Influence de la longueur d'onde sur la photoisomerisation de derives de l'hexatriene-1,3,5

Kinetic studies of photoisomerization of 1-phenyl 6-methyl-cyclohexa-1,3-diene, and of the four isomeric 5-phenylhepta-1,3,5-trienes, have been performed at 313, 254, 228 and 214 nm. Photoreactions of these compounds are dependent on the wavelength of irradiation. Photolysis at short wavelength of Z,E-5-phenylhepta-1, 3,5-triene initiates a new reaction, sigmatropic [1,5] hydrogen migration, forming an ene-allenic compound. Thermal reactivity data for polyenic compounds supports the assigned structures.     

Syntheses d'oxazolidines substituees par l'adamantane et de nitroxydes oxazolidines stables derives de l'adamantane

Adamantane oxazolidine derivatives have been prepared. The corresponding mono and biradicals have molecular shapes well suited to the study of the motion anisotropy in solution.     

Processus concurrents de photoisomerisation d'hexatrienes polymethyles: effet de longueur d'one

The ground-state conformer composition is shown to play a determining role in the orientation of the photochemical reactions of two hexatrienes. A pronounced wavelength effect is observed for Z-2,6-dimethyl-1,3,5-heptatriene; photocycloaddition leads to a bicyclo[3.1.0]hexene and [1,5] sigmatropic hydrogen migration occurs from an s-cis-s-trans conformation preferentially excited by longer wavelength light, whereas the Z-E interconversion is predominantly observed as a result of excitation of the s-trans-s-trans conformation by shorter wavelength light.     

Criteres d'evaluation de la position de l'etat de transition dans les additions conjuguees

The conjugate addition of substituted aryacetonitrile anions to α-enones implies a reagent-like transition state; it is insensitive to conjugation loss but slightly sensitive to steric factors.     

Influence de la solvatation par le tert-butanol sur la structure et la reactivite de l'enolate de potassium de l'acetylacetate d'ethyle en presence de couronne ou de cryptand. etude par spectrometrie infra-rouge et cinetique

The structure and the nucleophilic reactivity of crowned (18-crown-6) or cryptated {cryptand (2.2.2)} potassium ethyl acetoacetate enolate have been compared in tert-butanol and in DME (or THF). In the protic solvent tert-butanol, the crowned and the cryptated potassium enolate species both exist as loose ion pairs in which the enolate anion, strongly hydrogen-bonded to the solvent, is in a “transoid” (non chelating) conformation. Both species show similar reactivities towards alkylating agents but completely different reactivities are observed in aprotic weakly dissociating media (THF, DME). In contrast to what is observed in tert-butanol, the cryptated species and the crowned species have very different nucleophilic reactivities in THF or DME; in those solvents only the cryptated species retains a loose ion pair structure; the crowned species is a contact ion pair in which the enolate anion chelates the potassium cation. The solvation of this crowned chelate species by tert-butanol has been demonstrated in binary mixtures of solvents (C₆D₆-t-BuOH, THF-t-BuOH). The oxygen basicity of the enolate anion is very different in the crowned chelated ion pair compared with the cryptand separated ion pair.     

Sinthese de copolymeres sequences de l'acetate de vinyle et de l'acrylate d'ethyle par telomerisation

A new two-step procedure for the synthesis of ethyl acrylate - vinyl acetate block copolymers is described : the first stage is the telomerisation of ethyl acrylate with carbon tetrachloride by redox catalysis, the second being another telomerisation of vinyl acetate, using the trichloromethyl end group of the first block as the telogen agent, with peroxydic initiation.     

Synthese stereo et enantioselective par l'intermediare de complexes du fer de derives chrysanthemiques dieniques et d'aldehydes hemicaroniques cis et trans.

A stereo and enantioselective synthesis of cis and trans hemicaronic aldehydes, starting from the optically active sorbic aldehyde-irontricarbonyl comples, is described.     

Transposition des oxirannes-ethanols par l'intermediaire d'alcoxyetains. Influence de la configuration de l'oxiranne

The transposition of oxirane-ethanols, through alkoxytin compounds, into oxetane-2-methanols and/or oxolan-3-ols (tetrahydrofuran-3-ols) is dependent upon the oxirane configuration. Cis configuration is more suitable for the formation of the smallest ring. Steric hindrance is not sufficient enough to explain the results.     

Etude de la cristallinite de derives n-octadecyliques de l'alcool polyvinylique obtenus par greffage chimique

Differential scanning calorimetry has been used to study the heats of fusion and melting transitions of n-octadecyl derivatives of polyvinyl alcohol. These derivatives were obtained by grafting paraffinic chains on polyvinyl alcohol, at various proportions. The melting transitions are independent of the degree of substitution; the heats of fusion are proportional to the fraction of crystallizable units. Using Flory's equilibrium crystallization theory, it is possible to interpret these results assuming, as demonstrated previously, that the chemical grafting is sequential with two unreacted hydroxyl groups between two adjacent paraffinic side-chains.     

Effet α. influence de la substitution sur l'exaltation de reactivite d'oximes vis-a-vis de l'acetate de p-nitrophenyle

Kinetic measurements show the relation between the magnitude of the “α effect” exhibited by trifluoromethyloximate anions RR'C=N-O^? and the electron attracting properties of the substituents. Indeed, the reactivity enhancement which is observed for α-trifluoromethyloximes is similar to that found in earlier studies of α-hydroxyiminoketones. This characteristic behaviour is no longer observed for oximes bearing an electron releasing substituent at the α-position of the hydroxyimino group. On the basis of sodium oximate molecular orbital energies the “α effect” induced by electron attracting substituents appears to depend on both the interactions of nitrogen and oxygen lone pairs of electrons and the lowering of the π^* vacant orbital thus favouring a secondary interaction between the π^* anion orbital and the π orbital of p-nitrophenylacetate carbonyl group.     

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.