Olefin cross metathesis mechanism
This decrease was attributed to the transformation of the mono-oxo species into the di-oxo one Figure This increase of the di-oxo concentration could explain the higher activity of this system compared to that obtained after activation under air. Stair et al. Surprisingly, these catalysts can be regenerated by a treatment under nitrogen at high temperature.
The main problem is due to the low amount of active sites. As a consequence, it is very difficult to understand the activation mechanism of the catalyst and also its deactivation. The preparation of systems containing a higher amount of active sites could lead to more active and easily regenerated systems and could allow a better characterization and mechanistic study of the initiation and deactivation steps and their rationalization in terms of classical organometallic chemistry.
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Surface organometallic chemistry SOMC is a choice method for the preparation of silica supported complexes. Numerous tungsten complexes with a variety of ligands alkyl, carbene, carbyne, oxo, imido, aryloxy, etc. SOMC can be considered as a bridge between homogeneous and heterogeneous catalysis [ 55 , 56 , 57 ]. Its aim is to graft organometallic complexes on oxide surfaces silica, alumina, titania, zirconia, etc. In the case of oxides, the complex can be linked to the support by one or more bonds with surface oxygen atoms.
When the support has been previously functionalized, the bonding can be made via other atoms such as P, N, Si, etc. As it is the case in homogeneous catalysis, these surface organometallic species can be defined by their ligands around the metal. Two types of ligands can be considered, those which will be involved in the catalytic cycle and those which are only spectators such as oxo, alkoxo, amido, or imido groups.
The modification of both types of ligands can have a drastic effect on the activity and selectivity of a given catalytic reaction, allowing to establish structure-activity relationships. For example, pretreatment of the support at different temperatures will lead to the synthesis of surface complexes with one, two, or three bonds with the surface. This new approach has many advantages:.
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The metal complexes have a limited mobility on the surface, avoiding the bimolecular decomposition reactions which are often observed in homogeneous catalysis [ 58 ]. The catalysts can be characterized easily by use of spectroscopic methods, as all species are identical. The good knowledge of the structure of the active site allows to propose a reasonable catalytic cycle and to determine how deactivation and regeneration will proceed.
A lot of organometallic complexes of groups 4—8 were grafted on a variety of surfaces such as amorphous inorganic oxides [ 55 ], zeolites [ 59 ], or metals [ 60 , 61 ]. This is mainly due to a combination of organometallic synthesis and surface science. The catalytic efficiency of the materials prepared by this way depends on the coordination sphere around the metal, on the number, and the character ionic or covalent of the bonds with the support and on the nature of the oxide support silica, alumina, silica-alumina, etc.
In the case of tungsten SOMC, the choices of the organometallic precursor and of the support are mainly dependent on the expected catalytic reaction and on the intermediates involved in the postulated catalytic cycle. The high oxidation state of tungsten VI allows the possibility of a number of ligands in the coordination sphere leading to both spectators and reactive species in the catalytic cycle. The reactive species will be hydrides, alkyl, carbenes, and carbynes.
During the last few years, many studies were made with such surface complexes in olefin metathesis. We will review here only those containing the oxo ligand as they could be considered as models of the industrial heterogeneous catalysts. There are two principal methodologies which have been developed to achieve well-defined tungsten oxo species on oxide: i grafting of a reactive tungsten carbyne complex followed by transfer of oxygen from the support and ii grafting of an organometallic complex bearing oxo ligand.
In order to avoid this deactivation, these complexes were heterogeneized. Weiss et al.
The carbenic ligand was evidenced by its reactivity with acetone via a pseudo-Wittig reaction and indirectly by the catalytic activity in olefin metathesis. This study showed the existence of interactions between protons of residual hydroxyl groups and the alkyl ligands of the supported species. Species 18 shows a good activity in propene metathesis initial TOF 5.
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Two mechanisms were proposed explaining the formation of the carbenic ligand. The other possibility is to form directly the carbene by metathesis between the olefin and the carbyne: a metallacyclobutene is formed which decomposes into a carbene-alkenyl tungsten complex Figure Alumina is a complex support as aluminum can be tetra-, penta-, or hexacoordinated and its surface hydroxyl groups can be bound to one, two, or three aluminum atoms.
