取代基对有机金属铼化合物中分子内α-氢转移反应势垒的影响

使用B3LYP方法研究了有机铼化合物R³R⁵(NHR⁴)Re(＝CHR¹)(＝NR²)中分子内α-氢转移反应, 探讨了不同取代基对α-氢转移反应势垒的影响. 研究发现, 可以通过改变取代基来影响过渡金属Re有机化合物中的α-氢转移反应. R¹位置的取代基为Me或CMe₃时, 可以较大程度降低α-氢转移反应的势垒. R²为H时, α-氢转移反应势垒最低. R³和R⁵位置为SiH₃时的反应势垒最低. R⁴为CMe₃时, α-氢转移反应势垒最低. 研究结果还表明, 取代基对于反应势垒的影响有加和性.     

Dispersion interaction between helium atoms

Accurate values for the coefficients of the R^?6, R^?8 and R^?10 in the series representation of the dispersion interaction between two helium atoms at distance R are obtained by a simple variation method.     

Proton affinities of phenylalkylamines by the kinetic method

The kinetic method was used to determine the proton affinities of phenylalkylamines of general formula R¹R²C₆H₃CHR³(CH₂)_nNR⁴R⁵ where R¹ = H or OH; R² = H, F, NO₂, OH or OCH₃; R³ = H or OH; R⁴ and R⁵ = H and/or CH₃; n = 1−3. Amines were used as reference bases and the proton affinities of the phenylalkylamines were bracketed by a pair of reference bases that give rise, in the MIKE spectra of the heterodimer, to more or less intense signals than the compound under study. The influence of the aliphatic chain length and of the substituents on the aromatic ring, on the proton affinities of the phenylalkylamines is presented and discussed. The formation of an hydrogen bond between the amino group and the aromatic ring is proposed to explain the results obtained.     

13C NMR spectra of 2,3-dihydro-1H-benzo[b]azepines: Differentiation of diastereomers

The ¹³C NMR spectra of 23 2,3-dihydro-1H-benzo[b]azepines, including nine pairs of diastereomers separated by chromatography, [(2R*, 3R^*) and (2R^*), 3S^*)] are hereby assigned and discussed. The relative configurations of the diastereomers were assigned by two methods. The first, based on the chemical shifts of the asymmetric carbons C-2 and C-3 (with regression analysis), shows that the values for (R^*, R^*) are approximately 1 ppm lower than those for (R^*, S^*) diastereomers. The second method uses the chemical shifts, δ₃, of the R₃(CH₃) substituents. When these δ₃ values are compared by means of the δ₃-δ_m difference (δ_m is the mean value obtained from compounds where R₂=H), the difference is always negative for (R^*, R^*) and positive for (R^*, S^*). This is attributed to a γ-gauche effect between R₂ and R₃ in the case of (R^*, R^*) diastereomers (R₂ and R₃ are cis). The results corroborate those already obtained by ¹H NMR [J(23)(R^*, R^*)<J(23)(R^*, S^*)] and are a confirmation of the results of a radiocrystallographic examination carried out on two nitrogen acetylated diastereomers.     

A test of the R-centroid method

The R-centroid method is shown to be exact when the molecular dipole transition moment, D(R), varies linearly with internuclear distance R, but may fail to give accurate results in the region of a maximum in D(R). Using individual line strengths computed for the X¹Σ⁺—A¹Σ⁺ system in LiH in which D(R) goes through a maximum, it is shown that good results can still be obtained if only the strongest transitions are used in the R-centroid analysis.     

Hydroboration d'amines insaturees—II: Régio et stéréosélectivité de l'hydroboration-alkylation d'amines acétyléniques

The hydroboration reaction of acetylenic amines R¹R²NCH(R³)CCR⁴ was studied. We report the first results of a study of the reactivity of dialkylborane R₂BH towards these amines which allow us to propose a new method for the synthesis of differently substituted β-ethylenic amines. The regioselectivity and the stereoselectivity of this reaction are examined and allow us to set out the possibility of a trans hydroboration.     

Spin chemistry of organometallic compounds 5. Interaction of N-bromohexamethyl disilazane with substituted silyl hydrides

Reaction of N-bromohexamethyl disilazane (Me₃Si)₂NBr with substituted triorganyl silanes R¹R²R³SiH results in asymmetric disilazanes Me₃SiNHSiR¹R²R³ and bromination product, bromotrimethyl silane Me₃SiBr. The reaction has demonstrated an unusual dependence on specific solvation. In benzene, bromination occurs immediately after mixing of the reagents, while in cyclohexane, the reaction products are formed only under UV-irradiation. Application of photoinduced CIDNP method has shown that the mechanism of bromination of triorganyl silanes is comprised of a series of consecutive radical stages involving N-centered disilazanyl (Me₃Si)₂N and Si-centered silyl R¹R²R³Si radicals.     

