Effect of π-accepting substituent on the reactivity and spectroscopic characteristics of triarylbismuthanes and triarylbismuth dihalides

Competitive chlorination of p-substituted triarylbismuthanes 1 [(p-XC₆H₄)₃Bi; a: X = OMe, c: Cl, d: CO₂Et, e: CF₃, f: CN, g: NO₂] and trimesitylbismuthane (2,4,6-Me₃C₆H₂)₃Bi 1h by sulfuryl chloride was carried out against 1b (X = H) and the effect of these substituents on the formation of triarylbismuth dichlorides 2 was studied. The relative ratios 2/2b decreased with increasing electron-withdrawing ability of the substituents (2a/2b = 53/47, 2c/2b = 33/67, 2d/2b = 35/65, 2e/2b = 29/71, 2f/2b = 16/84, 2g/2b = 0/100, 2h/2b = 46/54), indicating a lowering of reactivity of the lone pair on the bismuth atom. Pd-Catalyzed degradation of 2a-g and their difluorides 3 giving biaryls 4 was promoted by the electron-withdrawing p-substituents in the equatorial aryl groups but suppressed by the more electronegative fluorine atoms in the apical positions. This is in fairly good accord with the stability of the trigonal bipyramidal geometry. The ¹³C NMR study of 1-3 showed that the signals due to the ipso carbons (C1) attached to the bismuth atom shift downfield with increasing electron-withdrawing nature of the p-substituents. No such tendency was observed in other aromatic ring carbons. The electronic effect on the C1 atoms, similar to that on the chlorination of 1 and degradation of 2 and 3, indicates the significant participation of the C1 atoms in these reactions through the Bi-C1 bonds.     

NR transfer reactivity of azo-compound I of P450. How does the nitrogen substituent tune the reactivity of the species toward C-H and C=C activation?

We studied electronic structures and reactivity patterns of azo-compound I species (RN-Cpd I) by comparison to O-Cpd I of, e.g., cytochrome P450. The study shows that the RN-Cpd I species are capable of C=C aziridination and C-H amidation, in a two-state mechanism similar to that of O-Cpd I. However, unlike O-Cpd I, here the nitrogen substituent (R) exerts a major impact on structure and reactivity. Thus, it is demonstrated that Fe=NR bonds of RN-Cpd I will generally be substantially longer than Fe=O bonds; electron-withdrawing R groups will generate a very long Fe=N bond, whereas electron-releasing R groups should have the opposite effect and hence a shorter Fe=N bond. The R substituent controls also the reactivity of RN-Cpd I toward C=C and C-H bonds by exerting steric and electronic effects. Our analysis shows that an electron-releasing substituent will lower the barriers for both bond activation reactions, since the electronic factor makes the reactions highly exothermic, while an electron-withdrawing one should raise both barriers. The steric bulk of the substituent is predicted to inhibit more strongly the aziridination reactions. It is predicted that electron-releasing substituents with small bulk will create powerful aziridination reagents, whereas electron-withdrawing substituents like MeSO(2) will prefer C-H bond activation with preference that increases with steric bulk. Finally, the study predicts (i) that the reactions of RN-Cpd I will be less stereospecific than those of O-Cpd I and (ii) that aziridination will be more stereoselective than amidation.     

Apparent molar volumes and compressibilities of electrolytes and ions in γ-butyrolactone

The densities of tetraphenylphosphonium bromide, sodium tetraphenylborate, lithium perchlorate, sodium perchlorate and lithium bromide in γ-butyrolactone at (288.15, 293.15, 298.15, 303.15, 308.15 and 313.15) K and speed of sound at 298.15 K have been measured. From these data apparent molar volumes V_Φ at (288.15, 293.15, 298.15, 303.15, 308.15 and 313.15) K and the apparent molar isentropic compressibility K_S,Φ, at T = 298.15 K of the salts have been determined. The apparent molar volumes and the apparent molar isentropic compressibilities were fitted to the Redlich, Rosenfeld and Mayer equation as well as to the Pitzer and Masson equations yielding infinite dilution data. The obtained limiting values have been used to estimate the ionic data of the standard partial molar volume and the standard partial isentropic compressibility in γ-butyrolactone solutions.     