As a result, there is a great variety of hydroxyl groups with variable acidity. The infrared study showed that there is a partial consumption of the hydroxyl groups and that only those linked to one tetrahedral aluminum have reacted [ 72 ]. DFT calculations show that this broadening is due to an interaction between these methylene groups and the residual surface aluminum groups. The mechanism of this reaction involves three steps: dehydrogenation of the alkane, olefin metathesis, and hydrogenation of the resulting olefin.
This example shows that the support can have a nonnegligible effect on the catalysis. On silica sintering is observed and TEM shows that tungsten particles size 0. On alumina no sintering is observed and a hydride species is obtained [ 71 , 73 ]. This hydride species is characterized by a small peak at All these data combined to DFT calculations allowed to propose that the surface hydride is a tris-hydride oxo tungsten VI complex stabilized by coordination of the oxo ligand to one surface aluminum Figure 17 [ 75 ].
The mechanism of formation of this surface species was elucidated by use of DFT calculations. These calculations suggested that the oxo species is formed by reaction of an unstable tungsten hydride species with one oxygen atom of the alumina surface. The TON can reach after h [ 80 ]. The mechanism of this reaction passes probably via the same ethyl ethylidene oxo tungsten complex than above as ethane is also detected at the first stages of the reaction.
This complex can then convert ethylene to propene via three successive reactions: i dimerization of ethylene to butene; ii isomerization of butene to butene; and iii cross-metathesis between ethylene and butene Figure It is the first example of use of a supported tungsten complex for this reaction, which is very difficult, due to the high steric hindrance in the gem-tetra-substituted metallocyclobutane intermediate.
All these works show the beneficial effect of the oxo ligand even if it is only spectator in the coordination sphere of tungsten for metathesis of olefins. In this case, the oxo ligand was formed by extraction from the surface of alumina toward the xophilic tungsten center after its reduction under hydrogen. However, oxo alkylidene tungsten complexes are very unstable and there are only few reports on them.
The first supported tungsten oxo alkyl species active in olefin metathesis was achieved by grafting reaction of the complexes synthesized by Osborn et al. Figure 6 [ 26 ]. These results were also comforted by DFT calculations. The initial activities are quite similar for the two surface species: 4. The main difference is the evolution with time of stream: Complex 23 deactivates rapidly Figure 22a while complex 22 remains stable.
As a result, after 95 h the TON is 22, for 22 and ca. In terms of selectivity, complex 22 is stable with equimolar amounts of ethylene and butene while in the case of compound 23 isobutene and higher olefins pentenes and hexenes are also formed Figure 22b. Metathesis of propene catalyzed by complexes 22 and a Conversion as a function of time; b isobutene selectivity as a function of time for complex This hydride is then converted into an inactive tungsten IV species with the evolution of isobutene by reductive elimination Figure 23 [ 83 ].
This result shows also that the oxo ligand has a nonnegligible effect on the activity and stability of the tungsten catalysts.
Recently, Eisenstein et al. However, the real model for the industrial catalyst being a bipodal tungsten oxo carbene species was not achieved with the former organometallic complex grafted on SiO 2— support that frequently yields bipodal species. University of Regensburg. Stemoamide Total Synthesis of Stemoamide using ring-closing-metathesis - a comparison. Olefin metathesis The Olefin metathesis transalkylidenation is an chemical organic reaction that causes the reallocation of alkylene fragments by the decomposition of carbon - carbon double bonds in olefins.
Online available information resources on olefin metathesis transalkylidenation. The author- or copyrights of the listed Internet pages are held by the respective authors or site operators, who are also responsible for the content of the presentations. To include your website to the Internetchemistry directory, please use our registration form or send us an eMail. Olefin, metathesis, transalkylidenation, chemical, reactions, mechanisms, organic, synthesis. Related Books. German Version. Chemistry News in German. Chemistry Books. Olefin metathesis: Reaction and Mechanism.
University of Regensburg Stemoamide Total Synthesis of Stemoamide using ring-closing-metathesis - a comparison. When well-defined olefin metathesis catalysts started to come on scene, the … [Continue reading].
Conrad, J. Oligomers as Intermediates in Ring-Closing Metathesis. Reading this communication changed the way I … [Continue reading]. Careful combination of multiple polymerization mechanisms can generate truly unique architectures. Incorporation of chain-segments generated by metathesis polymerizations have been explored by a number of researchers particularly the Matyjaszewski … [Continue reading].