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.     

Heterocyclisation intramoleculaire d'alcools possedant un squelette cedranique. Oxydation par le tetraacetate de plomb et par l'oxyde de mercure et le brome

Oxidation with lead tetra-acetate of neoisocedranol yields the corresponding ether in good yield. Under the same conditions, cedrol remains unchanged; however, when cedrol is reacted with mercuric oxide and bromine, the corresponding ether is obtained as major product. The structure of the substrates is linked with the mechanism of the two reactions and with the nature of the intermediates in the heterocyclisation reaction. The difference in behaviour between the two substrates towards the two oxidising agents is rationalized.     

Etude des reactions de substitution homolytique sur le noyau pyridinique; influence de l'acidite du milieu

Homolytic substitution by the l-n-undecyl radical at positions 2 and 4 of the pyridine nucleus results from thermal decomposition of dodecanoyl peroxide in acetic acid. Rate dependence on pH shows that pyridine protonation increases the rate of addition of the alkyl radical to the pyridine ring but decreases the rate of the reaction of the intermediate radical with the peroxide. Results are interpreted in terms of orbital interaction theory.     

Exemple de reaction a l'interface solide-liquide: oxydation de naphtalenediols par le superoxyde de potassium

The oxidation of 1,2 and 1,3-dihydroxynaphtalenes with potassium superoxide in aprotic media is a heterogeneous to reaction at the solid-liquid interface, leading to the 2-hydroxy-1,4-naphtoquinone with good yields even if the addition of crown-ether is omitted.     

Action des phenylacetonitriles α-substitues sur les tetrahalomethanes en milieu basique: Mecanismes de l'halogenation et de la duplication

Carbanions of substituted phenylacetonitriles are halogenated by carbon tetrachloride or carbon tetrabromide, and also converted into symmetrically-substituted succinonitriles. The halogenation occurs via an ionic process, and the formation of succinonitriles via a radical process, essentially a no-chain mechanism in the case of diphenylacetonitrile anion, confirmed by the course of the reaction of p-nitrobenzylchloride on the diphenylacetonitrile anion.     

Remarques sur la cinétique de l'oxydation à l'air du monoxyde de titane

Thermogravimetric results on the oxidation of TiO by air obtained by one of the authors (M. J. Blais) and untimely exploited by Garnaud¹ are placed in bibliographical outline, described more in detail and carefully rediscussed.     

Decomposition d'hypobromites prepares par action de l'oxyde de mercure et du brome sur des alcools bicycliques pontes

Hydrobromites of bridged bicyclic alcohols have been decomposed to yield tetrahydrofuran-type ethers as well as fragmentation products. The latter results from the cleavage of the carbon-carbon bond α, β to the hydroxyl group. In the case of 3-hydroxy-3-methyl bicyclo[3.2.1]octane, conjugated ketones and brominated olefins are formed, whereas tetrahydrofuran-type ethers are formed only under different experimental conditions different from those applicable to general cases. The reaction mechanisms have been established. The influence of the various reaction parameters has been studied in order to obtain selectively either tetrahydrofuran-type ethers or a mixture of conjugated ketones and brominated olefins.     

Obtention de selenopysultones lors de l'oxydation de thio-1 chromenes par le dioxyde de selenium

Selenium dioxide oxidation of 1-thiochromene leads to 3-formyl (5,6)benzo-1,2 thiaseleninne-1,1-dioxide, which is a representative of a hitherto unknown class of heterocyclic compounds. Pyrolysis of this compound leads to 2-formyl benzo(b)selenophen, by loss of sulphur dioxide. The reaction can be applied to other 1 thiochromenes but no similar behaviour has been observed for their selenium isologues.     

Selectivite de la reduction de l'androsten-4 dione-3,17 et de la progesterone par divers borohydrures: role de l'assistance electrophile par le cation ou un solvant hydroxyle

The reduction of Δ⁴-androsten-3, 17 dione 1 and of progesterone 2 by nBu₄NBH₄ is highly chemioselective: in THF only the a-enone moiety is reduced, the saturated C₁₇ or C₂₀ keto group being kept unchanged. When TMEDA is added, saturated alcohols are obtained, without any allylic alcohol when the reaction goes to completion. However this reduction is poorly stereoselective as 70:30 mixtures of A/B cis and trans ring junction compounds are obtained. In MeOH, the saturated keto group is more than 90% selectively reduced. However, the reduction of 1 and 2 by LiBH₄ and Zn(BH₄)₂ is poorly chemioselective. These results are interpreted in terms of competition between electrophilic assistance and steric effects.     

Polymerisation cationique de l'indene amorcee par le tetrachlorure de titane-II Influence de la temperature sur la vitesse de polymerisation

Polymerization rates in the indene-titanium tetrachloride-methylene chloride system, using rigorously dried reagents, have been measured at various temperatures. At sufficiently high temperatures (above ?10°C), polymerization yields may be incomplete and cocatalytic phenomena have been observed; at low temperatures, yields are always complete. The reaction order in monomer is unity at ?70°C, which may be explained by a constant concentration of active centres during the major part of the reaction. At ?70°C, the reproducibility shows that residual water has no cocatalytic effect. The measured values of polymerization initial rates at various temperatures show that the apparent overall activation energy is negative. Various interpretations of this result are discussed, but it may be more easily reconciled with both the existence of a quasi-stationary state and an equilibrium between different types of active species.     

Reduction asymetrique de cetones par des alcoxyaluminohydrures chiraux prepares a partir de l'ephedrine et divers alcools ou phenols achiraux

The effect of the nature of the achiral component R′OH on the stereoselectivity of the asymmetric reduction of acetophenone by [LiAlH(OR′)₂(N-methylephedrine)] is examined. 3,5-Dimethylphenol as achiral component proved to be very suitable for making optically active secondary alcohols in high optical yield.     

Influence des substituants sur l'activite catalytique des composes arene-chrome tricarbonyle,mise en evidence de l'imfortance d'un α-heteroatome (O,N)

Substituents bearing a heteroatom in α position to an Cr(CO)₃-complexed arene group induce a strong perturbation in the six-electron ligand—metal bond allowing an increased catalytic effiency of these derivatives under mild conditions.     

Effets de complexation de l'ester lors de la catalyse par l'eau ou les sels de l'hydrolyse alcaline des esters maloniques: etude cinetique,spectroscop

When water or salts are added, catalytic effects on alkaline hydrolysis of malonic esters in methanol are observed; they are definitely greater than those observed with monoesters. Kinetic data in the presence of salts suggest that ester-M⁺ complexation phenomena is predominant and prevails over HO^-, M⁺ ionic association. Infrared and Raman spectra of esters in water-alcohol mixtures, with or without added salts, indicate the formation of 1:1 and 1:2 complexes with each carbonyl of the diester and one proton donor molecule as well as with Li⁺.Ab initio calculations of these diesters and of their complexes allow the estimation of the positive charge density at the carbonyl group carbon as well as the LUMO energy levels; they lead to an interpretation of these entities reactivities according to perturbation theory. Only orbital control of alkaline hydrolysis of esters gives an account of the relative reactivities of mono- and diesters with or without added salts. The only consideration of charges at the carbonyl carbon is, on the other hand, inadequate for justifying all the observed phenomena.