Metathesis homogeneous catalysts

The metathesis polymerisation of dicyclopentadiene (endo, exo) yields a linear polymer when carried out with single-component homogeneous metathesis catalysts of the metallacyclobutane or metal alkylidene type in diluted toluene solutions [142, 143]. 

There are several types of well-defined homogeneous metathesis catalysts ... 

Discussion Points DPI What are the main concerns with homogeneous metathesis catalysts that currently prevent their application in large scale processes What are the current issues regarding the heterogeneous catalysts How would you tackle these issues ... 

RUTHENIUM-VINYLIDENE COMPLEXES AN EFFICIENT CLASS OF HOMOGENEOUS METATHESIS CATALYSTS... 

Generally, there are two possibilities how to use mesoporous molecular sieves as supports for metathesis catalysts. First, to apply them as supports for heterogeneous metal oxide-based catalysts instead of classical silica or alumina. Second, to use them for preparation of heterogenized versions of originally homogeneous metathesis catalysts, especially for anchoring well-defined carbene complexes. 

W(OAr)2(Cl)(CH2R )(=CHR)(0R 2 is a well defined, Lewis acid free, homogeneous metathesis catalyst whose activity varies with OAr. The bis-alkyne complex M(X)(CF3CCCP3>2Cp (X = CF3, Cl, SR) reacts with Bu NC initially to form the n -vinyl Mo(SR) (CF3CCCF3) [ ti -C(CF3)C(CF3) (CNBu ) ] which converts to a complex metallacycle. 

Typical Examples of Catalyst Systems for the Homogeneous Metathesis of Pentene... 

Polar-substituted alkenes where the functionality is not attached to a strained ring are considerably more discriminating in their compatibility with metathesis catalysts and as a rule require relatively high catalyst charges. In the aliphatic series, unsaturated esters have received the most attention. Boelhouwer reported in 1972 the metathesis of the ester methyl oleate and its trans isomer, methyl elaidate, with a homogeneous catalyst based on a 1/1.4 molar combination of WCl6/(CH3)4Sn (23). At 70°C and an ester/W molar ratio of 33, near-thermodynamic equilibrium was attained, and 49 and 52% of the respective esters were converted to equal amounts of 9-octadecene and the dimethyl ester of 9-octadecene-1,18-dioic acid. 

The formation of carbon-carbon bonds using olefin metathesis methodology is a powerful technique in fine organic synthesis and polymer chemistry. The increasing importance of these reactions is reflected by the numerous publications over the last few years. Many of these pubhcations deal with the design and apphca-tion of polymer-supported olefin metathesis catalysts with the aim to overcome the common drawbacks of the homogeneous catalysts low thermal stability and difficulties associated with their recovery from the reaction mixtures. The modem state of art in this important field is described in chapter 11 of this volume. 

All these catalytic results, however, were usually achieved at very low (2-3%) conversions. The only exception is a paper reporting up to 80% selectivity at 20% conversion over a M0CI5—R4Sn-on-silica olefin metathesis catalyst (700°C, 1 atm, CH4 air = l).42 In general, higher temperature and lower—about ambient— pressure compared to homogeneous oxidation, and high excess of methane are required for the selective formation of formaldehyde in catalytic oxidations.43 The selectivity, however, decreases dramatically at conversions above 1%, which is attributed to the decomposition and secondary oxidation of formaldehyde.43,44 It is a common observation that about 30% selectivity can be achieved at about 1% conversion. 

C. Coperet, and J.-M. Basset, Strategies to Immobilize Well-Defined Olefin Metathesis Catalysts Supported Homogeneous Catalysis vs. Surface Organometallic Chemistry, Adv. Synth. Catal. 349, 78-92 (2007). 

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