Interpretation theorique de la reaction de diels-alder dans la serie des dienes fonctionnels dissymetriquement substitues-I : Structure electronique et reactivite

The chemical reactivity of two series of butadienes in Diels-Alder reactions is given. It shows a substitution dissymmetry dependent on the substitution sites (1 or 2) of the functional group (ethoxy) on the butadiene skeleton. The differences in the behaviour of the two series are shown by means of semi-empirical correlations between Log K and theoretical data derived from CNDO/S calculations within the frame of perturbation theory. Furthermore, the existence of an excellent correlation between Log K and electronic affinity for a series of ethylene dienophiles reacting with the ethoxy-1 butadiene is demonstrated.     

Interpretation theorique de la reaction de diels-alder dans la serie des dienes fonctionnels dissymetriquement substitues-II : Etude des mecanismes de reaction par calcul mindo des etats de transition

The different behaviour of two 1- or 2-ethoxy butadiene isomers with respect to acrolein in the Diels-Alder reaction is explained on the basis of reaction mechanisms. Using MINDO/2 calculations, various models of the transition state are investigated and geometric parameters for each reaction path coordinate are optimized. The choice of a model describing the transition state is given by the corresponding potential barrier height, which characterizes the approach of reagents and the site of the functional of the diene. Its lowest value corresponds to a concerted reaction mechanism with acrolein; a synchronous mechanism is found for 2-ethoxy butadiene, and a two-stage one for 1-ethoxy butadiene. The variation in barrier height in the two systems is well correlated with the corresponding change in the observed reaction rate.     

Relation entre la solvatation preferentielle et la densite en segments de polymeres: Generalisations et limitations

This paper presents results on viscosity and preferential solvation for the ternary systems: polystyrene (PS)-carbon tetrachloride-methanol, PS-benzene-cyclohexane, PS-chloroform-ethanol and poly(2 vinylpyridine)-ethanol-cyclohexane using linear and branched polymers. For a given ternary system the representation Δλ′ (variation of the preferential solvation coefficient) as a function of Δρ (variation of the segment density) already proposed gives a unique curve. This shows that this relation is not sensitive to branching or to molecular weight of the polymer when the dimensions of the polymer in the mixed solvent are lower than in the pure good solvent. However, when the dimensions of the polymer in the mixed solvent are higher than in the pure good solvent, the relation between Δλ′ and Δρ cannot be applied.     

Accroissement de la reactivite des phosphorines en tant que dienes et philodienes par complexation du phosphore

The 2-Phenyl-4,5-dimethylphosphorin P-W(CO)₅ complex reacts easily as a dienophile with 2,3-dimethylbutadiene through its 1,6-positions and as a diene with N-phenylmaleimide, dimethyl acetylenedicarboxylate and cyclopentadiene, through its 1,4-positions.     

Telomerisation de l'isoprene en catalyse redox avec CCl4. Determination de la structure des telomeres etude des parametres de la reaction

There has been a study of the redox telomerization of isoprene using transition metal salts, particularly the influence of reaction parameters on the nature, yield and structure of the telomers.     

Epoxydation en milieu heterogene solide-liquide faiblement hydrate : etude de la reaction autour de la structure du sel de sulfonium

Epoxide formation by sulphur y1id insertion into aldehydes under heterogeneous conditions in the presence of small quantities of water is described. Trimethylsulphonium and S-methylthiolanium bromide and iodide have been studied. For all alkyl substituents the bromide gives better results than the iodide, this is interpreted in terms of the reaction mechanism. Under optimum conditions high yields of epoxides are obtained.     

Une etude de la conjugaison entre un systeme aromatique et un cycle a trois chainons

Electron delocalization between a three membered ring (aziridinyl, cyclopropyl, oxiranyl) and an aromatic group was revealed by magnetic rotatory polarization. This conjugation varies according to the relative conformations of these two groups.     

Applications de la sonde moléculaire aux composites a teneur elevee en solides—I. Determination des temperatures de transition vitreuse et etude des phenomenes de surface

The molecular probe technique has been shown to be useful for measuring the glass transition temperature of a highly-filled composite, or of a polybutadiene prepolymer covering surfaces of varying types. The measured Tg of 193°K is independent of the curing stage as well as of the nature of functional groups. Transitions between 349 and 364°K were detected in a Fe₂O₃-containing composite, showing the general applicability of the technique. Studies of the slope of the curve relating the logarithm of the specific retention volume to the reciprocal of the absolute temperature can lead to useful information concerning the orientation of the polybutadiene prepolymer at the polymer-solid interface.     

Etude de la Fragmentation en Spectrographie de Masse de Spiropyrannes Benzodithioliques. Influence de la Position et de la Nature des Substituants

The mass spectra of 12 benzodithiolic spiropyrans are reported. Their fragmentation pathways have been determined by exact mass and metastable transition measurements. The nature of the fragmentation is strongly dependent upon the position and nature of the substituents. A comparison is made with their photo and thermochemical decompositions.     

