂O)Co(-OH)Co(H₂O)(nta)]^– was determined as 3.09(3) and the pK _a of the cis-[Co(nta)(H₂O)₂]/[Co(nta)(H₂O)(OH)]^– equilibrium was determined as 6.71(1). cis-[Co(nta)(H₂O)₂] undergoes ring-opening upon acidification below pH = 2.0. The formation of [Co( ³-nta)(H₂O)₃]⁺ was also studied. The substitutions between cis-[Co(nta)(H₂O)₂] and NCS^– ions were investigated in the pH = 2–7 ranges. [Co(nta) (H₂O)(OH)]^– reacts ca. 70 times faster at 24.7 °C with NCS^– ions than cis-[Co(nta)(H₂O)₂], indicating a cis-labilising effect of the OH^– ligand.     

Kinetic analysis of the phenyl-shift reaction in β-O-4 lignin model compounds: a computational study

The phenyl-shift reaction for the β-radical of phenethyl phenyl ether (PhCH(2)C?HOPh, β-PPE) is an integral step in the pyrolysis of PPE, which is a model compound for the β-O-4 linkage in lignin. We investigated the influence of natural occurring substituents (hydroxy, methoxy) on the reaction rate by calculating relative rate constants using density functional theory in combination with transition state theory, including anharmonic correction for low-frequency modes. The phenyl-shift reaction proceeds through an oxaspiro[2.5]octadienyl radical intermediate and the overall rate constants were computed invoking the steady-state approximation (its validity was confirmed). Substituents on the phenethyl ring have only little influence on the rate constants. If a methoxy substituent is located in the para position of the phenyl ring adjacent to the ether oxygen, the energies of the intermediate and second transition state are lowered, but the overall rate constant is not significantly altered. This is a consequence of the dominating first transition from reactant to intermediate in the overall rate constant. In contrast, o- and di-o-methoxy substituents significantly accelerate the phenyl-migration rate compared to β-PPE.     

Infrared and Raman study of alunite—jarosite compounds

Infrared and Raman spectra of alunite and jarosite compounds, AR₃(SO₄)₂(OH)₆, where R is either Fe³⁺ or Al³⁺ and A is a monovalent cation, have been obtained between 4000 and 40 cm⁻¹. The observed number and position of the i.r. and Raman bands have been analysed under the centrosymmetric D⁵_3d space group. In addition, some relationships between the crystallochemical characteristics of these compounds and their vibrational properties are presented.     

Isotopic labelling of peptides and isotope ratio analysis using LC–ICP–MS: a preliminary study

Bradykinin and substance P have been derivatised with cyclic diethylenetriaminepentaacetic anhydride (cDTPA) and subsequently labelled with natural and isotopically enriched Eu³⁺. This enabled the detection and relative quantitation of the peptides using element-selective detection by high-performance liquid chromatography inductively coupled plasma mass spectrometry (LC–ICP–MS). Relative quantitation was achieved by differentially labelling two peptide sources, after derivatisation with cDTPA, using natural and enriched ¹⁵¹Eu respectively. The ¹⁵¹Eu:¹⁵³Eu isotope ratio was measured and used to calculate the original peptide ratio. The measured ratios came within 5.2% of the known ratio. Derivatisation and chelation reactions were additionally confirmed using LC–ESI–MS.     

Divergent reactivity of allene-containing α-diazoesters using Cu and Rh catalysis

A versatile intramolecular reaction of allene-containing diazomalonates that exhibits excellent chemoselectivity for either allenic C–H insertion or cyclopropanation is demonstrated. The identity of the product depends on the transition metal catalyst that is employed for the reaction. Rh catalysts promote exclusive C–H insertion with good diastereoselectivity for the trans product, while Cu catalysis enables intramolecular cyclopropanation to yield methylenecyclopropanes with moderate to good E:Z ratios.     

