Mécanisme de l'hydrolyse acide des carbamates

N, N-dialkyl carbamates decompose in strongly acidic media to carbon dioxide, olefin, alkyl halide and alcohol, the rate of reaction of the secondary esters closely following the acidity function. This fact, together with the variation in rate of hydrolysis of carbamates of cyclic alcohols with the ring size, shows that, unlike the solvolyses of the corresponding chloroformates which lead to carboxonium ions (\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm R - O -}\mathop {\rm C}\limits^{{\rm (+)}} = {\rm O} $\end{document}), these reactions involve the intermediate formation of carbonium ions.     

Influence de la complexation sur le transfert des cations en osmose inverse

The influence of complexation by different ligands on the transfer of metallic cations in reverse osmosis has been studied by using membranes composed of aromatic polyamides. In spite of the large volume increase of the diffusing solute, the addition of a complexant can significantly improve the transfer of metallic cations through a reverse osmosis membrane. The most important factor in the improvement of the transfer is the solubility in the membrane of the complexes created. This solubility is linked to the possibility of creating hydrogen bonds between the membrane and the complex. Moreover, the addition of substances which strongly solvate both the membrane and the solute leads to an improvement of transfer which increases considerably with the increasing pressure. These results suggest the possibility of separating metal cations by specific complexation.     

Sur le mécanisme de la réaction de meth-cohn-tarnowski de préparation des thiocoumarines

Reaction of 2-t-butylthiobenzaldehyde with N-(2-dialkylaminoethyl)-cyanacetamides followed by heating of the resulting 3-(2-t-butylthiophenyl)-2-cyanoacrylamides in polyphosphoric acid leads to the formation of 3-carboxamido-2-imino-2H-benzo[b][l]thiopyrann intermediates which afford thiocoumarins on hydrolysis. The authors postulate the intermediate formation of a thiophenate ion by an electronic mechanism involving six centers.     

L'action de l'ammoniac sur l'hydroxyde de cadmium et la stabilité des complexes en milieu aqueux

The solubility of precipitated Cd(OH)₂ was determined at 25°C in 1 M NaClO₄, as a function of pH and of the ammonia content of the solutions. Formation constants were obtained for the following hydroxo, ammine and hydroxo-ammine complexes: CdOH⁺, Cd(OH)₂, Cd(OH), CdNH, Cd(NH₃), Cd(NH₃), Cd(NH₃) and Cd(OH)₂NH₃. The solubility product of the hydroxide was also calculated. The presence of polynuclear species was investigated by titrimetric determinations of the hydrogen ion concentration at constant metal concentration.     

Note sur le mécanisme réactionnel de la transposition de Dimroth. Une étude ab initio

An ab initio study of the Dimorth rerarrangement of the -amino-1,2,3,4-tetrazole leads to the conclusion teht in vapour phase the rate determining step is not the ring-chain isomerism, but is either the Z-E iomerism around the C?N double bond or the 1,3-sigmatropic shift of the proton.     

Réarrangements en milieu acide trifluoroacétique de deux alcaloïdes indoliques: la desformocorymine et la dihydrocorymine

The rearrangement in trifluoroacetic acid of two indole alkaloids of the echitamine series, desformocorymine (14) and dihydrocorymine (9) , has been investigated. Desformocorymine (14) was tranformed into a mixture of carbinolamines 17a , b , with the akuammiline skeleton, which were reduced (Et₃SiH, CF₃CO₂H) into an isomer 12 of cathafoline (6). This sequence constitutes the first example of an interconversion of the corymine skeleton into the akuammiline skeleton (Scheme 2). In the case of dihydrocorymine (9) , the rearrangement followed a different pathway owing to the formation of a hemiacetal between the primary alcohol CH₂(17)-OH and a carbonyl formed at C(3). Treatment of this hemiacetal 26 with aqueous base led to its opening with concomitant formation of a lactam. ¹³C-NMR seems to indicate that this lactam exists under a hydrated form 27. This highly unstable intermediate was cleanly transformed (MeONa-MeOH) into a 2-acyl indole 30 (Scheme 4), the structure of which was determined by X-ray crystallography. The formation of this acylindole involves the rupture of the C(7)? C(16) bond; it is the reverse of the reaction generally postulated as occurring in the biogenesis of the pentacyclic alkaloids. The structure of a by-product 34 was established as 17-hydroxymethylvincoridine by X-ray crystallography. The acid-catalyzed rearrangements involve the rupture of the Ph-N? C? N chromophore, with formation of a carbonyl at C(3). The reversibility of these steps is used in an easy correlation of dihydrocorymine and of 3-epidihydrocorymine via their trifluoroacetates.     

