Showing papers on "Metathesis published in 1981"

Molecular complexes for efficient metathesis of olefins. The oxo-ligand as a catalyst–cocatalyst bridge and the nature of the active species

Abstract: Three families of molecular complexes for the homogeneous metathesis of olefins are described; the role of the oxo-or imido-ligand as bridge for binding the Lewis acid and the nature of the active species in meta-thesis are discussed.

Olefin metathesis over well-defined active fixed molybdenum catalysts. Structure and oxidation state of the active site and reaction mechanism

Abstract: Fixed Mo catalysts (supported molecular catalysts), synthesised using the facile reaction between Mo (π-C3H5)4 and the surface acidic OH groups of alumina, silica and silica–alumina, were found to be very active for propene and but-1-ene metathesis in the temperature range 268–308 K. The isolated and coordinatively unsaturated Mo4+ species which are chemically bonded to the support surface are the active species. The nature of the ligands and supports coordinated to the Mo4+ ions produce different catalytic activities and selectivities. The support also works as a reservoir/supplier of reactants. Metathesis on fixed Mo catalysts is explained by a one-site chain mechanism involving carbene intermediates.

Activity of supported tungsten oxide catalysts for the metathesis of propene

Abstract: A high-temperature activation under inert gas of a WO 3 SiO 2 catalyst greatly increases the break-in rate and the catalytic activity for the metathesis of propene. During activation oxygen atoms and hydroxyl groups are eliminated from a tungsten oxide surface compound. As a result, tungsten ions become more exposed; some of these also undergo a change in oxidation state. The results suggest that the level of catalytic activity as well as the break-in rate depend on the number of these ions and on the rate of their formation. For a given catalyst type and structure, it depends primarily on the conditions of the activation whether a break-in appears or not. WO 3 Al 2 O 3 catalysts are characterized by a maximum in the reaction rate as a function of the reaction temperature. They attain, under the conditions of the proposed activation, high activity in a very short time and, under certain circumstances, are affected by product hindrance.

Structures of rhenium(VII) oxides on alumina and their catalytic activities for metathesis of olefins.

Abstract: IR studies revealed that the Re2O7/Al2O3 metathesis catalyst has two different surface rhenium species, [ReO4]ads− and [Re2O7]ads, which are similar in both structure and behavior to those in the aqueous solution of Re2O7. [Re2O7]ads, which is present only in high loading catalysts, should be responsible for their high catalytic activity for the metathesis of olefins.

The Characterization of a Novel Metathesis Catalyst, β-Titanium Oxide-supported Molybdenum Oxide

Abstract: The results of the X-ray photoelectron spectroscopy of a novel metathesis catalyst, MoOx/β-TiO2, are discussed with reference to its catalytic behavior. It is concluded that the active center for the metathesis reaction of olefin is the molybdenum ion with an oxidation higher than +4 on the average, and that the hydrogen scrambling of olefin during the metathesis is caused by the OH group on the TiO2 surface. During the preparation of this catalyst, the decomposition of titanic acid to TiO2 is found to be accelerated by MoO3; a similar thing can be seen of the reduction of MoO3 to zero-valent molybdenum by TiO2.

13C NMR spectra of polymers made by ring‐opening polymerization of (±)‐endo‐5‐methylbicyclo[2.2.1]hept‐2‐ene using metathesis catalysts

Abstract: A range of olefin metathesis catalysts has been used to prepare ring-opened polymers of (±)-endo-5-methylbicyclo[2.2.1]hept-2-ene (1), having cis double bond contents of 22–100%. The 13C NMR spectra of these polymers are interpreted in terms of TH, TT, HH, HT and tt, tc, ct, ccstructures (T = tail, H = head, referring to the methyl group orientation; t = trans, c = cis, referring to the double bond configuration). The fine structure of the olefinic region at 75 MHz indicates that the cis/trans double bond distribution is blocky when the cis content is more than 50%. The chemical shifts of the ring carbons indicate puckering of the cyclopentane rings. The cis relationship of the methyl carbon and the adjacent olefinic carbons causes an upfield shift in both cases.

