Inequivalence magnetique en serie imidazolidine : Application a la determination de la configuration relative de diastereoisomeres

The magnetic inequivalence of benzylic protons in N,N′-dibenzyl 2- or 4-monosubstituted and 2,4-disubstituted imidazolidines has been used to assign cis configuration to substituents at position 2 and 4.     

Étude thermodynamique de la complexation de l'argent par la thiourée et certains de ses derives alkylsubstitués

In order to find support for the validity of the hypothesis proposed by one of us regarding the different natures of the bonds in the four successive complexes formed between Ag⁺ and thiourea, the corresponding enthalpies are verified by direct calorimetry.Furthermore, because the influence of substituent groups on the nitrogen atoms may help to characterize the donor atom. the calorimetric enthalpies and potentiometric free energies of complexation between Ag⁺ and N-methylthiourea, N,N′- dimethylthiourea and N,N′-diethylthiourea have also been determined at 25°C, in aqueous medium of ionic strength μ = 0.5 M KNO₃.     

Application de la Résonance Magnétique de l'Azote 15 à l'Etude de la Délocalisation Electronique dans les Anilines,Aminopyridines et Enamines

¹⁵N spectroscopy at natural abundance has been applied to the study of electronic delocalization in the N? C bond of a series of substituted anilines. The relationships which offer a good way for evaluating the extent of electronic delocalization from ¹⁵N chemical shift data in anilines have been extended to aminopyridines, aminopyrimidines and nucleosides. The results obtained for C? N bonds in the case of anilines and aliphatic enamines are consistent with those concerning amides and analogous compounds. The validity of ΔG^≠-δ¹⁵N relationships for predicting π delocalization is discussed in terms of the relative contribution of δ- or π-density and non-bonded atom interactions to the total barrier height.     

Polymerisation de monomeres a structures biologiques

Fixation of biological compounds (cholesterol and testosterone) on (2-hydroxyethyl)methacrylate has been realized by a carbonate linkage. These new monomers have been characterized by N.M.R. and i.r.; they have been polymerized by a radical process. Resulting homopolymers have been analysed and compared with polymers bearing similar active compounds, previously synthesized by chemical modification of poly (2-hydroxyethyl)methacrylate.     

Structure de benzoates de fluoroalkyles

The crystal and molecular structure of 2,2,3,3-tetrafluoropropyl-parachlorobenzoate (C₁₀H₇ClF₄O₂) has been determined by X-ray diffraction. The crystals are monoclinic, P2₁, a = 4.826(1), b = 9.036(2), c = 12.442(4)Å, β = 94.64(2)°, V = 540.8 ų, Z = 2, D_x = 1.66 Mg m^?3. Starting from the ¹³C data of this compound, the identification of five other fluoroalkylbenzoates has been achieved by N.M.R. spectroscopy.     

Etude cristallochimique de LiPN2: Une structure derivée de la cristobalite

LiPN₂ has been prepared by reaction between Li₃N and P₃N₅ nitrides. The unit cell is tetragonal with a = 4.567(1) and c = 7.140(4) Å. The space group is $\text{I}\text{4}\text{2d}$. The structure is related to the β cristobalite type and is isostructural with CaGeN₂. It is built up from a PN₄ tetrahedra framework in the holes of which the lithium atoms are localized. The values of the rotation angle ф of tetrahedra, $\text{c}\text{a}$ ratio and θ (NPN) angle have been discussed in relation to the parameter x of the nitrogen atoms.     

Étude de quelques dérivés de la pyrazoline-2: Spectres infrarouge et methode CNDO/2

The infrared spectra of some derivatives of 2-pyrazoline have been studied in the region of the CN and N-H stretching bands.The N-H stretching frequency seems to be related to Taft's σ^* constants of the substituents bound to the intracyclic carbon atoms. The “acidity” of this group seems to be considerably less than in pyrrole or in carboxylic amides.Absorption spectra of the pure compounds or of moderately concentrated solutions suggest the existence of intermolecular hydrogen bonding.The hybridization state of nitrogen atom (1) seems to be approximatively sp³, without any noticeable conjugation with the CN(2) double bond. In those cases where two conformers could be assumed to exist only one has been observed.These results are in accordance with those of a molecular orbital calculation by the CNDO/2 method. This calculation suggests that in 5-methyl-2-pyrazoline, the hydrogen atom bound to N(1) is trans with respect to the methyl group.     

