Showing papers on "Double bond published in 1990"

Iridium(III) hydride complexes for the catalytic enantioselective hydrogenation of imines

Abstract: Considerable research during the past 20 years has produced remarkable catalysts based on Rh and Ru for enantioselective homogeneous hydrogenation of olefins and ketones where in many cases optical yields > 95% have been observed. On the other hand, few publications have appeared that deal with asymmetric hydrogenation of the C{double bond}N bond of imines to form chiral amines, the greatest success having been obtained by using in situ mixtures of M/chiral diphosphine/X (M = Rh{sup I} or Ir{sup I}, X = NEt{sub 3} or halide) but the nature and the mechanism of action of the catalyst is not well-defined. The authors report herein the discovery of a new class of iridium(III)-diphosphine-monohydrido complexes that are effective for asymmetric reduction of prochiral imines to the corresponding amine with high turnover numbers and moderate to good enantioselectivity.

A product study of the gas-phase reaction of methacrolein with the OH radical in the presence of NOx

Abstract: The gas-phase reaction of methacrolein with the OH radical, in the presence of NOx, was investigated at 298 ± 2 K and atmospheric pressure of air. Hydroxyacetone, methylglyoxal, a peroxyacyl nitrate identified as CH2 C(CH3)C(O)OONO2 (peroxymethacryloyl nitrate), formaldehyde, CO, and CO2 were observed to be the major products. The product yield data for these compounds show that OH radical addition to the >C C< bond accounts for ca. 50% of the overall reaction, with the remaining ca. 50% proceeding via H—atom abstraction from the CHO group. The data suggest that the alkoxy radical formed following the addition of OH to the terminal carbon atom, decomposes primarily to give the formyl radical plus hydroxyacetone. A lower limit ratio of 5: 1 has been estimated for OH radical addition to the terminal carbon atom of the double bond relative to addition on the inner carbon atom.

Chemioselectivity in the catalytic hydrogenation of cinnamaldehyde. Effect of metal particle morphology

Abstract: The selectivity of platinum and rhodium catalysts for the hydrogenation of cinnamaldehyde into cinnamyl alcohol is much higher on large, facetted metal particles them on small (<3 nm) particles. This is attributed to a steric effect of the phenyl group which hampers the molecule to adsorb parallel to the flat metal surface thus favouring the adsorption and hydrogenation of the carbonyl group with respect to the C = C double bond.

Oxidative addition of carbon-oxygen and carbon-nitrogen double bonds to WCl2(PMePh2)4. Synthesis of tungsten metallaoxirane and tungsten oxo- and imido-alkylidene complexes

Abstract: WCl 2 L 4 (1, L=PMePh 2 ) reacts rapidly with a variety of ketones and aldehydes to form bis(η 2 -ketone) or bis(η 2 -aldehyde) complexes WCl 2 (η 2 -O=CRR') 2 L 2 (2,3). When the ketone is part of a five-membered ring (cyclopentanone, 2-indanone, etc.), 2 rearranges with loss of ketone to give tungsten (VI) oxo-alkylidene products, W(O)(CRR')Cl 2 L 2 (4) in high yield. The net reaction is insertion of the tungsten center into the keton carbon-oxygen double bond, a four-electron oxidative addition. Insertion into the carbon-nitrogen double bond of N-cyclopentyl-p-toluidine yields the analogous imido-alkylidene complex W(N-Tol)[C(CH 2 ) 4 ]Cl 2 L 2

Synthesis and immunosuppressive activity of some side-chain variants of mycophenolic acid.

Abstract: The syntheses and immunosuppressive bioassays of 12 side-chain variants of mycophenolic acid are described. The compounds were made either from mycophenolic acid itself or from 5-(chloromethyl)-1,3-dihydro-4-hydroxy-6-methoxy-7-methyl-3-oxoisoben zofuran, a versatile intermediate for the synthesis of diverse side-chain variants. Replacement of the methylated E double bond of the natural product with a triple bond, a Z double bond, a saturated bond, or a sulfur atom, with overall chain lengths equal to or greater than that of mycophenolic acid, produced compounds devoid of significant activity. Replacement of the side-chain double bond with difluoro, dibromo, or unsubstituted cyclopropane rings also removed most activity. Replacement of the double bond with an allenic linkage yielded a compound with about one-fifth of the immunosuppressive activity of mycophenolic acid. Some possible causes for the unusual specificity of structure and activity are discussed.

