Selectivity under microwave irradiation. Benzylation of 2-pyridone: an experimental and theoretical study

The reaction of 2-pyridone with benzyl bromide in the absence of base and under solvent-free conditions has been studied experimentally and by computational methods. This reaction was one of the first reported examples in which modification of selectivity under microwave irradiation was observed. C- and/or N-alkylations were obtained depending on the benzyl halide and the heating system. N-Alkylation through mechanism A (S_N2 mechanism) is kinetically favoured while C-alkylation through an S_N1-type mechanism is thermodynamically favoured and is observed under microwave irradiation. Two S_N1-type mechanisms (mechanisms B and C) have been calculated, mechanism C being a kind of S_Ni. The influence of the pyridone/benzyl bromide ratio was studied. A second molecule of pyridone stabilizes the transition state and assists the leaving of the bromide ion. The occurrence of C-alkylation under microwave irradiation is explained by the predominance of the thermodynamic control in these conditions. Under microwave irradiation N-alkylation through an S_N1-type mechanism (mechanism C) can also occur. The dependence of the outcome of N-alkylation on the benzyl bromide ratio has been explained by a shift in the mechanism from S_N2 to S_N1 under microwave irradiation. Computational calculations have shown to be a useful tool for determination of the origin of the selectivity under microwave irradiation.     

Acid-catalysed hydrolysis of N-sulphonyl sulphilimines—II

The basicity and the acid-catalysed hydrolysis of ph(R)SNTs and o-HC₆H₄(Me)SNTs sulphilimines have been studied by UV spectrophotometric and kinetic methods, respectively, in aqueous HClO₄ (1–10 M) and 1:1 (v/v) EtOH/H₂O-HClO₄ (0.5–6 M). Depending on the constitution of the substrates, sulphilimine hydrolysis can follow three different courses, according to rate-acidity profiles, Bunnett-Olsen's treatment, activation parameters and product analysis. Most typical for sulphilimines is S_N2 hydrolysis with S^IV-N bond cleavage. In this case the reaction starts with the nucleophilic addition of water and is promoted by acid-base catalysis. If a relatively stable carbenium ion can be formed from R group, an S_N1 reaction with S^IVC bond cleavage takes place. Sulphilimine with X = o-CO₂H due to neighbouring-group participation hydrolyses very rapidly via acyloxy-sulphurane and acyloxy-sulphonium ion intermediates with five-memembered ring (S_Ni reaction involving S^IVN bond cleavage).     

Coordination compounds of disulphur dinitride

Summary Coordination compounds of the S₂N₂ molecule including methods for their preparation, reactivities, i.r. data, structures, and aspects of chemical bonding are reviewed. Methods of synthesis include reactions of S₂N₂, S₄N₄ or (NSCl)₃ with metal halides, metal complexes such as carbonyls, or even metals themselves. In all cases, the planar S₂N₂ ring is coordinated, usuallyvia both, of its nitrogen atoms so that S₂N₂ acts as a bridging ligand between two metal centres; short contact distances imply that halogen atoms linked to the metal atoms show some interaction with the sulphur atoms. The stability of S₂N₂ is greatly enhanced by coordination. In the i.r. spectra, two characteristic S₂N₂ vibrations assist identification of the S₂N₂ species, a ring stretching mode being observable atca. 850 cm^–1 and the out-of-plane deformation at 450–490 cm^–1.     

Conformational variation and crystalline phase transformation of low syndiotactic polypropylene films in stretched and stress‐relaxed states

Low syndiotactic polypropylene (sPP; rrrr = 80%) films were isothermally crystallized at 0 °C (sample S₀) and 90 °C (sample S₉₀) for 65 h, respectively. Fourier transform infrared spectroscopy, differential scanning calorimetry, and wide‐angle X‐ray diffraction were used to characterize the structure transformation and orientation behavior of samples S₀ and S₉₀ at both stretched and stress‐relaxed states. It was found that stretching (λ = 0–700%) induces the transformation of the chain conformation from helical to trans‐planar form for both S₀ and S₉₀ films. The stretched S₀ and S₉₀ samples show well oriented trans‐planar chains as well as partially retained helices. Simultaneously, crystalline phase transformation occurs during the stretching and relaxing processes of the investigated sPP samples, i.e., stable form I crystals can be transformed into metastable form III or mesophase by stretching samples, and vice versa. For stretched S₀ sample, form III with trans‐planar conformation, which generally exists in highly stretched sPP, cannot be observed, even at higher strains. For sample S₉₀, however, stretching might induce the formation of both the form III crystals and mesophase with trans‐planar chains; releasing the tension, form III again gets converted into trans‐planar mesophase and form I crystals. In the stretched and stress‐relaxed states of samples S₀ and S₉₀, the difference of the delicate orientation behavior and relative content of chain conformation and crystalline form can be attributed to the different heat‐treating methods of the low syndiotacticity sPP. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2924–2936, 2005     