Heterogeneous metathesis of unsaturated nitriles

Abstract: Unsaturated nitriles of general formula CH2CH[CH2]nCN, where I ⩽n⩽ 4, undergo metathesis in the presence of the catalyst system tungsten hexachloride-tetramethyltin with maximum reactivity for n= 2

Process for the preparation of linear olefins from triethylaluminum and tripropylaluminum via growth, isomerization and metathesis

Abstract: Internal olefins are subjected to metathesis closely coupled after an isomerization reaction to obtain desired molecular weight range linear olefins. Specific process steps and separation conditions are necessary to obtain the desired linear olefins.

Norbornene polymers containing bound antioxidant

Abstract: A polymeric composition of matter formed by ring opening copolymerization using a metathesis catalyst comprising bound antioxidant which is a reaction product of a norbornene-type monomer and a norbornenyl phenolic antioxidant.

Metathesis of dialkenylsilanes and polymerization of 1,1-dimethyl-1-sila-3-cyclopentene in presence of aluminum-rhenium catalysts

Abstract: 1. It was shown that the efficient metathesis of dialkenylsilanes of variable structure in the presence of aluminum-rhenium catalysts is possible. 2. Conditions were found for the preparative synthesis of 4,9-disila-4,4,9,9-tetramethy1-1,6,11-dodecatriene and 1,1-dimethyl-1-sila-3-cyclohexene via the inter- and intramolecular metathesis reaction. 3. 1,1-Dimethyl-1-sila-3-cyclopentene polymerizes to a linear polysilapentenomer in the presence of Al-Re catalysts.

Low Temperature Photoreduction-A New Approach to Preparation of Low-Coordinated Ions of Transition Metals on Surface of Oxide Catalysts

Abstract: Low temperature photoreduction of silica supported V5+, Mo6+ and Cr6+ ions in H2 results in formation of tetrahedral V4+, Mo5+ and Cr5+. In CO low coordinated Mo4+, V3+ and Cr2+ are formed. They exhibit unusually strong reducing properties. Photoreduced samples posess high activity in hydrogen-deuerium isotopic exchange, ethylene polymerisation and propylene metathesis in certain cases exceeding the activity of thermally reduced catalysts.

Stereoselectivity in the cross-metathesis of oct-1-ene and cis- or trans-oct-2-ene

Abstract: The yields and the initial ratios of cis and trans geometries of non-2-ene, tridec-6-ene, or tetradec-7-ene, formed by the cross-metathesis of oct-1-ene and cis- or trans-oct-2-ene catalyzed by tungsten hexachloride–tetraphenyltin (A), hexaphenoxytungsten–ethylaluminium dichloride (B), or tungsten hexachloride–triethylaluminium (C), have been correlated to the interactions of alkyl substituents on tungstacyclobutane. The results strongly suggest that the 2,4-interaction, between the alkyl substituent on C2 and the tungsten moiety, plays an important part in determining the distribution of products.

Hydrogen exchange, isomerization, hydrogenation and metathesis of olefins on solid surfaces：unsaturated coordination model of active sites

Abstract: A review is given on the results ever obtained III the author's laboratory with olefin reactions over nickel, sulfided nickel, molybdenum disulfide and titania-supported molybdenum oxide catalysts. Some detailed discussions are given on the stereoselectivity observed in [D41-2-butene formation by metathesis of a [Dol/[D81 (1: 1) mixture of cis or trans2-butene, and it is suggested by the extended HUckel theory (EHT) calculation that this stereoslectivity may be caused by the restricted orientation of C 2-carbene and metallacyclobutane intermediates on surface molybdenum ion surrounded by lattice oxygen. Taking account of the results obtained by X-ray photoelectron spectroscopy of the catalyst, it is emphasized that specially unsaturated coordination of legands (e.g. sulfur, oxygen etc.) to central metal ion is essential for an active site for the selective catalyses over these solid surfaces.