Average life of oxygen vacancies of quartz in sediments

Average life of oxygen vacancies of quartz in sediments is estimated by using the ESR(electron spin resonance) signals of E' centers from the thermal activation technique. The experi-mental results show that the second-order kinetics equation is more applicable to the life estima-tion compared with the first order equation. The average life of oxygen vacancies of quartz from4895 to 4908 deep sediments in the Tarim Basin is about 10~(18) a at 27℃.     

The reactivity of tert-butoxyl radicals in reactions of hydrogen abstraction and β-elimination

The transition states of competing reactions of abstraction of the H atom from hydrocarbon molecules by tert-butoxyl radical and its -elimination were studied by the semiempirical quantum-chemical AM1 method. The calculated activation enthalpy and entropy of these reactions correlate with the experimental values. A correlation between the reactivity in homolytic hydrogen abstraction and the parameters of the chemical structure of the reactants was found.     

Generation and Dienophilic reactivity of α-oxoselenoaldehydes and ketones

The reaction of elemental selenium with sulfur ylides stabilized by electron-withdrawing substituent(s) affords a facile method for generation of functionalized selenocarbonyl compounds, which can be effectively trapped by Diels-Alder reaction with 1,3-dienes.     

Transmission of substituent effects through unsaturated systems Part 71. Influence of substituent on infrared and 13C NMR properties in rigid cisoid and transoid Conformations of α-enones

Dual substituent parameter (DSP) analysis is applied to model substituent effects on ¹³C substituent chemical shifts (SCS) and ν_CO stretching frequencies in two series of α-enones with rigid cisoïd and transoïd conformations. The regression coefficients of DSP analysis for the ¹³C SCS of the conjugated systems are discussed in terms of the relative importance of the various mechanisms of transmission of electronic effects. The differences between the regression coefficients relative to the field and resonance effects for the cisoïd and transoïd systems reflect very well the variations of geometries between both series. Concerning the i.r. data, the DSP analysis leads to an apparently unprecedented observation since it appears that the stretching frequencies of the carbonyl group are more sensitive towards substituent effect in the aplanar cisoïd system than in the more planar transoïd system. This surprising result is discussed.     

The reactivity of TMSN3 with ruthenium cyclopropenyl complexes containing different ligands and different substituent at Cγ

Ruthenium vinylidene complexes 2a-2c containing indenyl and bidentate dppe ligands can be obtained in efficient yields. Treatment of the cationic ruthenium vinylidene complexes with n-Bu₄NOH in acetone yields the neutral cyclopropenyl products (η⁵-C₉H₇)(dppe)Ru-CC(Ph)CHR (3) (3a, R = Ph; 3b, R = CN; 3c, R = p-C₆H₄-CN) via the deprotonation reaction. Reaction of complexes 3a and 3c with Me₃SiN₃ (TMSN₃) the N-coordinated complexes 4a and 4c can be obtained as stable products. Complex 3b containing CN group at Cγ in the cyclopropenyl ring reacts with TMSN₃ yielded the tetrazolate complex 5b. Similar cyclopropenyl products containing indenyl and two triphenylphosphine ligands 3′ can also be synthesized. Reaction of complex 3b′ with TMSN₃ also yielded the tetrazolate complex as the major product. And the minor products are [Ru]-N₃ and organic compound 6b. Reaction of 3a′ and 3c′ with TMSN₃ yielded [Ru]-N₃. The corresponding organic products can also be obtained via the N₃⁻ attacking the metal center in the N-coordinated complexes 4a′ and 4c′.     

Effect of pH on the reactivity of dipeptides and α-amino acids in the N-acylation

The reaction kinetics of Gly, L-??-Ala, Gly-Gly, L-??-Ala-L-??-Ala and ??-Ala-??-Ala with picryl benzoate in water (40 wt %)-2-propanol was investigated. At pH = 4?C8 the rate constants of N-acylation of the anionic form of dipeptides are less than those of the corresponding amino acid anions, in agreement with their basicity, whereas the relative effective rate constants of reactions depend on pH: in acidic, neutral and slightly alkaline media the k _ef values are higher for the dipeptides, and in a strongly alkaline medium, for the amino acids. These differences are due to the changes in the concentrations of reactive forms of amino acids and dipeptides in the system at varying the medium pH.     