Chiral Separation of (R,R)-Tadalafil and Its Enantiomer in Bulk Drug Samples and Pharmaceutical Dosage Forms by Chiral RP-LC

A new simple isocratic chiral RP-LC method has been developed for the separation and quantification of the enantiomer of (R,R)-tadalafil in bulk drugs and dosage forms with an elution time of about 20 min. Chromatographic separation of (R,R)-tadalafil and its enantiomer was achieved on a bonded macro cyclic glycopeptide stationary phase. The method resolves the (R,R)-tadalafil and its enantiomer with a resolution (R _s) greater than 2.4 in the developed chiral RP-LC. The mobile phase used for the separation and quantification of (R,R)-tadalafil and its enantiomer involves a simple mixture of reverse phase solvents and the cost of analysis was drastically decreased. The test concentration is 0.4 mg mL⁻¹ in the mobile phase. This method is capable of detecting the enantiomer of (R,R)-tadalafil up to 0.0048 μg wrt test concentration 400 μg mL⁻¹ for a 20 μL injection volume. The drug was subjected to stress conditions of hydrolysis, oxidation, photolysis and thermal degradation. There was no interference of degradants with (R,R)-tadalafil and its enantiomer in the developed method. The developed chiral RP-LC method was validated with respect to linearity, accuracy, precision and robustness. The percentage recovery for the enantiomer of (R,R)-tadalafil in bulk drug samples and in dosage forms ranged from 97.0 to 102.5%. The test solution was found to be stable in the mobile phase for 48 h after preparation.     

Metal‐Free Addition/Head‐to‐Tail Polymerization of Transient Phosphinoboranes,RPH‐BH2: A Route to Poly(alkylphosphinoboranes)

Mild thermolysis of Lewis base stabilized phosphinoborane monomers R¹R²P? BH₂?NMe₃ (R¹,R²=H, Ph, or tBu/H) at room temperature to 100 °C provides a convenient new route to oligo‐ and polyphosphinoboranes [R¹R²P‐BH₂]_n. The polymerization appears to proceed via the addition/head‐to‐tail polymerization of short‐lived free phosphinoborane monomers, R¹R²P‐BH₂. This method offers access to high molar mass materials, as exemplified by poly(tert‐butylphosphinoborane), that are currently inaccessible using other routes (e.g. catalytic dehydrocoupling).     

Polarity and Conformatios of Phosphorylethylenes and Phosphorylacetates in Solution

Abstract

A series of derivatives R¹R²P(X)R³, where R¹=R²=Ph. R³= -CH=CH-Me, X=O(I); R¹=Me, R²=Ph, R³= -CH=CH₂, X=O(II); R¹=R²=Ph, R³= -CH=CH₂, X=Se(III) and R¹R²P(O)-CH₂C(O)OX, where R¹=Ph, R²= -CH=CH₂, X=Ment?(IV); R¹=Ph-2-OMe, R²=Ph, X=Ment?(V); R¹=R²=CH₂Ph, X=Et(I), were investigated by means of dipole moments method. The problem of conjugation in phosphorylethylenes and conformation behaviour of phosphorylacetates was considered. DM (exp.) of (I-IV), determined in CC1₄ solution are 4.48(I), 4.27(II), 4.97(III), 4.21(IV), 5.21(V) and 4.02 D (VI). The intramolecular electronic interactions of phosphoryl group with unsaturated fragment did not displays in polarity properties of compounds (I-III). The experimental dipole moments of derivatives (I-III) are equal to the calculated values of DM. DM (IV-VI) is very sensitive to orientation of the P=O and C=O polar bonds. Because DM (exp.) of these compounds very sensitive to its orientation. DM (calc.) for cis- and trans- orientation of P=O and C=O dipoles are really different, that allows to drow the conclution that, in the contrast to the crystal state, the corresponded dipoles prefer an anti array in solution.     

