Etude conformationnelle par RMN de phosphines ethyleniques cis et trans: Influence de l'orientation du doublet libre du phosphore sur le couplage 2JP-H transmis a travers un carbone sp

NMR spectra of variously substituted cis and trans ethylene phosphines have been taken at different temperatures. When a bulky group is bound to the double bond, the cis phosphines adopt a preferred conformation, but a conformational equilibrium applies to some trans phosphines.The calculated values of ²P_P-H for the different conformers are dependent on the phosphorus lone pair orientation: ²J_P-H is large and positive when the CH bond and the lone pair direction are parallel; it is small and may become negative when these directions are antiparallel.     

Hydroboration d'amines insaturees—II: Régio et stéréosélectivité de l'hydroboration-alkylation d'amines acétyléniques

The hydroboration reaction of acetylenic amines R¹R²NCH(R³)CCR⁴ was studied. We report the first results of a study of the reactivity of dialkylborane R₂BH towards these amines which allow us to propose a new method for the synthesis of differently substituted β-ethylenic amines. The regioselectivity and the stereoselectivity of this reaction are examined and allow us to set out the possibility of a trans hydroboration.     

Alcaloides steroidiques—CLXIV : Isomerisation en presence d'une base ou d'un acide d'oxazirannes steroidiques derives de la conanine

Isomerisation into an imino-carbinol or a nitrone is the first step in the hydrolysis of an oxazirane. This isomerisation, studied on three steroidal compounds lead to the following results: compound 6, bearing cis-oriented 18H and oxazirane ring, gives rise only to the nitrone 13. Compound 1 which bears trans 18H may isomerise either into the imino-carbinol 5 or into the nitrone 11 (11 is quantitatively obtained in acidic conditions with minimum basic catalysis). The case of compound 7 which isomerises whatever the conditions into the α hydroxylated imino-ether 8 shows the influence of structural factors which enhance the mobility of the 18H.     

Reactivite d'hydrocarbures methylenecyclo-hexaniques—I : Stereochimie de la reaction d'oxymercuration-demercuration

Methylenecyclohexanes, prepared in excellent yield by Corey and Chaykovsky then Cornforth reactions, give on oxymercuration followed by reductive demercuration tertiary alcohols resulting from axial attack by solvent. The attack takes place trans to the substiuent when the latter is at the 2 position. α-Hydroxy or methoxy compounds behave similarly to each other. For acetoxy compounds, the cis alcohols are obtained preferentially, by intramolecular attack of acetoxy group on the mercurinium ion intermediate. The results can be explained satisfactorily on the assumption that the factors inducing stereochemistry are short distance interactions.     

Stereochimie de la stannylation d'halogenures allyliques en serie cyclohexenique

cis- and trans-5-Methyl-2-cyclohexenyl chlorides were matallated by trimethylstannyllithium and tributylstannyllithium giving the corresponding allyltin compunds with inversion of configuration as shown by ¹³C and ¹¹⁹Sn NMR spectroscopy.     

Transpositions spinales in vitro en milieu acide—VII : Isomerisation d'hydroxy-17 steroïdes ethyleniques: possibilite d'un mecanisme concerte au niveau des cycles c et d

Synthesis and rearrangement of ethylenic 17-hydroxy steroids, with a cis or trans configuration of the 18-methyl and 17-hydroxy groups, is described. The cis compounds lead, in acidic media, to backbone-rearranged steroids, whereas the trans compounds lead to C-nor spiroketones. A concerted mechanism is postulated for the rearrangement of the C and D rings, in every case different from the one occurring for the transposition of ABC rings.     

Stereochimie et mecanisme de l'ouverture par les amines d'ions N-alcoxypyridinium : Formation competitive d'un ylure endocyclique; application a une nouvelle conversion heterocyclique

The N-alkoxypyridinium salt 1, with pyrrolidine, piperidine and diethylamine, undergoes reversible ring opening in acetonitrile. This competes with irreversible cyclization of an ylide to the bicyclic intermediate 3 which suffers subsequent ring opening to 3-ω-aminobutadienylisoxazol-4 ones in methanol. The different courses of the reaction is tentantively explained by means of HSAB theory. Stereochemistry of the primary open products is consistent with a disrotatory ring opening of cis-dihydropyridine adducts resulting from amine addition to the salt 1 or to the bicyclic ion 3. These primary products are further isomerized and all-trans structures have been isolated in the case of isoxazoline derivatives. Conversion of an N-alkoxypyridinium salt to substituted butadienylisoxazolones exemplifies a new type of heterocyclic ring transformation that can be described by the PARC-ANRO sequence: Proton Abstraction, Ring Closure—Addition of a Nucleophile, and Ring Opening.     