Synthese de 2h-pyrroles et de pyrroles: action de carbanions sur des 2H-azirines ou des sels de N,N,N-trimethylhydrazonium

Dimsyl sodium reacts with 2H-azirines 2 to give amino ethylenic sulfoxide 5E. The same reaction is observed from N,N,N-trimethylhydrazonium salt 1 using an excess of dimsylsodium. Carbanions of ketone or nitrile react with 2H-azirine or N,N,N-trimethylhydrazonium to produce 2H-pyrroles 6–13, 20 or pyrroles 18, 19. This reaction is the most convenient general route to 2H-pyrroles.     

Thermodynamique de composes azotes. VII. Etude thermochimique du pyrazole et de l'imidazole

Enthalpies of sublimation for pyrazole and imidazole have been obtained by calorimetry at 298.15K. The ΔH⁰_sub (298.15 K) values for these two compounds are, respectively, 69.16 ± 0.32 and 74.50 ± 0.40 kJ mole^?1. From literature data obtained by combustion calorimetry for ΔH⁰_f (c, 298.15 K), the enthalpies of formation of these compounds in the gaseous state (pyrazole: 185.1 ± 2.3 kJ mole^?, imidazole: 133.0 ± 1.7 kJ mole^?1) have been derived. Several energy values related to the molecular structure of these two compounds (as resonance energy, enthalpy of isomerization, …) have been determined. The study of pyrazole has enabled us to contribute to the evaluation of some characteristics of the NN bond.     

Etude par Résonance Magnétique Nucléaire de dioxaphosphorinanes-1,3,2 à liaison P?N extracyclique. Exemple de changement d'orientation de la liaison P?N

The ¹H NMR spectra of several 1,3,2-dioxaphosphorinanes bearing an extracyclic P? N bond have been analysed. The ³J(POCH) couplings are strongly dependent upon the orientation of the bond around the phosphorus atom. Depending upon the nature of the bonds attached to the nitrogen atom, the dioxaphosphorinane ring may adopt either a fixed chair conformation with the P? N bond in the axial or equatorial orientation, or it may be in equilibrium between two chair conformations where the P? N bond is alternately axial or equatorial. The equilibrium is fast on the NMR time scale.     

Application des mesures de couplages 15N?H à la détermination de configurations d'oximes

Several ¹⁵N enriched oximes of heterocyclic aldehydes have been prepared in syn and anti forms. The less stable form may be obtained by UV irradiation of the other one. The geminal ¹⁵N? H coupling in the R? CH?¹⁵N? OH fragment allows an immediate and unambiguous assignment of the configuration to be made, being 13 to 16 Hz and 2 to 3 Hz for the anti and syn forms, respectively. Whereas oximes 1 to 4 are preferentially in the anti form, the N-methylpyridinium aldoxime iodides (2-PAM, 3-PAM, 4-PAM) are found to be syn in the stable form and not anti as previously thought. This reassignment is of special interest, since 2-PAM ( 8 ), which is an excellent antidote against alkyl phosphate nerve poisoning, has been used to study the geometry of the acetylcholinesterase active site of the enzyme.     

Analyse isotopique de l'azote par spectrometrie optique,pour de faibles teneurs en 15n

For mixtures of ¹⁴N₂ and ¹⁴N¹⁵N low in ¹⁵N (isotopic abundances less than 2%), previous optical spectrometric methods do not produce rapid results of sufficient precision. In this paper, operating conditions are described for the determination of the isotopic content to ±3% near the natural abundance and to ±1% when the sample contains an atom ratio of ¹⁵N: ¹⁴N of 1.5:100. The time required for a determination is ca. 10 min. A spectrometer is modified by the substitution of an exit slit and a photomultiplier in place of a photographic plate. The excitation of the electrodeless discharge tube is made by a high-frequency field. The method has been applied to samples of ammonium salts, fertilisers and plant material. In all cases, the precision of the isotopic measurements was greater than that required for most agricultural studies (5–6%).     

Etude par RMN de derives phosphores : Etude de diazaphospholanes-1,3.2

The NMR Spectra of six 2-R-1,2-dimethyl-1,3,2-diazaphospholane (R = Cl, F, Me, Ph, OMe, N(Me)₂) have been analyzed in terms of AA′BB′X systems.     

La fluoration de l'hexachlorobenzene et de la pentachloropyridine en milieu de fluorure de potassium solide

The fluorinations of hexachlorobenzene and pentachloropyridine were carried out in sealed tubes with KF in presence of inert gas; the fluorinated derivatives C₆F_xCl_y x + y = 6 0?x?6 and C₅F_xCl_yN x + y = 5 0?x?5 are obtained. The influence of contact time, temperature and the concentration of the ion F^? are investigated and compared; the molar yield varied from 45% to 90%. It is possible to get directly and selectively some fluorinated derivatives as C₅Cl₂F₃N. The fluorinations in liquid KFKCl and solid KF are compared.     