Rearrangement of bicyclo[2.2.1]heptane ring systems by titanocene alkylidene complexes to bicyclo[3.2.0]heptane enol ethers. Total synthesis of (±)-Δ^(9(12))-capnellene

Abstract: A variety of ester-substituted norbornenes react with titanamethylene complex (Tebbe's reagent) to yield stable titanacyclobutanes. Endo esters do not react with the reagent in competition with the norbornene double bond. The X-ray structure of the metallacycle formed from titanacene methylene complex and 1-methylbicyclo[2.2.l]hept-5-ene-2,3-dicarboxylic acid diisopropyl ester was determined. On heating, the metallacycle rearranged to a carbene-olefin complex. The ratio of productive opening, cleavage of the bicycloheptane ring system, to nonproductive opening, regeneration of the starting materials, is controlled by a variety of steric factors that were studied and analyzed. The productive opening was detected by the formation of the product resulting from the intramolecular trapping of the intermediate titanium alkylidene by the endo ester functionality in a Wittig-like reaction to yield substituted bicyclo[3.2.0]heptenes. Rearrangement of the titanacycle formed from 4,4-dimethyltricyclo[ 5.2.1.0^(1,5)]dec-8-ene-6-carboxylic acid tert-butyl ester yielded 10,10-dimethyl-3-methoxy-7- vinyltricyclo[5.3.0.0^(2,5)]dec-2-ene, which was transformed into Δ ^(9(12))-capnellene in good yield.

Stereoselective nucleophilic additions to the carbon-nitrogen double bond. 3. Chiral acyliminium ions

Abstract: On propose un modele de l'etat de transition en accord avec la theorie des orbitales moleculaires et les preferences conformationnelles du substrat

CC Bond Formation by Addition of Carbenium Ions to Alkenes: Kinetics and Mechanism

Abstract: The addition of carbenium ions to CC double bonds, a key step in many syntheses in organic and macromolecular chemistry, is analyzed using the Lewis acid promoted reactions of alkyl chlorides with alkenes as an example. Stereochemical and kinetic experiments suggest that the transition state is slightly bridged and product-like. Rearrangements of the carbenium ions that result from the electrophilic attack can be minimized by adding salts with nucleophilic counter ions. The thermodynamics of the addition reactions are analyzed, and the conditions necessary in order to observe the back reaction (i.e. the Grob fragmentation) are discussed. Multiparameter equations that predict rate constants are derived from kinetic studies on the reactivities of carbenium ions and alkenes. Reactivity-selectivity relationships over a reactivity range that covers eight orders of magnitude show that the structure of the transition state is only changed by variation of substituents in the immediate vicinity of the reaction center.

Kinetic and ab initio study of the prooxidant effect of vitamin E. Hydrogen abstraction from fatty acid esters and egg yolk lecithin

Abstract: A kinetic and ab initio study of the prooxidant effect of vitamin E derivatives has been carried out. Rates of hydrogen abstraction from various fatty acid esters and egg yolk lecithin by tocopheroxyl radicals have been determined spectrophotometrically. The second-order rate constant for the hydrogen abstraction dramatically increases as the number of π-electron systems activating the hydrogen (C=C double bonds) increases from zero to two. The rate constants per an active hydrogen for the various fatty acid esters that contain hydrogens activated by two π-electron systems are similar to one another. In order to interpret the observed features of the rate constants, ab initio calculations of models for the fatty acid esters have been carried out. From the calculation results, it is shown that the observed features of the rate constant can be explained in terms of the pseudo-π-conjugation between the C=C double bond and the active hydrogen-carbon bond. The bulkier the substituent groups at the 5- and 7-positions of the tocopheroxyl radical, the smaller the rate constant of the hydrogen abstraction