Theoretical studies on the acid hydrolysis of methyl carbamate

The mechanism of the A2 acid hydrolysis of methyl carbamate was investigated using MNDO method. The reaction was found to proceed in two steps: (1) the rate-determining nucleophilic attack of water on the carbonyl carbon of the N-protonated tautomer involving the tetrahedral TS; and (2) the fast subsequent proton abstraction by the leaving group, NH₃, to form products. The mechanism is similar to that involved in the A2 hydrolysis of acetamide. Effects of substituents, R¹, R², and R³ in R¹OCONR²R³, on rates can be predicted by the changes in electron densities on alkoxy oxygen and N, in complete agreement with the experimental results. We concluded that there is no need for invoking two different mechanisms for amides and carbamates since a common mechanism can easily accommodate all the experimental results.     

取代基对有机钼化合物中α-氢转移反应势垒和产物稳定性的影响

使用B3LYP方法研究了发生在有机钼化合物R³R⁴Mo(≡CH)(CHR¹R²)和R³R⁴Mo(＝CH₂)(＝CR¹R²)之间的α-氢转移反应, 探讨了R¹, R², R³和R⁴位置上不同取代基对α-氢转移反应势垒和产物稳定性的影响. 研究发现, 金属钼有机化合物中, 发生α-氢转移的碳原子在过渡态中采用sp²杂化. R¹和R²位置上取代基对α-氢转移反应势垒的影响取决于取代基对过渡态中碳原子的未参与sp²杂化的p_z轨道上单电子的离域作用. 当R¹, R²位置是甲基时, 由于碳原子的p_z轨道与甲基的一个C—H键轨道间存在强的超共轭效应, 从而可以较大程度地降低α-氢转移反应的势垒. 研究结果还表明, 当R³, R⁴位置为SiH₃时的反应势垒较低. 所以当R¹和R² 位置为Me, R³和R⁴位置为SiH₃时, 反应势垒最低. 第一个甲基取代R¹或R²位置的H时, 反应势垒降低很大; 第二个甲基继续取代时, 反应势垒的降低约为第一个甲基的一半. 第一个SiH₃取代R³或R⁴位置的甲基时, 反应势垒降低较大; 第二个SiH₃继续取代时, 反应势垒的降低小于第一个SiH₃的一半. 对反应物和产物的相对稳定性的研究表明, 第一个甲基和第二个甲基对产物的相对能量的降低几乎相同; 第一个SiH₃降低产物的相对能量, 但是第二个SiH₃使产物的相对能量升高, 从而抵消了第一个SiH₃对产物的稳定作用.     

Enantiomeric Separation of Moxifloxacin and Its (R,R)-Isomer by Ligand-Exchange Chiral Chromatography

A new stereospecific LC method for the separation and quantification of moxifloxacin and its (R,R)-enantiomer in bulk drug was developed and validated by ligand-exchange liquid chromatography on a reversed phase column using aqueous mobile phase containing the chiral reagent l-isoleucine-Cu(II). The UV detector was operated at 293 nm. The flow rate of mobile phase was set at 0.9 mL min^?1. The achiral ODS column offers good separation of the two enantiomers in less than 20 min. The test concentration was 1,000 μg mL^?1 in the mobile phase. This method was capable of detecting the (R,R)-enantiomer of moxifloxacin up to 0.1 μg mL^?1 for a 20 μL injection volume. The drug was subjected to stress conditions of hydrolysis, oxidation, photolysis and thermal degradation. There was no interference of degradants with the (R,R)-enantiomer in the developed method. The developed chiral RP-LC method was validated with respect to linearity, accuracy, precision and robustness. The percentage recovery for the (R,R)-enantiomer in bulk drug samples ranged from 98.1 to 104.4%. The test solution was found to be stable in the mobile phase for 48 h after preparation.     

Organoborazines. III. Homo- and copolymerization of p-vinylphenylcyclotriborazines

The polymerization and copolymerization with styrene of a series of unsymmetrically B-p-vinylphenyl-N-methyl and N-phenyl borazines [R³(R²)₂B₃N₃(R¹)₃; R¹ = methyl, phenyl, R² = methyl phenyl, R³ = p-vinylphenyl] has been studied. The polymerization of these monomers yielded both tractable and crosslinked materials. The polymers obtained were characterized by ¹H- and ¹³C-NMR spectroscopy, elemental analysis, gel permeation chromatography, and thermogravimeteric analysis. The reactivity ratios for the copolymerization reaction were calculated by the Mortimer-Tidwell method.     

Amperometric titrations of organolead and organotin ions

Mixtures of R₂Sn²⁺ and R₃Sn⁺ compounds can be analysed by titrating their total amount potentiometrically with alkali, and then determining R₂Sn²⁺ in another aliquot by amperometric titration with standard 8-hydroxyquinoline solution. In mixtures of R₂Pb²⁺ and R₃Pb⁺ compounds, dialkyllead ion can be titrated amperometrically with ferrocyanide solution and trialkyllead ions with tetraphenylboron solution. A potentiometric method is described for the determination of small amounts of lead chloride in the presence of any alkyllead chloride.     