Etude de la stabilite de la conformation de copolypeptides alternes

Conformation stability of alternating copolypeptides with the repeating units $\text{Nε-}\text{carbobenzoxy}\text{-}\text{l}\text{-}\text{lysyl}\text{, β-}\text{benzyl}\text{-}\text{l}\text{-}\text{aspartate}\text{(}\text{CLL}\text{-β}\text{BLA}\text{)}\text{and}\text{Nε-}\text{carbobenzoxy}\text{-}\text{l}\text{-}\text{lysyl}\text{, γ-}\text{benzyl}\text{-}\text{l}\text{-}\text{glutamate}\text{(}\text{CLL}\text{-γ}\text{BLG}\text{)}$ has been studied by NMR 220 MHz and by viscosity in deuterochloroform/trifluoroacetic acid (TFA) pair at 25°. NMR spectra show a helix-coil transition at low TFA contents for both copolypeptides as compared to other types of copolypeptides. In the case of poly (CLL-γBLG), the helix begins to collapse at 4% TFA and the transition is complete at 6% TFA. Poly(CLL-βBLA) helix is still more fragile since the transition is complete at 3% TFA. Viscosity measurements seem to indicate helix coil transition to take place at lower TFA content but the disagreement between the two sets of results is only artificial. This low helix stability of both copolypeptides is discussed according to the Lifson—Roig—Cortijo theory taking into account formation of H-bonding between side-chains having different chemical structures. This low helix stability would be due to a co-operative collapsing effect.     

Sur la separation simultanee du11C et de13N par fusion oxydante,en vue de la determination de deux elements legers par une seule activation au moyen de photons γ ou de particules chargees

A method of simultaneous¹¹C and¹³N separation by oxidizing fusion is described. It can be used in particular to determine, from a single irradiation with γ photons or charged particles (p, d,³He or α), two light elements amongst traces of Be, B, C, N and O in certain highly pure metals and semi-conducting materials.     

Substitution nucleophile aromatique. Etude cinetique de la C-arylation de cyanoacetamides par les halogenonitrobenzenes dans le N,N-dimethylformamide

The C-arylation of the sodium salt of N,N-dimethylcyanoacetamides, in DMF, has been investigated. The kinetics can be explained if two forms of the nucleophile, the isolated carbanion and the less reactive ion pair, are involved by use of Acree's theory, it has been possible to elucidate constants k_i (for isolated carbanion) and k_p (for ion pair) for different aromatic substrates. The variation of k_i/k_p may account for the selectivity observed in some cases.     

Les liaisons triangulaires changent les perspectives de la chimie

Triangular bonds change the shape of chemistry. One of the most exciting and far-reaching developments that have taken place in chemistry during the past few decades has been the recognition of triangular (three-centre, two-electron) bonds. Such bonds are often associated with “electron deficiency” and they result in the formation of novel polyhedral clusters of atoms. The boron hydrides are the classic examples of such three-centred (or multicentred) bonding, but examples are now known involving over half the elements of the periodic table – both metals and nonmetals – including carbon. The extraordinary diversity of the previously unimagined molecular shapes of such compounds and their unusual physical and chemical properties will be reviewed. To cite this article: N.N. Greenwood, C. R. Chimie 6 (2003) 000–000.     

Triazènes et benzotriazines dérivés de sucres

Sugar triazenes and benzotriazines Several triazenylsugars have been prepared, generally in good yields, by treating an amino sugar with a substituted benzenediazonium salt. The triazenylsugars bearing a hydrogen atom on the triazenyl group are acetylated on the nitrogen atom closest to the glycosyl moiety (N(1)), even when the proton exchanged sites between N(1) and N(3). When an acetyl or a methoxycarbonyl group was present in the ortho position of the benzene ring cyclization took place leading to a 3,4-dihydro-4-methylidene-1,2,3-benzo[d]triazine or a 3,4-dihydro-1,2,3-benzo[d]triazin-4-one respectively.     

Aminophosphines: Influence des Substituants lies au Phosphore sur la Barriere a la Rotation Autour de la liaison P?N et sur la Nucleophilie de L'atome D'azote

The ΔG* values for the barrier to rotation around the P? N bond have been determined in some aminophosphines and are dependent on the substituents bound to the phosphorus atom. The comparison between ³J(P? N? C? H) and ΔG* values indicates that p_π—d_π overlapping between nitrogen and phosphorus is the main effect. The nucleophilic behaviour of the nitrogen atom has been measured by the rate of the chemical exchange between methyl-trifluoroacetate and various aminophosphines: the results of this kinetic study are in good agreement with the NMR conclusions.