Ring-closing metathesis polymerization

Figure 3 Chiral molybdenum alkylidene complexes used in asymmetric ring closing metathesis polymerization...

Scheme XXVI. The ring-closing metathesis polymerization of diethyl...

When a mixture of alkenes 1 and 2 or an unsymmetrically substituted alkene 3 is treated with an appropriate transition-metal catalyst, a mixture of products (including fi/Z-isomers) from apparent interchange of alkylidene moieties is obtained by a process called alkene metathesis. With the development of new catalysts in recent years, alkene metathesis has become a useful synthetic method. Special synthetic applications are, for example, ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROM) (see below). 

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

Ring-closing metathesis (RCM), 432, 434 Ring-opening metathesis polymerization (ROMP), 432, 435, 436 Ring-opening polymerization, 18,... 

The ruthenium catalyst system, 14, shown in Fig. 3, also carries out ADMET condensation chemistry, albeit with higher concentrations being required to achieve reasonable reaction rates [32]. The possibility of intramolecular compl-exation with this catalyst influences the polymerization reaction, but nonetheless, ruthenium catalysis has proved to be a valuable contributor to overall condensation metathesis chemistry. Equally significant, these catalysts are tolerant to the presence of alcohol functionality [33] and are relatively easy to synthesize. For these reasons, ruthenium catalysis continues to be important in both ADMET and ring closing metathesis chemistry. 

Scheme 10. Olefin metathesis RCM (ring closing metathesis), ROMP (ring opening metahesis polymerization), ADMET (acyclic diene metathesis), CM (cross metathesis).

A high-throughput colorimetric assay was applied to identify catalysts by combining metals (Pd, Rh, Ru, Ir) and various phosphines for the hydroamination of dienes.217 Combinatorial catalysis was successfully used to find active catalysts in the Ru-catalyzed ring-closing metathesis reaction218 and the olefin polymerization by Ni and Pd.219... 

SCHEME 1. Types of alkene and alkyne metathesis reactions. DBC, double bond cleavage TBC, triple bond cleavage ADMET, acyclic diene metathesis RCM ring-closing metathesis ROMP ring-opening metathesis polymerization... 

Atropisomeric biaryl bisphosphines, in hydrogenation, 10, 2 Atropisomerism, dynamic NMR studies, 1, 421 ATRP, see Atom transfer radical polymerization type reactions rj3-l-Azaallyl complexes, with nickel, 8, 158 6-Aza[3.2.1]bicyclooct-3-ene, via ring-closing metathesis,... 

Dienes are cyclized by intramolecular metathesis. In particular, cyclic alkenes 43 and ethylene are formed by the ring-closing metathesis of the a,co-diene 46. This is the reverse reaction of ethenolysis. Alkene metathesis is reversible, and usually an equilibrium mixture of alkenes is formed. However, the metathesis of a,co-dienes 46 generates ethylene as one product, which can be removed easily from reaction mixtures to afford cyclic compounds 43 nearly quantitatively. This is a most useful reaction, because from not only five to eight membered rings, but also macrocycles can be prepared by RCM under high-dilution conditions. However, it should be noted that RCM is an intramolecular reaction and competitive with acyclic diene metathesis polymerization (ADMET), which is intermolecular to form the polymer 47. In addition, the polymer 47 may be formed by ROMP of the cyclic compounds 43. 

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