Ligand Substituent Effects on Ruthenium (III/II) Redox Properties: An Advanced Inorganic Laboratory Experiment

A convenient inorganic laboratory exercise is presented that exposes the student to the influences of ligand substituents on a metal centers redox properties. Students prepare a series of compounds with the general formula [Ru(bpy)₂(R¹,R²mal)](PF₆), where bpy is 2,2-bipyridine and R¹,R²mal is an anionic -diketone ligand with various R¹ and R² substituents. Each complex is prepared in a single synthetic step, and the Ru(III)/Ru(II) couple is characterized by cyclic voltammetry. The potential of the Ru(III)/Ru(II) couple shifts upon varying R¹ and R² from 0.64 to 1.10 volts vs. SSCE in the following order: (R¹ = R² = Me) < (R¹ = R² = Ph) < (R¹ = Me, R² = CF₃) < (R¹ = R² = CF₃). The inductive effects of the R substituents on the Ru(III)/Ru(II) couple are consistent with their electron-donating and electron-withdrawing properties. Hammett constants for the substituents provide an approximate measure of these effects. A linear relationship is observed when the Hammett constants are plotted against the Ru(III)/Ru(II) potential. This laboratory exercise applies a versatile electroanalytical method, cyclic voltammetry, to measure the substituent effects on a metal centers electron density. Student results of the syntheses and redox characterizations of a series of [Ru(bpy)₂(R¹,R²mal)]⁺ complexes are presented and discussed.     

Extra-atomic relaxation energies for the 3d-transition metal series obtained with SCF hole-state calculations

A method to calculate extra-atomic relaxation energies is presented. It is based on the use of SCF hole-state calculations. The E_R^ea (2s) and E_R^ea (2p) values obtained for the 3d-transition metal series are in fair agreement with experimental values.     

Basicity and Reactivity of Alpha-Aminophosphonates: Theoretical Investigation

Abstract

Previously [1] we have found the possibility of the mechanism alteration for addition of substituted aminophosphonates (RO)₂P(O)CHR¹NHR² to PhNCO (where R = alkyles). which depends on the substituents at phosphorus and carbon atoms. We have performed theoretical investigation of basicity and reactivity of alpha- arninophosphonates by PM3 method using HyperChem molecular modeling package, We found that the calculated basicity parameters are in good correlation with our previous experimental data. From the results of the calculations, the whole series can be divided into three subsets: S1 with R¹=H and R²=n-Bu; S2 with RI=Ar with electronodonating substituent, R²=cycloHex. S3 with R¹=Ar with acceptor substituents, R²=cycloHex. The molecules, which belong to different series, differ in MO distribution on the reaction center NH. The aminophosphinates. containing electrono- withdrawing substituents at both centers (nitrogen and alpha-carbon) are deactivated completely.     

Improved detection limit for ammonium/ammonia achieved by Berthelot's reaction by use of solid-phase extraction coupled to diffuse reflectance spectroscopy

The proposed procedure is based on the extraction of the indothylmol blue into C₁₈ solid-phase extraction (SPE) membranes and direct quantification on the membrane surface by diffuse reflectance spectroscopy. The analytical performance of the proposed method has been evaluated for standard solutions of ammonium using reflectance values, R, as well as the Kubelka-Munk function, F(R). The results have been compared with those obtained by the conventional method, which uses UV-vis absorption spectroscopy with a sensor-based method. The described methodology provided satisfactory linearity and reproducibility within the ammonium concentration intervals 25-250 μg L⁻¹ and 25-500 μg L⁻¹ when using R and F(R), respectively. The limit of detection was around 10 μg L⁻¹, which is markedly lower than that of the classical procedure and than those provided by Nessler and OPA/thiol fluorimetric methods. For air samples the linear interval expressed as μg of ammonia is 0.24-2.4 or 0.24-4.7 employing R or F(R), respectively. The effect of potential interferences such as metals and aliphatic amines has also been evaluated. Finally, the proposed methodology has been adapted to the determination of ammonia in air and water samples. The method can be also used as a detector support for visual estimation.     

New synthesis of secondary carboxamide by using 2-methyl-2-oxazoline as a building block

New method for the synthesis of secondary carboxamides of type, R²NHCOR¹, which utilizes 2-methyl-2-oxazoline as a carboxamide building block and various halides, R²X, has been developed.