Etude du comportement photochimique de complexes {PtCl2(oléfine)(amine)}, influence de la nature de l'olefine et de la configuration du complexe

Irradiation of complexes {PtCl₂(olefine)(amine)} leads to the departure of the olefinic ligand forming a dimeric compound {PtCl₂(amine)}₂ which has a trans configuration. The same behaviour is observed at different irradiation wavelengths, with variation of the nature of the coordinated olefin and with a cis or trans configuration of the starting compound. Experimental results show that the primary photochemical reaction pathway arises probably via a (d → π^★ C₂H₄) charge transfer existed state. A simultaneous cis → trans photoisomerization is observed in the case of the cis complexes.     

Reaction de metathese des olefines induite photochimiquement en presence d'un complexe de metal de transition: III. Caracterisation generale de la transformation

Activated sodium hydride (NaHRONa) was treated with Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Zr, Mo, Pd, W salts to give the new complex reducing agents NaHRONaMX_n (CRA). Their reactivities were examined for reduction of 1-bromonaphthalene. In most cases, the CRA exhibited very good reducing abilities, which can be varied by changing the solvent, the alkoxide, or the temperature. Moreover, some CRA act as coupling agents for aromatic halides. Easy to prepare and handle on a preparative scale, CRA must be regarded as very promising tools for organic synthesis.     

Detection d'une activite optique au niveau d'un atome de vanadium a environnement pseudo-teraerique chiral

Dissymmetrical vanadocenes CpCp′V are prepared by reduction of the corresponding dichlorides CnCp′VCl₂ (Cp = C₅H₅ ; (Cp′ = C₅H₄R). Like vanadocene they display a carbeno? behavior and can be coordinated to CS₂ and acetylenic compounds. The addition of dissymmetrical acetylenes involve the formation of a chiral center at vanadium. This chirality is detected by partial chromatographic separation of diastereoisomers CpCp″V* (RCCR*) which bear a second chiral center on acetylenic ligand.     

Cis-homo addition-1,4 thiophile d'organomagnesiens avec des composes thiocarbonyles comportant un groupe carbonyle en β: Synthese stereoselective d'alcoylthio-2 et de bis (alcoylthio)-2,2 cyclopropanols

Aliphatic Grignard reagents react stereoselectively with β thioxo carbonyl compounds to give substituted cis 2-alkylthio or 2,2-bis (alkylthio) cyclopropanols with good yields. Two types of non-enethiolisable thiocarbonyl compounds of formulae MeCOC(Me)₂CSR undergo this reaction a thioketone (R = Me) and a number of dithioesters (R = SMe, SEt, S iso Pr). The cis configuration of cyclopropanols has been assigned by ¹H NMR-Eu(fod)₃. The cyclopropane ring closure is a concerted cis-1,4-homo addition.     

Etude du mecanisme de photolyse des complexes cis- et trans-[PtCl2(C2H4)(4-CH3C5H4N)]. Influence de la concentration et de la longueur d'onde

Photodimerization, photoisomerization and photosubstitution quantum yields are measured for cis- and trans-[PtCl₂(C₂H₄)(4-CH₃C₅H₄N)], at various concentrations and wavelengths. Dissociation of the platinumethylene bond o?curs with a quantum yield nearly unity when the cis-compléx is irradiated in the charge transfer bands 5d → π^*(C₂H₄). Dissociation is also observed, but with a lower efficiency, at longer wavelengths. A cis → trans-photoisomerization reaction, probably via a low energy dd excited state is observed at 313,366 and 405 nm, with a constant quantum yield.     

Dosage du platine par spectrométrie d'absorption atomique: Partie II. Résultats analytiques sur les complexes or-ganométalliques d'intérêt biologique: effet cis-trans

An atomic-absorption spectrometric method is reported for the determination of platinum in a series of platinum complexes, with and without the use of spectrochemical buffers. The use of potassium phosphate and lanthanum chloride as buffer eliminated interference effects. The results were reproducible and accurate Platinum phosphine complexes were easily analysed. The cis and trans PtL₂L^'₂ (L,L^' are ligands) complexes soluble in water gave significant differences in absorbance in the absence of spectrochemical buffers, so that absorbance measurements differentiated cis- and trans-compounds in a given series. The cis-trans effect was also detected in organic solvents.     

Rearrangements des halohydrines en presence de carbonate d'argent sur celite-II: Halohydrines cyclohexaniques substituees

Silver carbonate on Celite reacts with trans-diaxial halohydrins of the cyclohexane series, giving epoxides. cis-Bromohydrins, where the bromine atom is axial, lead to ketones.     