Thermal decomposition of tris(piperidyldithiocarbamates) of As(III), Sb(III) and Bi(III)

Thermal studies by TG, DTG and DTA of tris(piperidyldithiocarbamates) of arsenic(III), antimony(III) and bismuth(III) of the general type M[S₂CN(CH₂)₅]₃ (M=As, Sb and Bi) have been carried out in nitrogen and air, as well as under vacuum, to determine their modes of decomposition. The apparent activation energies were determined by graphical methods and the TIN temperatures were calculated from the TG profiles. A possible mechanism of the decomposition reaction is suggested on the basis of the results of their pyrolysis and their mass spectral data. The intermediates obtained at the end of various decomposition steps were identified via their elemental analysis, i.r. spectral data and X-ray diffraction studies. A dimeric structure of type M₂[S₂CN(CH₂)₅]₄ (M=As, Sb) is proposed.     

Molekulare Zusammensetzung von flüssigem Schwefel. Teil: 3 Quantitative Analyse im Bereich 115–350°C

Molecular Composition of Liquid Sulfur. Part 3: Quantitative Analysis in the Temperature Region 115–350°C Relative concentrations of S₆, S₇, S₈, S_x (x > 8) and S_μ (insoluble sulfur) in equilibrium melts of elemental sulfur have been determined from i. r. and Raman spectra. At the freezing point (115°C) the melt consists of 0.6% S₆, 2.8% S₇, 1.5% S_x, and 95.1% S₈. – The solubility of S₇ in CS₂ has been determined at −77 to −26°C; the solubilities of both S₇ and S₈ in CS₂ are considerably enhanced by the presence of S_x. The thermal decomposition of S₈ and S_μ formation from S₆, S₇, and S_x has been investigated.     

Chemistry of dithiocarbamate derivatives of amino acids. Part III. X-ray photoelectron spectroscopy of Ba(S2CNHCH2CO2)·3H2O. I.r. and e.s.r. studies of α-amino acid-dithiocarbamate complexes of copper(II)

Summary Coordination compounds [Cu(S₂CNHCHRCO₂H)₂], were obtained by reaction of dithiocarbamate derivatives of amino acids with copper(II), and the products were characterized by i.r. and e.s.r. spectroscopy. The xps spectrum of Ba(S₂CNHCH₂CO₂)·3H₂O is reported.     

Conformational Study of Heteropentapeptides Containing an α‐Ethylated α,α‐Disubstituted Amino Acid: (S)‐Butylethylglycine (=2‐Amino‐2‐ethylhexanoic Acid) within a Sequence of Dimethylglycine (=2‐Aminoisobutyric Acid) Residues

Heteropentapeptides containing the α‐ethylated α,α‐disubstituted amino acid (S)‐butylethylglycine and four dimethylglycine residues, i.e., CF₃CO‐[(S)‐Beg]‐(Aib)₄‐OEt ( 4 ) and CF₃CO‐(Aib)₂‐[(S)‐Beg]‐(Aib)₂‐OEt ( 7 ), were synthesized by conventional solution methods. In the solid state, the preferred conformation of 4 was shown to be both a right‐handed (P) and a left‐handed (M) 3₁₀‐helical structure, and that of 7 was a right‐handed (P) 3₁₀‐helical structure. IR, CD, and ¹H‐NMR spectra revealed that the dominant conformation of both 4 and 7 in solution was the 3₁₀‐helical structure. These conformations were also supported by molecular‐mechanics calculations.     

Ab initio investigation on the structures and spectra of the firefly luciferin

The ground state (S₀) geometry of the firefly luciferin (LH2) was optimized by both DFT B3LYP and CASSCF methods. The vertical excitation energies (T _v) of three low-lying states (S₁, S₂, and S₃) were calculated by TD-DFT B3LYP//CASSCF method. The S₁ geometry was optimized by CASSCF method. Its T _v and the transition energy (T _e) were calculated by MS-CASPT2//CASSCF method. Both the TD-DFT and MS-CASPT2 calculated S₁ state T _v values agree with the experimental one. The IPEA shift greatly affects the MS-CASPT2 calculated T _v values. Some important excited states of LH2 and oxyluciferin (oxyLH2) are charge-transfer states and have more than one dominant configuration, so for deeply researching the firefly bioluminescence, the multireference calculations are desired. Supported by the National Natural Science Foundation of China (Grant No. 20673012) and the Major State Basic Research Development Programs (Grant No. 2004CB719903)     

