Metathesis reactions Subject

A different approach, taken by both Monsanto (58) and Gulf Research and Development Company (59), involved the oxidative coupling of two molecules of toluene to yield stilbene. The stilbene is then subjected to a metathesis reaction with ethylene to yield two molecules of styrene. 

Representatives of the bridged sulfone system 70 have been subjected to ruthenium catalysed ring-closing metathesis reactions (Grubbs catalyst) and shown to afford, in low yields, a few selected cyclic dimers and trimers, of all the possibilities available. The diastereoselectivities observed were rationalised in terms of kinetic control involved with internal ruthenium/sulfonyl oxygen coordination . 

This review focuses on the cross-metathesis reactions of functionalised alkenes catalysed by well-defined metal carbene complexes. The cross- and self-metath-esis reactions of unfunctionalised alkenes are of limited use to the synthetic organic chemist and therefore outside the scope of this review. Similarly, ill-defined multicomponent catalyst systems, which generally have very limited functional group tolerance, will only be included as a brief introduction to the subject area. 

Direct metathesis reactions are characterized by tight transition states and involve the transfer of atoms or radicals as a consequence of the close proximity of the two reactants, and they are the only type of reactions that are not subject to becoming energy-transfer-limited at high temperatures and low pressures. A typical energy diagram for metathesis reactions is shown in Fig. 8c. Examples of such reactions include... 

The marine natural product ascidiatrienolide A 220 is a strong inhibitor of phospholipase A2. Compound 220 and the closely related didemnilactones 221-223 feature a common hydroxy-substituted anti to the ring oxygen) (Z)-nonenolide core. Lactone 219, which constituted the key intermediate in a previous total synthesis of 220 and can also be elaborated to lactones 221-223, has been the subject of an interesting study by Flirstner s group, that revealed once more the very subtle and cooperative influence of different parameters on the stereochemical course of metathesis reactions. Thus, it was shown that the ElZ ratio obtained in an RCM step is not only dependent on the relative configuration of the cyclization substrate, but also on the chosen catalyst (Scheme 41). When applied to the... 

The metathesis reaction consists of a movement of double bonds between different molecules, as shown in Figure 1.4. Thus, the metathesis reaction can be addressed as a transalkylideneation reaction. The cleavage of the carbon-carbon double bonds was established using isotopic labelled compounds that were subjected to ozonolysis after reaction (9). 

The chiral Mo-alkylidene complex derived from AROM of a cyclic olefin may also participate in an intermolecular cross metathesis reaction. As depicted in Scheme 16, treatment of meso-72a with a solution of 5 mol % 4a and 2 equivalents of styrene leads to the formation of optically pure 73 in 57% isolated yield and >98% trans olefin selectivity [26]. The Mo-catalyzed AROM/CM reaction can be carried out in the presence of vinylsilanes the derived optically pure 74 (Scheme 16) may subsequently be subjected to Pd-catalyzed cross-coupling reactions, allowing access to a wider range of optically pure cyclopentanes. 

This work consists of an overview of the major developments in the alkene metathesis reaction since 1997. In view of the breadth of the subject area and the rapid pace of advancement in the field in recent years, this review is not intended to serve as a comprehensive survey, but rather as an account of how the development of novel catalyst systems has made a dramatic impact on the reaction in terms of scope and efficiency/selectivity. 

Ruthenium-based metathesis reactions have also been subject to a number of ESl-MS studies. We note that these systems have been studied extensively in the gas phase by Chen however, this work is beyond the scope of our review. Chen also pioneered the study of these systems in the condensed-phase using a charge-tagged substrate (see Section 1.1.3) [69],... 

The bridging chloride ligands in these [Ir(olefin)2Cl]2 compounds are susceptible to metathesis reactions, yielding new dimeric compounds of the form [Ir(olefin)2B]2 where B represents a new bridging ligand. AUcoxides, thiolates, and carboxylates have all been employed successfully in the replacement of chloride. The complexes with B = Br, I have also been prepared, both by metathesis reactions and by direct reaction of cyclooctene or cyclooctadiene with IrBrs or Iris The olefin complexes also provide excellent starting materials for the syntheses of arene and cyclopentadienyl iridium complexes, a subject that will be discussed in the next section. 

Metathesis, which is reversible and can be catalyzed by a variety of organometallic complexes, has been the subject of considerable investigation, and many reviews on this topic have been published.In 1970, Herisson and Chauvin proposed that these reactions are catalyzed by carbene (alkylidene) complexes that react with alkenes via the formation of metallacyclobutane intermediates, as shown in Figure 14-20. This mechanism, now known as the Chauvin mechanism, has received considerable support and is believed to be the pathway of the majority of transition metal-catalyzed olefin metathesis reactions. 

Glycals can also be obtained from suitable substrates, by ring-closing olefin metathesis reactions. In a general approach to variously linked C-disaccharides, illustrated in O Scheme 48, nonreducing-end glycals are typical intermediates, finally subjected to hydroboration or dihy-droxylation to afford the desired C-analogs of O-disaccharides [238,239]. 

First the lower C4-C10 a-olefins are stripped off. In a heavy ends column the C20+ a-olefins are removed from the desired C12-C20 a-olefins. Finally the middle-range products meeting the market needs are separated into the desired cuts and blends. The very high flexibility of the SHOP results from the following steps. The C4-CJ0 and the C2o+-fractions are combined to be isomerized to internal linear olefins (eq. (4)) and then subjected to a metathesis reaction (eq. (5)). Both steps require about 80-140 °C and 0.3-2 MPa. Isomerization is accomplished by a typical isomerization catalyst such as Na/K on AI2O3 or a MgO catalyst in the liquid phase [42], where about 90% of the a-olefins are converted to internal... 

During modei studies for the synthesis of botrydiane sesquiterpene antibiotics, B.M. Trost and co-workers prepared a compiex 1,6-enyne precursor for transition metal catalyzed enyne metathesis reactions. The 1,6-enyne was prepared from a heavily substituted alkynal, which was synthesized via the Eschenmoser-Tanabe fragmentation of an epoxy ketone. The resulting alkynal was unstable, so it was immediately subjected to a Wittig oiefination to afford the desired 1,6-enyne. 

R. L. Banks takes up the subject of olefin metathesis previously discussed by J. J. Rooney and A. Stewart in Volume 1 and gives an authorative review of the very substantial literature which has appeared in the last four years. Naturally his account covers both heterogeneous and homogeneous catalysis and he summarizes as well the industrial applications which have been made to date of metathesis reactions. S. Malinowski and J. Kijeriski review the specialist field of very highly basic catalysts largely developed by the work of the Polish school. In their chapter they discuss the evidence for the nature of catalysts such as alkali-treated magnesium and other oxides and the kind of reactions that take place thereon. J. M. Winterbottom in a chapter with emphasis on the literature since 1973 concentrates mainly on the dehydration of alcohols as the fundamental studies on dehydration far exceed those on hydration, which features mainly in the patent literature. His chapter dis-... 

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