Types of Metathesis

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. 

As shown by reaction 7.3.1.5, apart from Vestenamer , another example of the industrial application of ROMP is the conversion of norborene to Norsorex . Conversion of 1,6-heptadiene to cyclopentane and ethylene, as shown by reaction 7.3.1.6, is an example of an RCM reaction. 

It should be noted that many monomers used in ROMP have strained structures. On polymerization, the excess strain energy of the monomer is released. This makes ROMP a thermodynamically favorable reaction. 

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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). 

Metathesis reactions are now widely used in natural product synthesis. Novel retrosynthetic analyses were developed because a carbon-carbon single bond can be formed after hydrogenation of a double bond constructed by metathesis. Although many types of metathesis are now known, the reaction is classified by olefin, enyne, and alkyne metatheses in this chapter. 

Metal-metal bonding is also known between different metals. Me3Sn—Fe(TPP) made from [Fe (TPP)]- is diamagnetic and probably contains an Sn,v—Fe" bond.194 123 Reaction of anionic metal carbonyls and In(Por)Cl gives metal-bonded complexes which show characteristic hyper-type spectra (L., 440 nm) (Scheme 26).124 The precedent of this type of metathesis is the unexpected formation of (TPP)Sn—Mn(CO)4—Hg—Mn(CO)s from Sn(TPP)Cl2 and sodium amalgam reduced [Mn(CO)5]. 125 In this compound, the linear Mn—Hg—Mn moiety is bonding to the Sn atom through the tetracarbonylated Mn and parallel to the macrocycle (Scheme 27). 

This chapter concerns the preparation of macrocyclic products by ring-closing metathesis (RCM) or related processes combining RCM with other types of metathesis, starting from suitably substituted diene, ene-yne, or diyne precursors. Macro-cyclic rings of 10 or more members have been taken in consideration. Such macrocycles can exist individually or as part of a polycyclic system of the bridged, fused, or ansa type. 

Along with diene and diyne metathesis, ene-yne metathesis has also been employed to form macrocycles. This type of metathesis is performed with the catalysts used for olefin metathesis, and the yields are improved in the presence of ethylene, which forms the highly reactive [Ru]=CH2 species. Shair and coworkers took advantage of this reaction twice in the course of their total synthesis of longithorone A [40]. When ene-ynes 51 and 52 are treated with ruthenium complex G1 under an atmosphere of... 

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. 

Polymers with carbon-carbon double bonds in their backbone can undergo two types of metathesis, both leading to degradation. In an intramolecular reaction cyclic oligomers are formed, while many unsaturated polymers can be degraded by intermolecular cross-metathesis with low-molecular-weight olefins. Identification of the degradation products provides valuable information on the microstructure of the polymer [7] (cf Section 3.3.10.1). 

A type of metathesis that does not involve redox change is illustrated by the [V(0)(0Pr-i)2(NEt2)] -f CS2 reaction, which gives [V(0)(SCSNEt2)3] and [V(0)(0Pr-O3] ". 

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. 

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