The incongruent effusion of ZnGa8S13(s)

The chemistry and thermodynamics of vaporization in a Knudsen effusion cell of the phase ZnGa₈S₁₃ was studied in the range 1115–1246 K where the vapor pressure ranged from 0.25 to 10.0 Pa. Samples prepared by sealed-tube methods and characterized by X-ray diffraction were studied by the simultaneous Knudsen and torsion (Volmer) effusion method. ZnGa₈S₁₃(s) effused incongruently in equilibrium with a saturated solid solution (sss) of ZnS in Ga₂S₃, Ga₂S(g), Zn(g), and S₂(g), always on the ZnSGa₂S₃ join. Ga₂S₃(sss) then effused with net loss of ZnS to the vapor until the composition of Ga₂S₃ was reached. Vapor analysis by combining measured rates of mass loss and momentum loss allowed analysis of the vapor and calculation of equilibrium constants of the effusion equations. Thermodynamic treatment by the third-law method yielded the ΔH° (298 K) of ZnGa₈S₁₃(s) with respect to those of ZnS(s) and 4Ga₂S₃(s), − 52 ± 22 kJ mol ^{− 1}.     

Synthesis,Characterization and Crystal Structure of Some Novel 1‐Aryl‐2‐Thioxo‐2,3‐Dihydro‐1H‐Quinazolin‐4‐Ones

The base catalyzed intramolecular nucleophilic cyclization of 1‐(2‐haloaroyl)‐3‐aryl thioureas ( 1a‐i ), in the presence of DMF afforded the 1‐aryl‐2‐thioxo‐2,3‐dihydro‐1H‐quinazolin‐4‐ones ( 2a‐i ). The structures were confirmed by spectroscopic data, elemental analyses and in case of the 2c by single crystal X‐ray diffraction data. The mechanistic studies support an intramolecular nucleophilic substitution (S_NAr mechanism) rather than intramolecular aromatic substitution (S_RN1 mechanism).     

Electrolytic properties and stability of solid solutions of ytterbium sulfide in calcium thioytterbate

Solid solutions of Yb₂S₃ based on CaYb₂S₄ (Yb₃S₄) were prepared by the ceramic and sol-gel methods. The samples were identified by XRD. The electric conductivity of complex sulfide phases with different backgrounds was studied. The length of phases based on CaYb₂S₄ was determined. The thermal stability of the samples was tested and the range of working temperatures was determined.     

Neutron activation R-matrix elements and κ-matrix elements and neutron energy spectra

In the present paper, the principle of using some -matrix elementsK _ij, such as¹⁷⁵Yb (i or j)-¹⁶⁹Yb (j or i),¹⁵³Gd (i or j)-¹⁵⁹Gd (j or i),¹⁰³Ru (i or j)-⁹⁷Ru (j or i) and⁹⁵Zr (i or j)-⁹⁷Zr (j or i), as neutron energy spectra monitors or indicators is discussed in detail. Some expoerimental results showed the tendency of change ofK _kj withK _ij of¹⁶⁹Yb (i)-¹⁷⁵Yb (j) and the ratios of infinitely dilute resonance integrals S_i, S_j and S_k to the respective thermal neutron capture cross-sections, where k represents nuclides other then i and j.     

Stereoselective synthesis,solution structure and metal complexes of (1S,2S)-2-amino-1-hydroxyalkylphosphonic acids

The highly stereoselective synthesis of (1S,2S)-2-amino-1-hydroxyalkylphosphonic acids was achieved by addition of dimethyl phosphite to N-protected aminoaldehydes. Relative configuration and solution conformations of (1S,2S)-2-amino-1-hydroxy-alkylphosphonic acids (in D₂O) and their dimethyl esters (in CDCl₃ and CD₃OD) were established by means of NMR basing on the dependence between observed values of coupling constants (³J_HH, ³J_PC, ³J_HP) and corresponding dihedral angles. Potentiometric and spectroscopic methods were used for the evaluation of the structure of the complexes of (1S,2S)-2-amino-1-hydroxy-alkylphosphonic acids with Zn(II) and Cu(II) ions in aqueous solutions.     

Adsorption-calorimetric determination of the surface energy of dispersed oxides

A new method is proposed for the determination of the surface free energy σ_S and interfacial free energy σ_SL, based on their linear dependence on the specific heat of wetting of dispersed materials by water. The σ_S and σ_SL values of Silochrome, silica gel, and γ-aluminum oxide were determined first by this method. The obtained results are compared with data obtained from the similarity method and the empirical relationship between σ_S and the surface pressure. It is demonstrated that the methods can be used to determine σ_S for the investigated sorbents. Conclusions about the hydrophilicity-hydrophobicity of their surface were reached on the basis of the results. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 2, pp. 75–80, March–April, 2007.     