Strukturuntersuchungen an mono- und bis-alkoxy(triphenylsilyl)-carben-komplexen des rheniums

Steric and electronic influences on bond lengths and angles at the carbene carbon atoms of cis-Re₂(CO)₉C(OR)SiPh₃ (I: R = CH₃, II, R = C₂H₅) and cis,trans-Re₂(CO)₈[C(OEt)SiPh₃]₂ (III) are discussed based on their structural analyses. I (ReRe 305.2(1) pm; ReC(carbene) 209(2) pm) and II (two independent molecules; ReRe 305.0(3) and 305.2(4) pm; ReC (carbene) 208(5) and 210(5) pm) differ by the cis and trans positions of the alkyl groups at the partial C(carbene)O double bonds. The change in configuration affects the bond angles at the carbene carbon. In III the carbene ligands are bonded to different rhenium atoms; cis to one Re atom and trans to the other Re atom (ReRe bond 309.1(2) pm). The ReC(carbene length of the trans- carbene ligand is significantly shorter (185(3) pm) than that of the cis-carbene ligand (208(3) pm).     

Etude de la complexation des lanthanides trivalents par les isomères de l'acide diaminocyclohexanetétraacétique: Partie 2. Les Constantes d'Acidité et les Constantes de Formation des Complexes 1:1 avec l'Acide trans-1,4-Diaminocyclohexane-N,N,N′,N′-Tétraacétique

(Study of the complexation of trivalent lanthanides by the six isomers of diaminocyclohexanetetraacetic acid. Part 2. Acidity constants and formation constants of the 1:1 complexes of trans-1,4-diaminocyclohexane-N,N,N′,N′-tetraacetic acid)Potentiometric measurements of the acidity constants of trans-1,4-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (trans-1,4-DCTA) and of the stability constants of its 1:1 complexes with the trivalent lanthanides are reported for an ionic strength of 1 (KCl) at 25°C. The behaviour of this ligand is similar to that of monoaminodiacetic acids, suggesting that only one N(CH₂COO^?)₂ group participates in chelation. The selectivity of trans-1,4-DCTA for the lanthanides is better than that reported for the monoaminodiacetic acids.     

Chimie organometallique XIII. Preparation et determination structurale d'un complexe μ-alkyldene (octacarbonyle) ditungstenique (WW)

The reaction of MeLi with MeOC(Me)W(CO)₅ (I) gives an unexpected binuclear complex [(CO)₄W]₂HCCHC(Me)₂ (II), the X-ray structure which shows a very short HCCH bond. A mechanism for the formation of II is outlined.     

The trans-strengthening of the SnO bond in six-coordinated complexes of tin(IV): Crystal and molecular structures of SnI4·2DPSO and C2H5SnI·2DPSO

An X-ray study of SnI₄·2diphenylsulphoxide and EtSnI₃·2diphenylsulphoxide indicates octahedral geometry for all the tin atoms in both complexes, the two DPSO ligands being cis to each other and in the second complex the ethyl group being trans to one of the DPSO ligands. In the second complex, under the influence of the ethyl group lengthening of the cis SnI bonds by 0.06 Å and shortening of the trans SnO bond by 0.06 Å occur. The mutual dependence of the SnO and OS bond distances is noted.     

Additions electrophiles sur les esters acetyleniques et alleniques—IV : Addition des halogenes et des halogenures de sulfenyle sur le butyne-3 oate d'ethyle

Ethyl but-3-ynoate undergoes electrophilic additions of BrCl and of ICl to give CHXCClCH₂COOEt E (X = Br or I). The addition of sulfenyl halides RSY follows the opposite orientation and leads to CHYCSRCH₂COOEt E (Y = Cl or Br, R = Et or Ph). In the case of IBr, both CHICBrCH₂COOEt E and CHBrClCH₂;COOEt E are obtained.A mechanistic interprétation of these results is supported by both the orientation and the kinetic order of the addition: PhSCl reacts by an Ad_E2 process, whereas ICl addition involves a combination of Ad_E3 and Ad_E4 mechanisms.     

Etude de la complexation des lanthanides trivalents par les six isomeres de l'acide diaminocyclohexanetetraacetique: Partie 1: synthèses et détermination des constantes d'acidité et des constantes de formation des complexes 1:1 avec les lanthanides trivalents de l'acide trans-1,2-diaminocyclohexane-N,N,N′,N′-tétraacétique

(Study of the complexation of trivalent lanthanides by the six isomers of diaminocyclohexanetetraacetic acid. Part 1. Syntheses, acidity constants of trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid, and formation constants of its 1:1 complexes.)The syntheses of five isomers (1,2 cis, 1,3 cis, 1,3 trans, 1,4 cis, 1,4 trans) of diaminocyclohexanetetraacetic acid which are not commercially available are described. For comparison with literature vlaues, potentiometric measurements of the acidity constants of the sixth isomer (1,2 trans) and of the stability constants of its 1:1 complexes with trivalent lanthanides are reported for an ionic strength of 1 (KCl) at 25°C. In contrast to some literature data, the stability constants found, plotted as a function of Z, display only one discontinuity at the level of gadolinium.