Nitrogen-15 nuclear magnetic resonance studies on the tautomerism of 8-hydroxy-2'-deoxyguanosine, 8-hydroxyguanosine, and other C8-substituted guanine nucleosides

Abstract: The favored tautomeric and ionic structures were examined for the oxidative DNA damage adduct 8-hydroxy-2'-deoxyguanosine and its RNA analogue 8-hydroxyguanosine by 15N NMR spectroscopy. In addition, 15N chemical shifts and coupling constants from 13 different guanine nucleosides, including a wide variety of C8 substitutions (OH, SH, Br, OCH2C6H5, OCH3, SCH3, and SO2CH3), have been analyzed with respect to their tautomeric structures. A -98.5-Hz proton-nitrogen coupling constant observed for the N7 resonance of 8-hydroxyguanosine in dimethyl sulfoxide was evidence for 8-keto substitution, which is contrary to the structure implied by the generally used nomenclature. The pH dependence of 15N NMR spectra of 8-hydroxyguanosine in aqueous solution showed downfield shifts of the N1 and N7 resonances that were greater than 50 ppm, which indicated the conversion from a neutral 6,8-diketo to a 6-enolate-8-keto (pKa1 = 8.6) and finally to a 6,8-dienolate structure (pKa2 = 11.7). There was no evidence of an 8-enol substituent in the absence of ionization. It is proposed that the syn conformation of these oxidized bases in duplex DNA and RNA can be further stabilized by abnormal hydrogen bonding or mispairing that involves N7-H. The combined data show that 15N NMR is a sensitive probe to examine tautomerism of the guanine ring system. The analysis indicates that the change from a single to a double bond for the C8 substituent, and the accompanying removal of the normal double bond between N7 and C8 on the imidazole ring system, has no detectable effect on the tautomerism at the N1-O6 site of the pyrimidine ring system for both the 8-keto and 8-thio substitutions. In addition, large differences in electronegativity of the C8 substituents do not alter the N1-O6 tautomerism.

A novel oxidation of internal alkynes with hydrogen peroxide catalyzed by peroxotungsten compounds

Abstract: Internal alkynes underwent a novel oxidation with aqueous hydrogen peroxide catalyzed by peroxotungstophosphates under two-phase conditions using chloroform as the solvent, affording α, β-epoxy ketones and α,β-unsaturated ketones as principal products. The epoxidation of α, β-unsaturated ketones by this catalyst oxidant system was considered to proceed by electrophilic attack of the peroxo species to the double bond.

Synthesis, structure, and reactivity of (phosphoranylidenephosphine)pentacarbonyltungsten complexes. Another access to the phosphorus-carbon double bond

Abstract: The reaction of trialkylphosphines with 7-phosphanorbornadiene PW(CO) 5 complexes affords the corresponding phosphoranylidenephosphine complexes R 3 1 P=P(R)→W(CO) 5 . One such complex (R=CO 2 Et, R 1 =Et) has been investigated by X-ray crystal structures analysis. It shows a zwitterionic character with a long P=P bond. These phosphoranylidenephosphine complexes cleanly react at room temperature with aldehydes to give the corresponding phosphaalkene complexes via a «phospha-Wittig» reaction. These phosphaalkene complexes are either isolated as such or trapped by MeOH, dimethylbutadiene, or benzoic acid

Sheet or film composed of cyclo-olefin polymer

Abstract: A sheet or a film composed of at least one cyclo-olefin polymer selected from the group consisting of ring-opened polymers derived from at least one cyclo-olefin of formula (I) ##STR1## wherein R1 to R12, independently from each other, represent a member selected from the class consisting of a hydrogen atom, halogen atoms and hydrocarbon groups; two of R9 to R12 may be linked to each other together with the carbon atoms to which they are bonded to form a monocyclic or polycyclic group which may contain a double bond; R9 and R10, or R11 and R12 may each form an alkylidene group together with the carbon atoms to which they are bonded; and n is 0 or a positive integer, ring-opened copolymers derived from the cyclo-olefins of formula (I), hydrogenation products of said polymer or copolymers and addition polymers of the cyclo-olefins of formula (I) with ethylene, said cyclo-olefin polymer having an intrinsic viscosity [η], measured in decalin at 135° C., of 0.01 to 10 dl/g and a softening temperature of at least 70° C.