Substitution électrophile aromatique dans l'anhydride sulfureux liquide. Influence de la concentration en bromure et rôle du solvant sur la réactivité des anisoles au cours de la réaction de bromation

Electrophilic Aromatic Substitution in Liquid Sulfur Dioxide. Kinetic Dependance of Rate on the Bromide Concentration and Influence of the Solvent during the Course of the Reaction On the reported data for bromination of anisole and eleven of its derivatives in liquid SO₂, it was shown that, with a large excess of bromide, the rate of reaction, obeys a first-order law. Rate constants thus obtained do not discriminate between the two different forms of bromide, e.g. Br₂ and Br^?₃ present as the A⁺Br^?₃ form, and corrections were made by use of the apparent equilibrium constant K′ for tribromide formation. The variations of rate constants with initial concentration of bromide has been studied and the effect results in a retardation of the bromination rate. Moreover, the ratio [Br₂] [A⁺Br^?]_T, which is constant during an experiment, varies with initial bromide concentrations, this variation affecting the total rate. To account for the bromide effect on the reactivity, variations of k^o,p_g {1 + K′[A⁺Br^?]_T}VS[A⁺Br^?]_T were studied over a 0.01 to 1M range of bromide concentration. The mechanism proposed shows that liquid SO₂ helps the reactive intermediate to be deprotonated and because of solvation of reactive species this step would probably be rate determining. Bromination by molecular bromine is more sensitive to substituent effects in liquid SO₂ than in water. This result is ascribed to the +M effect of the methoxy group which increase the conjugation of ortho-substituted derivatives (p⁺_p = ?7.83; p⁺_o= ?10.47).     

Précisions apportées au mécanisme de la polymérisation du chlorure de vinyle par le chlorure de tertiobutyle-magnésium en milieu tétrahydrofurannique. Etude par résonance magnétique nucléaire

NMR analysis of the low molecular weight poly(vinyl chloride) obtained from polymerizations initiated by t-BuMgCl in THF solution suggests that all the polymer chains have the structure: Enhanced signal/noise ratio by NMR spectrum accumulation confirms this structure in products of higher molecular weights. Examination of the NMR spectrum of vinyl chloride monomer in THF solution reveals the formation of a monomer–THF complex; it seems that the initiation step of the polymerization is governed by this complexation phenomenon.     

Action de l'anhydride acétique sur les (aziridinyl-1)-2 tétra-hydropyrannes et sur les dialkylamino-2 tétrahydropyrannes: Influence des substituants de l'azote sur l'orientation de la réaction

The 2-(1-aziridinyl)tetrahydropyrans cannot be obtained by aminolysis of the 2-dialkylaminotetrahydropyrans. These compounds are synthesized by the Hofmann reaction between 2-chlorotetrahydropyrans and ethylenimine. The effect of acetic anhydride on the 2-(1-aziridinyl) and 2-dialkylaminotetrahydropyrans was studied. The 2-dimethylaminotetrahydropyrans were converted into 2-acetoxytetrahydropyrans and dimethylacetamide by cleavage of the carbon-nitrogen bond. Ring opening of the oxygen heterocycle gives the corresponding dialkylacetamides and polymerised products. The 2-(1-aziridinyl)tetrahydropyrans allow the synthesis of the N-(2-tetrahydropyrannyl)-N-(2-acetoxy-ethyl)acetamides. Acetic anhydride ring opens the nitrogen heterocycle.     