A comparative study in structure and reactivity of “FeO_x-on-Pt” and “NiO_x-on-Pt” catalysts

Oxide nanostructures grown on noble metal surfaces are often highly active in many reactions,in which the oxide/metal interfaces play an important role.In the present work,we studied the surface structures of Fe Ox-on-Pt and Ni Ox-on-Pt catalysts and their activity to CO oxidation reactions using both model catalysts and supported nanocatalysts.Although the active Fe O1x structure is stabilized on the Pt surface in a reductive reaction atmosphere,it is prone to change to an Fe O2x structure in oxidative reaction gases and becomes deactivated.In contrast,a Ni O1x surface structure supported on Pt is stable in both reductive and oxidative CO oxidation atmospheres.Consequently,CO oxidation over the Ni O1x-on-Pt catalyst is further enhanced in the CO oxidation atmosphere with an excess of O2.The present results demonstrate that the stability of the active oxide surface phases depends on the stabilization effect of the substrate surface and is also related to whether the oxide exhibits a variable oxidation state.     

Structures of α-type ions formed in the 157 nm photodissociation of singly-charged peptide ions

One hundred fifty-seven nm photodissociation of singly-charged peptide ions induces the cleavage of alpha-carbon to carbonyl-carbon bonds along the backbone. a(n) + 1 radical ions are observed as the primary photolysis products of peptides with N-terminal arginines in a linear ion trap mass spectrometer. The radical elimination pathways undertaken by the a(n) + 1 radical ions to form more stable even-electron species are studied in hydrogen-deuterium (H/D) exchange experiments. Two types of a(n) ions along with d-type ions are observed as secondary elimination products. The relative abundance of each depends on the C-terminal residue of the radical fragment ion.     

The substituent effect of π-electron delocalization in N-methylamino-nitropyridine derivatives: crystal structure and DFT calculations

Structural Chemistry - The crystal and molecular structures of 3-(N-methylamino)-2-nitropyridine, 5-(N-methylamino)-2-nitropyridine and 2-(N-methylamino)-5-nitropyridine have been characterized by...     

Novel ions of quaternary ammonium halides in FDMS

Field desorption mass spectrometry exhibited novel ions [ Y-n H+n R ]+and [ Y-nH+nX]+as well as cluster ions [nY+(n-1)X-mH+mR]+and [nY+( n-1)X-mH+mX]+(n,m=0-3; X and Y represent the cation and anion moieties of quaternary ammonium halides),whose formation mechanism was discussed.     

Vibrational behavior of matrix-isolated ions in Tutton compounds. V. Infrared spectroscopic study of NH4+ and SO42− ions included in zinc sulfates and selenates

Infrared spectra of K₂Zn(SeO₄)₂·6H₂O and (NH₄)₂Zn(SeO₄)₂·6H₂O containing SO₄^2? guest ions and those of K₂Zn(SO₄)₂·6H₂O and K₂Zn(SeO₄)₂·6H₂O containing NH₄⁺ guest ions are presented and discussed in the region of the stretching modes ν₃ and ν₁ of the sulfate ions and in the region of asymmetric bending modes ν₄ of the NH₄⁺ ions, respectively. The SO₄^2? ions matrix-isolated in the selenate matrices (approximately 2 mol%) exhibit three bands for ν₃ and one band for ν₁ in agreement with the low site symmetry C₁ of the selenate host ions. The NH₄⁺ guest ions included in the potassium sulfate matrix are characterized also with three site symmetry components of ν₄. However, the ammonium ions in (NH₄)₂Zn(SeO₄)₂·6H₂O as well as those included in K₂Zn(SeO₄)₂·6H₂O display four infrared bands corresponding to ν₄ probably due to some kind of disorder of the ammonium ions. The extent of energetic distortion of the isomorphously included sulfate ions as deduced from the values of Δν₃ (site-group splitting) and Δν_max (the difference between the highest and the lowest wavenumbered components of the stretching modes) are commented. The spectroscopic experiments reveal that the SO₄^2? guest ions are weaker distorted in the potassium selenate matrix than the same ions in the neat potassium sulfate due to the larger unit-cell volumes of the selenate compounds. However, the SO₄^2? guest ions are stronger distorted in the ammonium selenate matrix as compared to the same ions in the neat ammonium sulfate owing to the formation of hydrogen bonds between the SO₄^2? guest ions and the NH₄⁺ host ions. The analysis of the spectra shows that the band positions of the water librations in the host potassium compounds are affected by the included ammonium cations. The formation of the hydrogen bonds between the NH₄⁺ guest ions and the XO₄^2? host ions leads to a decrease in the proton acceptor capabilities of the anions (anti-cooperative or proton acceptor competitive effect) and as a result the hydrogen bonds formed by the water molecules weaken on going from the neat potassium compounds to the mixed crystals K_1.8(NH₄)_0.2Zn(SO₄)₂·6H₂O and K_1.8(NH₄)_0.2Zn(SeO₄)₂·6H₂O (the bands corresponding to water librations are broadened and shifted to lower frequencies).     