Peptide compounds having substance p antagonism, processes for preparation thereof and pharmaceutical composition comprising the same

Abstract: Peptide compounds having Substance P antagonism of the formula: ##STR1## wherein R1 is lower alkyl, aryl, arylamino, pyridyl, pyrrolyl, pyrazolopyridyl, quinolyl, or a group of the formula: ##STR2## wherein the symbol of a line and dotted line is a single bond or a double bond; X is CH or N; Z is O, S or NH; R2 is hydrogen or lower alkyl; R3 is hydrogen or hydroxy; R4 is lower alkyl which may have suitable substituent(s); R5 is ar(lower)alkyl which may have suitable substituent(s) or pyridyl(lower)alkyl, or R4 and R5 are linked together to form benzene-condensed lower alkylene; A is an amino acid residue excepting D-Trp, which may have suitable substituent(s); and Y is bond, lower alkylene or lower alkenylene; and their pharmaceutically acceptable salts are disclosed.

Orientation of ethylene and propylene on Ag(110) from near edge x‐ray adsorption fine structure

Abstract: The π bond orientation of ethylene and propylene on the clean and atomic oxygen covered Ag(110) surfaces has been determined with near‐edge x‐ray absorption fine‐structure (NEXAFS) measurements. The π system of ethylene is tilted 20±5° and 18±5° from the surface normal for ethylene on the clean and atomic oxygen covered surfaces, respectively. The tilt angle of the π system is 20±5° on the clean surface and 31±5° on the atomic oxygen covered surface. On both the clean and atomic oxygen covered surface, the carbon–carbon double bond in ethylene and propylene appears to form a π‐donor type bond with the Ag(110) surface.

Synthesis and subsequent rearrangement of chloro(pentafluorophenyl)-1,5-cyclooctadienepalladium(II), an illustrative example of endo attack on a coordinated double bond

Abstract: The complexe Pd(C 6 F 5 )Cl(1,5-cyclooctadiene) rearranges slowly in solution by intramolecular double bond insertion into the Pd-C 6 F 5 bond to give an allyl complex Pd 2 (μ-Cl) 2 (6-C 6 F 5 -1-3-η 3 -C 8 H 12 ) 2 and σ,π-complex Pd 2 (μ-Cl) 2 (8-C 6 F 5 -1:4-5-η 3 -C 8 H 12 ) 2 . A derivative of the latter, Pd(8-C 6 F 5 -1:4-5-η 3 -C 8 H 12 )(F6-acac), has been characterized crystallographically: triclinic space group P1, a=10.360 (3) A, b=11.051 (2) A, c=11.084 (4) A, α=73.70 (2)°, β=61.41 (2)°, γ=66.08 (2)°, final R of 0.036 for 3341 independent reflections. The rearrangement of Pd(C 6 F 5 )Cl(1,5-COD) is catalyzed by its products and slowed down in coordinating solvents. Both products are the result of an endo attack of C 6 F 5 to COD and are formed competitively from a common intermediate

Products and mechanisms of the gas-phase reactions between NO3 and a series of alkenes

Abstract: Products and mechanisms for the gas-phase reactions of NO3 radicals with a series of alkenes in air have been studied. The experiments with propene, isobutene, trans- and cis-2-butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene were carried out at 295 f 2 K and 740 * 5 Torr in a 480-L Teflon-coated reaction chamber where NO3 was generated by the thermal dissociation of N2O5. Reactants and products were observed to produce nitroxy-nitroperoxy intermediates that decayed with formation of carbonyl, nitroxycarbonyl, nitroxy alcohol, and dinitrate species. The product distribution was found to be dependent on the alkyl substitution pattern around the double bond. Evidence was also found of a reaction between NO3 and organic peroxy radicals with a rate constant measured in the case of the 2,3-dimethyl-2-nitroxy-3-peroxybutane radical to be of the order of 5 X cm3 molecule-' s-l.