Zur Kenntnis des quaternären Systems Zn?In?S?Se Der Schnitt ZnIn2S4?Znln2Se4?In2S3?In2Se3

The Quaternary System ZnIn₂S₄? ZnIn₂Se₄? In₂Se₃? In₂S₃ The title system has been investigated on the indium rich side (ratio In/Zn ≥ 2) on samples quenched from 800°C to room temperature using x-ray methods. In this section 7 different phases could be identified the phase borders of which are given. ZnIn₂S₄-type and thiogallate type mixed crystals only show a small region of homogeneity while the monophase region of spinel type mixed crystals in the indiumsulfide rich part of the phase diagram has a larger extension. There is a new trigonal compound ZnIn₂S₂Se₂ (a_hex = 3.937, c_hex = 31.97 Å) with a large region of homogeneity. In the indiumselenide rich part there are two new phases: (i) Zn_0.4In₂Se_3.4 with unknown structure and (ii) a ternary phase of unknown structure in the system In₂S_3?xSe_x for 2.1 ≤ x ≤ 2.7.     

Hindered interdiffusion in asymmetric polymer-polymer junctions

We discuss theoretically the diffuse interface formed when a long (L) polymer is put into contact with shorter chains (S) of the same material (all chains being entangled). At time t shorter than the reptation time T_L of the long chains, the L chains behave like a gel swollen by the S chains. The “penetration factor” ψ (i.e. the volume fraction of S near the gel surface) is controled by a balance between the osmotic pressure of the swollen L chains, and the elastic tension ψ due to swelling. If t is larger than T_S (the reptation time of the short chains), ψ is of order N_e/N_S (where N_e is the number of monomers between entanglement points, and N_S is the degree of polymerisation of the short chains). On the other hand, if t < T_S, N_S must be replaced by the average number s (t) of monomers of an S chain which have entered the L region, and ψ ∼ N_e/ s (t) ∼t^−1/2. The width of the mixed region e(t) increases like s ^1/2(t) at T_S, and like (D_St)^1/2 (where D_S is the reptation diffusion constant of the S chains) at t>T_S.     

Phase equilibrium in the PbSnS2-PbSb2S4 system

The PbSnS₂-PbSb₂S₄ system was studied by physicochemical methods, and its phase diagram was plotted. The system is quasi-binary; solid solutions regions based on PbSnS₂ (6 mol % PbSb₂S₄) and PbSb₂S₄ (12 mol % PbSnS₂) were revealed. At a component ratio of 1: 1, a congruently melting compound Pb₂SnSb₂S₆ is formed. Pb₂SnSb₂S₆ single crystals were obtained by chemical transport. The unit cell parameters of Pb₂SnSb₂S₆, which crystallizes in orthorhombic system, were determined: a = 15.22 ?, b = 10.68 ?, c = 3.90 ?.     

Detection of transferrin isoforms in human serum: comparison of UV and ICP–MS detection after CZE and HPLC separations

Two methods for separation of transferrin (Tf) sialoforms, capillary electrophoresis (CE) and high performance liquid chromatography (HPLC) with conventional UV absorbance detection, have been investigated and compared. First, conditions affecting the separation of the Tf isoforms by capillary zone electrophoresis and HPLC were carefully optimized. The use of 15 mmol L⁻¹ borate buffer (pH 8.4) containing 3 mmol L⁻¹ diaminobutane (DAB) as additive enabled good separation of the Tf isoforms by CE (75 cm×50 μm i.d. fused silica capillary) at 25 kV. In HPLC, a gradient of ammonium acetate (from 0 to 250 mmol L⁻¹ in 45 min) buffered at pH 6 (Tris-HCl) proved suitable for separation of Tf isoforms on a Mono-Q HR 5/5 anion-exchange column. On-line specific detection of the iron associated with the different Tf isoforms, after Fe saturation, by inductively coupled plasma mass spectrometry (ICP–MS) was studied in detail to compare its analytical performance with UV detection. For both CE and HPLC an octapole reaction system (ORS) ICP–MS instrument was used to minimize polyatomic interferences on the ⁵⁶Fe major isotope. Limits of detection of the different isoforms were in the range of 0.02–0.04 μmol L⁻¹ Tf for HPLC–ICP (ORS)–MS. This hybrid technique proved more selective and reliable detection of transferrin isoforms with 2, 3, 4, 5, and 6 sialic acid residues (S₂, S₃, S₄, S₅, and S₆) in real serum samples. Interesting results from iron speciation of Tf in serum from healthy individuals and from pregnant women are given.