CARBENES, METATHESIS, AND POLYMERIZATION

We now look in detail at compounds with multiple bonds between metal and ligand. We are chiefly concerned with multiple bonds to carbon, as in metal-carbene complexes L M=CR2, which have a trigonal planar carbon and at least formally contain an M=C double bond, and metal-carbyne complexes, L MsCR, which are linear and contain an M=C triple bond, but we also look at complexes with multiple bonds to O and N. 

The simplest carbene is methylene, CH2, which has an sp hybrid orbital and a p orbital in addition to the two C—H bonds. As a 6e species, CH2 has 4e in the two C—H bonds and therefore two electrons remain to be placed in the sp and p orbitals on carbon. We will consider that both electrons are placed in the lower-lying sp orbital to give a singlet carbene, leaving the p orbital empty (see Fig. 11.1a). In the free carbene, the triplet state, where the two unpaired electrons have parallel spins, is also important, however. 

FIGURE 11.1 The relative energies of the M( /.) and the Cip ) orbitals control the electrophilic or nucleophilic character of the carbene. (a) The orbitals of free CH2. (6) If the M(d,) levels are lower in energy, a Fischer carbene will result, (c) If the M(d,) levels are higher in energy, a Schrock carbene will result. Shading represents occupied orbitals. 

M=C(Hal)2 because the halide has intermediate ir-donor strength between H and —OMe.  

The IT bond in the Schrock case is polarized in the M+—C direction, because the M(d ) levels now lie above the C(p ) level. One way of looking at this is to say that the two electrons originally in M(d ) transfer to the more stable C(p ) orbital, oxidizing the metal by two units and giving a CJc ligand. The system can therefore be seen as a metal-stabilized carbanion acting as both a a donor and a it donor to the metal, not unlike phosphorus ylidS such as PhjP+—CH . This oxidation of the metal translates into the Schrock 

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Updated and expanded coverage of the latest developments from the field, including IR, NMR, and mass spectroscopy catalysis carbene complexes metathesis and polymerization and applications to organic synthesis. 

Chapter 10 in the first edition covered metal carbene complexes, metathesis, and polymerization reactions. The chapter has now been split into two chapters. Chapter 10 now emphasizes the chemistry of carbene complexes new material on /V-heterocyclic carbene complexes, with applications in synthesis, has been introduced. Chapter 11 now considers metathesis and polymerization. The sections on the discovery and elucidation of n-bond metathesis have been rewritten and expanded. The discussion of both metathesis and Ziegler-Natta polymerization reactions has been considerably enhanced and brought up to date. 

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

According to this mechanism, olefin metathesis is a chain reaction with the carbene as chain carrier. It predicts complete randomization of alkylidene fragments from the first turnover. An important additional feature is that both metathesis and ringopening polymerization of cycloalkenes can be explained by this mechanism, which provides ready explanation of polymer molecular weight. 

In the previous chapters we discussed alkene-based homogeneous catalytic reactions such as hydrocarboxylation, hydroformylation, and polymerization. In this chapter we discuss a number of other homogeneous catalytic reactions where an alkene is one of the basic raw materials. The reactions that fall under this category are many. Some of the industrially important ones are isomerization, hydrogenation, di-, tri-, and oligomerization, metathesis, hydrocyana-tion, hydrosilylation, C-C coupling, and cyclopropanation. We have encountered most of the basic mechanistic steps involved in these reactions before. Insertions, carbenes, metallocycles, and p -allyl complexes are especially important for some of the reactions that we are about to discuss. 

Metal-carbene complexes undergo numerous reactions, several of which are useful in the synthesis of complex organic molecules. These complexes are also intermediates in processes such as metathesis and ring-opening metathesis polymerization (Chapter 11). In Chapter 10 we will discuss the structure, synthesis, and... 

Metals of Groups 5 and 6 (Nb, Ta, Mo and W) are known to form carbene complexes and are widely used in olefin metathesis [99, 100, 111]. Therefore, the polymerization of substituted alkynes with catalysts based on these metals is assumed... 

Reactivity characteristic of alkylidene complexes of tantalum is that the a-carbon is susceptible to electrophilic attack, in contrast to the electron-deficient a-carbon of Fischer-type carbene complexes of group 6 transition metals [62]. Based on this unique property of the alkylidene metal-carbon double bond, a range of new types of reactions has been developed. The discovery of the alkylidene complexes of tantalum was a key to understanding the mechanism of olefin metathesis, and they continue to play important roles in C—H bond activation, alkyne polymerization, and ring-opening metathesis polymerization. 

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