Species-specific isotope-ratio measurements of volatile tin and antimony compounds using capillary GC–ICP–time-of-flight MS

The analytical performance of an axial inductively-coupled-plasma time-of-flight mass spectrometer (ICP–TOFMS) as a detector for fast transient chromatographic signals resulting from the coupling to capillary gas chromatography (CGC) was investigated. A cryotrapping GC–ICP–TOFMS method for the determination of volatile metal(loid) compounds (VOMs) in gases was used and the suitability of the TOF mass analyzer for multi-elemental speciation analysis and multi-isotope ratio determinations was studied in terms of accuracy and precision. Isotope ratios ¹¹⁸Sn/¹²⁰Sn and ¹²¹Sb/¹²³Sb have been determined in in-house gas standard atmospheres in Tedlar bags at two different levels (100 pg and 1 ng) for different elemental species (SnH₄, MeSnH₃, Me₂SnH₂, Me₃SnH, BuSnH₃, SbH₃, and MeSbH₂). A limitation arising from counting statistics in both detection modes could be shown. A solution containing rhodium (10 ng mL^–1) and cadmium (40 ng mL^–1) was introduced simultaneously to the GC outlet. Rhodium acts as a continuous internal standard and Cd is used for mass-bias correction (by measuring the ¹¹¹Cd/¹¹³Cd ratio). The detection system in both pulse counting and analog mode was examined. The best attainable precision was established for Me₂SnH₂ (analog mode, 12 replicates, 1 ng, RSD 0.34%, accuracy 0.31%) whereas most other species ranged between 0.4 and 0.5% RSD if higher concentrations were used. The limitations of the pulse counting system are clearly seen, with peak heights of more than 2000 counts reaching saturation (for an integration time of 100 ms), which reduces the accuracy of isotope ratio determinations. A dozen VOM could be detected in an aged landfill gas sample; several unidentified Sn compounds were present. Although their isotope ratios are within the confidence value of the standards, it is not yet clear if the acquired precision is good enough to identify isotopic fractionation of metal(loid)s through biovolatilization processes. With the precision achieved, the combination of cryotrapping GC and ICP–TOFMS is a powerful tool for monitoring volatile multi-element species in multi-tracer experiments and isotope dilution methodology. Received: 23 November 2000 / Revised: 19 February 2001 / Accepted: 24 February 2001     

Synthesis and characterization of Nb–Ge doped titanosilicate nanoparticles and study of their selectivity for absorption of 137Cs and 90Sr

In this research, for the first time Nb and Ge were doped into titanosilicate nanoparticles up to 25% simultaneously. Crystalline phases and morphology of the synthesized samples were studied by X-ray diffraction (XRD) method and scanning electron microscope (SEM), respectively. Elemental analysis of the samples was performed using X-ray fluorescence (XRF) and Energy dispersive X-ray (EDX) techniques. Surface area of the samples was measured by BET method. Ion exchange potential of the synthesized samples for Sr²⁺ and Cs⁺ and effective parameters such as concentration, temperature, time, and pH were investigated. In addition,¹³⁷Cs and ⁹⁰Sr radio nuclides absorption in the best appropriate sample was examined. The selectivity of the samples for absorption of ¹³⁷Cs and ⁹⁰Sr was studied by gamma spectroscopy, liquid scintillation spectrometry, and atomic absorption spectroscopy methods. The obtained results showed that the prepared samples had good potential for absorption of ¹³⁷Cs and ⁹⁰Sr from the model solution. The sample containing equal amount of niobium and germanium, removed completely the ¹³⁷Cs within the waste water of Tehran nuclear reactor and ⁹⁰Sr in the desired solution.     

Synthesis and reactivity of enediyne–nucleobase hybrids: effect of intramolecular π-stacking

Three distinct classes of nucleobase-containing enediynes 1–9 with varying nature of the linker have been synthesized to explore the effect of π-stacking interaction in accelerating the rate of Bergman cyclization (BC). Chemical reactivity study, both experimental and computations demonstrated the important role that aromatic π-stacking interactions between the appended nucleobases within an enediyne frame play in lowering the activation barrier of Bergman cyclization.     

Sensitive CE–MS analysis of potentially genotoxic alkylation compounds using derivatization and electrokinetic injection

A CE–MS method has been developed to detect trace levels of potentially genotoxic alkyl halides. After derivatization of the target components with 4-dimethylaminopyridine (DMAP) or butyl 1-(pyridinyl-4yl) piperidine 4-carboxylate (BPPC), the natively positively charged derivatives are pre-concentrated by applying electrokinetic injection and separated by a highly efficient CZE method using a background electrolyte (BGE) consisting of 100 mM of TRIS adjusted to pH 2.5 with phosphoric acid. Using a sheath liquid interface, subsequent MS detection allows highly specific and sensitive analysis of alkyl halides.