Pyrolyse eclair de polyisoprenes. Essai de correlation entre les produits de degradation et la microstructure des polymeres

Flash pyrolysis between 500 and 600° of polyisoprenes with the three types of unit (1,4, 3,4 and 1,2) essentially yields isoprene and two cyclic dimers, viz. dipentene and 3,4-dimethyl, 4-vinyl, cyclohexene. These dimers are characteristic of 1,4 and 3,4 units respectively. The yield of dipentene is maximum when the 1,4 units are contained in long blocks; it is formed preferentially by cyclization of the biradical formed from two adjacent 1,4 units. The yield of the other dimer is maximum when the chain contains isolated 3,4 units; it is formed preferentially by Diels-Alder condensation between free isoprene and the pendant isopropenyl group of a 3,4 unit following chain scission. The 1,2 units thermally depolymerize to isoprene. Polyisoprenes made with alkaline earth metals are block copolymers of 1,4 and 3,4 units; polymers made with Ziegler-Natta catalysts have a random microstructure.     

Utilisation de l'ultracentrifugeuse pour la mesure des masses et pour l'etude de la polydispersite des polymeres de synthese

After a review of the two techniques based on sedimentation rate and equilibrium in binary medium, more recent methods involving the presence of a second non-macromolecular solute are described. Application of these methods to synthetic polymers is still rare but of increasing interest. The use of computers simplifies the exploitation of the measurements.     

Stereochimie de la deprotonation et de la reaction d'aldolisation des dithioesters

Cis lithium thioenolates are preferentially formed by deprotonation of dithiopropanoates with lithium diisopropylamide in tetrahydro-furan at -78°C. The cis selectivity observed, is unproved by increasing the alkylthio group size or by the ability of this group to chelate the lithium cation. When more bulkier bases such as lithium 2,2,6,6-tetramethylpiperi-dide or lithium hexamethyldisilazane are used, the selectivity is lowered. This lowering of selectivity is suppressed when the deprotonation is performed in presence of 12-crown-4. Addition of hexamethyl phosphoramide to the base does not invert the selectivity as it uas reported for the deprotonation of esters ; a rather better cis selectivity is reached in the case of methoxymethyl dithiopropanoate. These original results are well understood in terms of an open transition state model. Preformed lithium thioenolates are reacted uith a variety of aldehydes and afford stereo-specifically syn aldols. The influence of hexamethylphosphoramide and reaction time is also examined.     

Relations entre la structure et les proprietes des membranes de polytetrafluoroethylene greffe—III. Membranes non-ionisables contenant de la poly-n-vinylpyrrolidone

Equilibrium properties (swelling) and transport phenomena (dialysis) of non-ionizable membranes, obtained by radiation grafting of N-vinylpyrrolidone onto thin PTFE films, were studied. Grafting was conducted by the direct method with monomer solutions in benzene. The overall activation energy of grafting determined between 20 and 50° is high (12.5 kcal mol^?1): the reaction is controlled by the monomer diffusion into PTFE film. The length of grafted chains increases with temperature.The molality of the membranes decreases when the temperature of grafting increases; for a given grafting ratio, the swelling is higher for membranes containing a small number of long grafts than for membranes with numerous short branches. The transfer of both ions and water is a function of the free volume in the films, it depends on the grafting ratio and is not influenced by the preparation conditions.     

Etude de la chloration du PVC en masse—II. Influence et evolution de la stereoregularite de la chaine en rapport avec les mecanismes de reaction

Bulk chlorination of PVC has been studied by i.r. and NMR. The evolution of chain stereoregularity during reaction and its significance to reaction mechanism have been specified. In particular, for temperatures <80°, the ratio Cl/C cannot exceed 0.75 because changes of conformation are impossible. To exceed this ratio, additional energy is necessary but then there is a parallel loss of hydrogen chloride. In all cases isotactic diads of TG conformation would be first chlorinated. At temperature above 100°, the microstructure of bulk chlorinated products is similar to that of products prepared in solution while their configuration and conformation are different.     

Etude comparative de la reactivite des α-aminonitriles secondaires et tertiaires en solution aqueuse entre pH 10 et 14. Hydrolyse des α-aminonitriles secondaires et son importance dans la formation prebiotique des acides amines naturels

A kinetic study of the hydrolysis of secondary α-aminonitrile has been carried out. The results compared to those previously obtained for the decomposition reaction of tertiary α-aminonitriles, show the importance of solvation at the reactivity level of each compound.These results confirm that α-aminonitrile can act as prebiotic precursors of natural α-amino-acids.     

Contribution a l'etude des metallocenes : XXVIII. Reduction de cetones cycliques en series du ferrocene et du cymantrene. Etude de la stereospecificite. Assignation des configurations endo ou exo des alcools derives

The reduction of various 2,3-ferroceno-1-indanones and 1,2-(α-oxotetramethylene)-methylcymantrenes by metal hydrides or Grignard reagents is stereo-specific, except for LiAlH₄, the observed stereoselectivity being in all uses more than 95%. The endo configuration of the dominant alcohol is established from TLC, IR, RMN and mass data. These criteria for the assignment of the structure of these alcohols are shown to be general.