Types of metathesis reactions

In order to give perspective on the kinetic versus thermodynamic balance, the cis-trans selectivity of some commonly utilized Ru metathesis catalysts is presented. A number of catalysts with modified ligands that result in a distinct stereochemical preference are then compared with these original catalysts and their reactivity discussed. Finally, the successful implementation of ligand-driven selectivity has led to three families of Ru-based metathesis catalysts that can perform Z-selective metathesis. For each of these catalyst families, a model for the origin of Z-selectivity, the role of ligands in influencing stereochemistry and trends in their reactivity are examined. 

In a wide variety of cases, application of this model allows the desired cross product to be generated in high yield. In order to generate the product as a single stereoisomer, an understanding of the relationship between catalyst structure in Ru-based catalysts and cis-trans selectivity is necessary. This requires a closer examination of the olefin metathesis mechanism. 

In both side-bound and bottom-bound metallacycle orientations, disubstituted metallacycles can adopt one of two stereochemical configurations one where the substituents are syn to each other, which would lead to formation of the Z-alkene and 

It is expected that ligand-induced asymmetry with respect to the two faces of the metallacyde will have the most discernible influence on the syn-anti preference of disubstituted metallacycles. These effects will only be evident, however, if ligand rotations with respect to the metallacyde plane are slower than either productive 

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During our efforts in profiling (comparative investigations) of several commercial available metathesis catalysts bearing NHC ligands in different types of metathesis reactions remarkable solvent effects were observed [4], Interestingly, the efficiency of most transformations studied frequently depended more on solvent and temperature effects rather than on the nature of Ru precursor and NHC ligands. 

Several fundamental types of metathesis reactions for monoolefins or diolefins are shown in Eqs. 2-5. 

As will be discussed more thoroughly in Section 3.2.5, transition metal carbene complexes can mediate olefin metathesis. Because heteroatom-substituted carbene complexes are usually less reactive towards olefins than the corresponding nonheteroatom-substituted complexes, it is, e.g., possible to use enol ethers to terminate living polymerization or other types of metathesis reaction catalyzed by a non-heteroatom-substituted carbene complex. Olefin metathesis can also be used to prepare new heteroatom-substituted carbene complexes (Figure 2.15, Table 2.11). 

One of the three types of metathesis reactions is driven by the production of an insoluble solid. While we are going to go much more in depth about solubility in Chapter 15, there are some basic rules you can learn now that will provide you with more than enough information to predict the products of chemical reactions. These basic rules, usually referred to as solubility rules, are listed in Table 11.5. When reading the chart, keep in mind that all the solutions listed are aqueous solutions (water is the solvent). The term insoluble is not exactly correct. 

There are three main types of metathesis reactions, all distinguished by the products. One forms a precipitate, another a nonelectrolyte, and the third a gas. 

Figure 7.10 Different types of metathesis reaction. The first two have found significant use in industry.

A second type of metathesis reaction involves metals multiply bonded to nonmetals (equation 68). In one sense, this corresponds to a special case of dinuclear reductive elimination of neutral molecules in which multiply bonded species are involved. This reaction can occur in both directions. Multiply bonded metal-metal complexes can be cleaved by multiply bonded main group species. 

Recognize and describe classes of reactions decomposition reactions, displacement reactions, and various types of metathesis reactions... 

Metathesis reactions result in the removal of ions from solution this removal of ions can be thought of as the driving force for the reaction—the reason it occurs. The removal of ions can occur in three ways, which can be used to classify three types of metathesis reactions ... 

The formation of an insoluble or slightly soluble gas provides a driving force for a third type of metathesis reaction that we call a gas-formation reaction. The only common gases that are very soluble in water are HCl(g) and NH3(g). All other gases are sufficiently insoluble to force a reaction to proceed if they are formed as a reaction product. 

The fifth class of olefin metathesis in Scheme 21.1 is cross-metathesis. Selective cross metathesis is a developing type of metathesis reaction. Expulsion of ethylene usually provides the driving force for cross metathesis. In principle a statistical mixture of olefins would be formed by cross metathesis, as shown in the first reaction of Scheme 21.1. However, kinetic preferences for the reaction of a hindered carbene complex with an unhindered olefin or a kinetic preference for the reaction of an electron-rich carbene with an electron-poor olefin can provide the selectivity needed for the formation of one olefin over other potential homo- and cross-metathesis products. 

Olefin metathesis has proven to be useful for the synthesis of an enormous variety of unsaturated compounds, including cyclic, acyclic, and polymeric alkenes. The studies discussed in previous sections focused on fundamental reactivity patterns that apply to all types of metathesis reactions, from cross-metathesis to polymerizations. With many of the basic mechanistic features established, many... 

In the J. Heyrovsk Institute we performed a detailed study of (a) M0O3 supported on siliceous mesoporous sieves of several types and (b) rhenium (VII) oxide supported on organized mesoporous aluminas in order to (i) verify positive effect of these supports on catalyst activity, (ii) to find out the most convenient way for catalyst preparation, (iii) to receive detailed information about their catalytic activity and selectivity, and (iv) to show the applicability of these catalysts in different types of metathesis reaction. 

Six types of metathesis reactions are known, and all of them can be catalyzed by Schroek and Grubbs metathesis catalysts. They can lead to extensive organic transformations including the synthesis of low-dispersity polymers. ... 

RCM Reaction for the Formation of a Medium-Sized Ring System Among various types of metathesis reactions, the RCM reaction has been widely used by synthetic organic chemists for constructing small, medium. 

Alkylidene complexes of tantalum and ruthenium were discovered in the late 1970s and early 1990s, respectively. Following these discoveries, many new fuUy characterized alkylidene complexes were shown to be efficient precatalysts for metathesis reactions. In the following sections we discuss the different types of metathesis reactions and their mechanisms. 

Many different types of metathesis reactions are known. Reaction... 

Two other types of metathesis reactions have also been found to be very useful for the manufacture of specialty polymers, and syntheses of a wide variety of organic compounds. These are ROMP and ring-closing metathesis (RCM) reactions. General representations of ROMP and RCM are shown by reactions 7.3.1.3 and 7.3.1.4, respectively. 

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