Showing papers on "Metathesis published in 1979"

Cis‐trans double bond distribution in poly(1,3‐cyclopentylenevinylene); dependence on metathesis catalyst, cocatalyst, and additives

Abstract: About one hundred samples of poly(1,3-cyclopentylenevinylene) (2) were prepared by ring-opening polymerization of norbornene using a variety of metathesis catalysts based mainly on WCl6 or MoCl5, with EtAlCl2, BuLi, Ph4Sn, and (CH2CHCH2)4Sn as cocatalysts, and with methyl acrylate, diethyl maleate or diethyl fumarate as additives. Ir, Re, V, and Os compounds were also used as catalysts. The double bond pair sequences, trans-trans (tt), trans-cis (tc) etc. were determined from the 13C NMR spectra of these samples and the ratios rt(=tt/tc) and rc(=cc/ct) examined as a function of the fraction of double bonds σc having cis structure. For σc up to 0,35 all polymers showed random distribution of cis and trans structures (rtrc = 1), corresponding to a single type of propagating species. Polymers having σc = 0,35–0,85 invariably gave rtrc ≥ 1, with values greater than 5 in some cases. It is proposed that steric crowding of the active site leads both to high values of σc and to restriction of rotation about the metal-carbene bond of the propagating metalcarbene species, such as to give rise to kinetically distinct conformations which tend to regenerate their own kind on addition of monomer.

Properties and reactions of manganese methylene complexes in the gas phase. The importance of strong metal-carbene bonds for effective olefin metathesis catalysts

Abstract: In this communication the formation, properties and reactions of the gas phase carbenes MnCH/sub 2//sup +/, (CO)/sub 5/MnCH/sub 2//sup +/, and (CO)/sub 4/MnCH/sub 2//sup +/ are described. Reported results include observation of metathesis and abstraction reactions of the methylene ligand with olefins and the first experimental determination of metal-carbene bond dissociation energies. Important points are that: (a) metal-methylene bond energies are extremely strong; and (b) the Mn/sup +/-methylene bond energy is decreased substantially on addition of five carbonyls to the metal center. If the metal-carbene bond energy exceeds 100 kcal/mol, then transfer of the carbene to an olefin to give a cyclopropane or new olefin will be endothermic and thus will not compete with the metathesis reaction. In order to avoid low turnover numbers resulting from consumption of carbene intermediates, strong metal-carbene bonds are a desirable feature of practical metathesis catalysts. (DP)

Identification of catalytically active sites on reduced molybdena-alumina catalysts

Abstract: Nitric oxide selectively poisoned propylene metathesis over alumina- or silica-supported molybdenum carbonyl (Catalyst A). Nitric oxide selectively poisoned olefin hydrogenation, but not metathesis or isomerization, over reduced molybdena-alumina catalysts (catalyst B). IR spectroscopy revealed dinitrosyl surface complex on the poisoned catalysts. The over-all reaction rate of a 5:1 mixture of hydrogen-propylene at 200/sup 0/C was over 15 times higher on A than on B, but both catalysts yielded similar product distributions. The dimeric nature of the NO species on catalyst B was confirmed with nitrogen-15 labeled nitric oxide. The oxidation state of molybdenum in catalyst A was +2 and in catalyst B, near +4. The data suggested that the active site for hydrogen activation is multiply coordinatively unsaturated and probably involves molybdenum(III) centers. The study was partly based on available data.

Heterogeneous metathesis of alkenes having functional groups

Abstract: Various alkenes carrying functional groups are metathesized, with a high selectivity, with the catalyst system Re2O7–Al2O3, promoted by a small amount of tetramethyltin.

New efficient route to alkene metathesis catalysts. Electrogeneration of catalytically active species from tungsten hexachloride

Abstract: At ambient temperature, an active species generated by the electroreduction of tungsten hexachloride with an aluminium anode catalyses the metathesis of internal olefins in halogenated solvents with good activity and selectivity.

Metathesis in Costanoan Grammar

Abstract: 0. Metathesis, as a marker of grammatical categories, appears to occur rather infrequently in languages of the world. As Thompson and Thompson pointed out,1 metathesis is far more common as a form of morphophonemic alternation than as a grammatical process in its own right. Among those languages exhibiting synchronic metathesis are the various Sierra Miwok languages2 in which alternating stem shapes are based in part on quantitative ablaut and in part on an interchange of stem-final VC and CV. To a large extent, this metathesis is morphophonemic, the shape of the stem being determined by the following suffix. Nevertheless, metathesis may also serve a grammatical function, since it may distinguish noun stems from verb stems even in the absence of further suffixation. Within Penutian, Miwok forms a subfamily with Costanoan, and, because of 1 Laurence C. Thompson and M. Terry Thompson, \"Metathesis as a Grammatical Device,\" IJAL 35 (1969): 213-19. 2 Sylvia M. Broadbent, The Southern Sierra Miwok Language, UCPL 38 (Berkeley and Los Angeles: University of California Press, 1964), pp. 37, 89; L. S. Freeland, \"Language of the Sierra Miwok,\" IUPAL Memoir 6 (1951): 12, 93-96.

Isomerization and secondary metathesis of 2-butenes over a MoO3/Al2O3 catalyst

Abstract: Transformation of cis- and trans-2-butene at 40°C was investigated over a MoO3/A2O3 catalyst reduced with hydrogen and subjected to different pretreatments. Isomerization is accelerated by hydrogen preadsorbed either at low or high temperatures. The cometathesis of 2-butenes with the 1-butene intermediate, however, is enhanced only by hydrogen preadsorbed at low and suppressed by hydrogen adsorbed at high temperatures.

Selective syntheses of bipentaboranes: 1,2′-(B5H8)2 and 2,2′-(B5H8)2

Abstract: The Friedel–Crafts catalysed reaction of 2-Br-B5H8 with B5H9 produces 1,2′-(B5H8)2, whereas metathesis of KB5H8 with 2-BrB5H8 produces 2,2′-(B5H8)2.

Metathesis of 1,5-Cyclooctadiene over Supported Rhenium Oxide Catalysts

Abstract: The metathesis of 1,5-cyclooctadiene (COD) was investigated over supported rhenium oxide catalysts in terms of carrier effects, acid properties of the catalysts, product distributions, and the reversibility of the reaction. Only alumina plays an effective role as a support. The reaction of COD over rhenium oxide–alumina catalyst yields oligomers, C8n, sesqui-oligomers, C8n+4, and polymers. The important results are as follows: 1) The product distribution in terms of the carbon number of the products shows a decay curve with maxima around C12, C16, and C20. 2) The reaction of oligomers gives COD and some products similar to those of COD. 3) The reactivity of 1,5,9-cyclododecatriene (CDT) was low compared with COD, 1,5,9,13-cyclohexadecatetraene (CHT), and 1,5,9,13,17-cycloeicosapentaene (CEP). 4) A reaction of a mixture of macrocyclic polyenes and polymers gave appreciable amounts of CDT, CHT, and CEP, whereas no COD was obtained. On the basis of the results, we conclude that the products, oligomers, sesqu...

Hexakis(trimethylsilylmethyl)ditungsten as catalyst for metathesis reactions of olefins

Abstract: W2[CH2Si(CH3)3]6 initiates the polymerization of cyclic olefins, and in combination with WCl6 is active in the metathesis of both linear and cyclic olefins.