Hydro-metathesis

Hydro-metathesis of propene under hydrogen atmosphere, in the presence of TaH/ KCC-1 catalyst, proceeds smoothly under dynamic reaction condition at 150 C for 65 h with 750 TON [83]. In addition to the expected hydrogenation product, propane, ethane, and butane were formed as major products, and methane, isobutane, and isopentanes formed as minor products in case of propene. Similarly in the case of 1-butene, propane and hexanes were formed as major products, and ethane, propene, pentanes, and heptanes were formed as minor products. In case the of butene, the catalyst was found to be stable even after 75 h and cumulative TON up to 1,150 achieved after 75 h of the reaction [83]. The most important issue with this catalyst is the stability and reusability of this Ta-H/KCC-1 catalyst and the high turnover numbers reached compared with the turnover numbers reported for the Ta-H/Si02 catalyst in alkane metathesis reaction. 

As expected, this reaction was found to be faster in comparison to alkane metathesis because of the absence of C-H bond activation steps which are assumed to be the difficult step of alkane metathesis reaction. Besides thermodynamic factors, this could also explain the comparative ease for the hydro-metathesis because olefin hydrogenation is thermodynamically favored even at low temperatures. 

Based on the experimental fact and following the Chauvin mechanism for alkane metathesis, a probable mechanism was proposed for hydro-metathesis of propene (Scheme 28). 

Scheme 28 Possible mechanism of hydro-metathesis of propene with silica-supported tantalum hydride...

It is however noteworthy that the synthesis of all the latter salts implies the use of imidazole which, as previously evidenced, arises from petroleum. Really, imidazolium cations can be obtained also from natural compounds, as evidenced by Handy et al (Scheme 15) Fructose has been converted into hydro-methylimidazium based ILs after two sequential alkylations and an anion metathesis step. 

In some cases, rapid a-elimination reactions have been observed. These reactions most often occur with early metal complexes and form metal-alkylidene complexes. However, examples of this elimination process from complexes of later transition metals are now known. a-Eliminations from carbene complexes to form carbyne complexes and from amide complexes to form imido complexes are also now well established. Although a-eliminations typically occur with complexes that cannot undergo 3-hydrogen elimination, complexes are now known that undergo faster a-hydrogen elimination than p-hydro-gen elimination. Such a-elimination reactions give rise to the metal-alkylidene complexes that catalyze the olefin metathesis chemistry described in Chapter 21. 

Supported metal oxide catalysts are widely employed in industrial applications alkane dehydrogenation, olefin polymerization, olefin metathesis, selective oxidation/ammoxida-tion/reduction of organic molecules (alkyl aromatics and propylene), and inorganic emissions (N2O, NO , H2S, SO2, and VOC) [1,3,7,11-13]. The initial industrial applications of supported metal oxide catalysts were limited to hydrocarbon dehydrogenation/hydro-genation and olefin polymerization/metathesis reactions. In more recent years, the number of applications of supported metal oxide catalysts for oxidation reactions has grown significantly due to their excellent oxidation characteristics in the manufacture of certain... 

Mainly Th and U have been studied. The complexes [MCp 2R2] are the most common ones, when R lacks 3-H atoms (otherwise, they decompose by 3-elimination). They undergo hydro-genolysis by a-bond metathesis and CO insertion driven by the oxophilicity of the metal. They are efficient olefin hydrogenation and polymerization catalysts. 

Oils with high o. content are obtained from - olive, Euphorbia lathyris and high-oleic - sunflower. Simple ->hydrolysis yields o. concentrations up to 90%. This new quality o. will bring new aspects to old products and processes (- glyceryl monooleate, ->metathesis, - ozonolysis, - epoxidation, - hydroformylation and - hydro-carboxylation). 

Sch 351448 (72), isolated from Micromonospora, is a novel activator of low-density lipoprotein receptor (LDL-R) promoter with an IC50 of 25 nM, which features a 28-membered macrodiolide consisting of two identical hydroxy carboxylic acid units. In its synthesis, intramolecular olefin metathesis of 70 mediated by [Ru]-II proceeded smoothly, and the macrodiolide was obtained after a hydro-genation/hydrogenolysis sequence (Scheme 5.16) [39]. Other syntheses of this... 

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