ESI-TEM imaging of surfactants and ions sorbed in Stöber silica nanoparticles

The sorption of surfactants and NaCl in silica nanosized particles creates unexpected spatial distributions of solutes that were evidenced by electron spectroscopy imaging in the transmission electron microscope (ESI/TEM). The spectral images show that simple ions (Na(+), Cl(-), Br(-)) are actually absorbed within the particles irrespective of their charges, while surfactant chains are adsorbed at the particle surfaces. The expected effect of the surfactants on particle aggregation is also observed in the micrographs. In the case of salt, close-packed silica particle arrays are formed at low ionic strength, but only coarse aggregates form at higher salt concentrations. The particles absorb both Na(+) and Cl(-) ions in similar amounts, from 0.5 mol L(-)(1) NaCl, but Na(+) ions are depleted from the particles' immediate outer vicinity, where Cl(-) ions are in turn accumulated. These results confirm that St?ber silica nanoparticles are highly porous and reveal their potential usefulness as carriers of small molecules and ions, due to the small particle size, exceptional colloidal stability, and this newly found sorption behavior.     

Impact of substituent position in monosubstituted α-cyclodextrins on enantioselectivity in capillary electrophoresis

In this study, our three recently synthesized regiospecifically monosubstituted carboxymethyl-α-cyclodextrins (CMACDs) were successfully applied for the enantiomeric separation of several biologically important low-molecular weight compounds by capillary electrophoresis. The enantioselectivity of the individual monosubstituted CMACDs added into the background electrolyte (BGE) was studied and compared with the mixture of three monosubstituted CMACDs and with native α-cyclodextrin at pH of the BGE ranging from 2.5 to 11. Our experiments revealed a significant influence of the position of the carboxymethyl group on the α-cyclodextrin skeleton on the enantioselectivity for all the studied analytes. Interestingly, the least common 3(I)-O regioisomer was revealed as the most effective chiral selector.     

The role of metal ions in dopaminergic neuron degeneration in Parkinsonism and Parkinson’s disease

Abstract  

Parkinson’s disease is characterized by the selective degeneration of neuromelanin-containing dopaminergic neurons in the substantia nigra and locus coeruleus. Although the cause of this disease remains unknown, several transition metals, including manganese and copper, have been associated with the development of the atypical form of Parkinsonism, and iron accumulation has been associated with the development of Parkinson’s disease. Manganese³⁺ is a strong oxidizing agent, which oxidizes dopamine to aminochrome (dopaminochrome), the precursor of neuromelanin. Aminochrome formation in cell culture medium induces acute cell death in cells that uptake aminochrome, explaining the role of manganese in the development of atypical Parkinsonism. Copper accumulation in Wilson’s disease also induces Parkinsonism as one of the main symptoms, and an atypical Parkinsonism has also been observed in young copper miners. Interestingly, copper is able to complex with dopamine, which can be taken up by cells expressing the dopamine transporter, inducing caspase-independent cell death with formation of autophagic vacuoles. Iron is also able to form a complex with dopamine, the neurotoxic action of which also depends on the cellular expression of the dopamine transporter. The neurotoxicities of these transition metals to cells expressing the dopamine transporter all involve dopamine oxidation to quinones and require the inhibition of DT-diaphorase.     

Effects of substituent and solvent on the Sonogashira coupling reaction of β-bromoporphyrin

A series of experiments were designed to understand the effects of substituent and solvent on the Sonogashira coupling reaction of β-bromoporphyrin using a Pd₂(dba)₃/AsPh₃ system as the catalyst. Electron-efficient groups and aprotic solvents are conducive to the reaction. A possible explanation was given. A new family of β-pyrrole substituted porphyrins was also synthesized during the study.