Résines époxydes—2 Mécanisme réactionnel et cinétique de la réaction du diglycidyl amino-1-méthyl-2-benzène sur les amines aromatiques

The study of the polyaddition of diglycidyl-1-amino-2-methyl benzene (commercial grade and purified in the presence of primary, secondary and tertiary aminobenzenes) by HPLC, chemical measurements, high resolution NMR in the solid (CP MAS) and liquid (¹³C and ¹⁵N) states has allowed the reactivity of the N(epoxy)-2 group to be determined and a reaction mechanism to be proposed. Comparison of results obtained for the commercial grade and purified resins shows the role of impurities on reaction kinetics and on the structure of the final polymer network.     

Etude du mécanisme de la réaction de hantzsch des thiazoles. I. Mise en évidence et détermination des produits intermédiaires et de dégradation

The Hantzsch thiazole synthesis from α-haloketones and thioamides, proceeds via intermediates that can be isolated under certain conditions. These have been identified, mainly by nmr spectroscopy, as thioimidate derivatives. The chain or cyclic structures of these compounds depend upon the nature of the substituents. Their dehydration gives together with the expected thiazole, secondary products which have also been isolated and identified as α-mercaptoketones and α-ketothiolesters.     

Influence de la temperature sur les associations moleculaires et sur la polymerisation de l'acide acrylique en solution

Infra-red spectra of pure acrylic acid and solutions of monomer in toluene indicate that increasing the temperature from 20 to 60° displaces the equilibrium of H-bonded species from cyclic dimer to open oligomer. Viscosity measurements on the same solutions confirm this conclusion. In methanol solutions. alcohol-alcohol and alcohol-acid associations are found together with the acid-acid associations and the interpretation of the results is extremely difficult. Polymerization kinetics are directly influenced by the shift of association equilibria. In hexane, an increase in temperature results in a sudden increase in reaction rate as the linear oligomeric structures appear in the solution. In methanol increase in temperature reduces the kinetic anomalies observed at 20 and the polymerization no longer exhibits any stereospecificity. At the same time, a chain transfer process sets in. These various results confirm the earlier assumption of an assisted propagation step occurring in auto-associated aggregates of acrylic acid.     

L'action de l'ammoniac sur l'hydroxyde de cobalt(II) et la stabilité des complexes en milieu aqueux

From the solubility of precipitated Co(OH)₂ (s) determined radiometrically as a function of pH and ammonia content of the heterogeneous systems, the formation constants have been obtained for the following mononuclear hydroxo-, ammine- and mixed hydroxo-ammine-complexes: Co(OH)₂, Co(OH)₃^?, Co(NH₃)₂²⁺, Co(NH₃)₃²⁺, Co(NH₃)₄²⁺ and Co(OH)₂ (NH₃)₂. The solubility of cobalt(II) hydroxide has also been calculated. The medium was 1M NaClO₄ and the temperature 25° C.     

Chloration et S-oxydation du benzo[b]thiophène par l'hypochlorite de tertiobutyle. Mécanisme de la réaction

In the reaction with benzothiophene, t-butyl hypochlorite acts simultaneously as a smooth oxidizing agent and as a chlorinating agent since it is a source of both hypochlorous acid and chlorine. The mixture of all the products obtained imply that the reaction intermediate is the unstable benzothiophene I-oxide which was shown to be present by the stop flow technique. The chlorine formed traps the highly reactive benzothiophene 1-oxide in situ to give the observed products.     

Recherches sur la formation et la transformation des esters LXXVIII. Sur la réaction de l'isothiocyanate d'o-méthoxycarbonyl-phényle avec des diamines et l'hydrazine,et sur la cyclisation de quelques N-phényl-N′-hydroxyalcoyl-thio-urées

1-Amino-2-dimethylamino-ethane and 1-amino-3-dimethylamino-propane react with o-methoxycarbonylphenyl isothiocyanate (I) to yield the corresponding 3-(ω-dimethylaminoalkyl)-2-thiono-1,2,3,4-tetrahydroquinazolin-4-ones (V). The tertiary amino group of V is quaternized to the corresponding ammonio derivative VI when treated with methyl iodide in methanol.