Routes using olefin metathesis

Since ring-opening olefin metathesis polymerizations (ROMPs) [111, 112], yield polymers which contain at least one double bond in the backbone per monomer unit (Fig. 10-17), it is 

Durham polyacetylene was the Hrst processable polyacetylene, and its use has been taken up by a number of groups [41,115]. One physical property which distinguishes it from 

Shirakawa polyacetylene is its decreased crystallinity [103,116]. This is manifested not only by its diffractive properties, but also by a slightly higher energy absorption maximum in the solid state. It was found, though, that stretching the precursor polymer before elimination induced crystallinity and yielded a more Shirakawa-like polyacetylene. 

The most recent application of olefin metathesis to the synthesis of polyenes has been described by Tao and Wagener [105,117], They use a molybdenum alkylidene catalyst to carry out acyclic diene metathesis (ADMET) (Fig. 10-20) on either 2,4-hexadiene or 2,4,6-octatriene. The Wagener group had earlier demonstrated that, for a number of nonconjugated dienes [118-120], these polymerizations can be driven to high polymer by removal of the volatile product (e. g., 2-butene). To date, insolubility limits the extent of polymerization of unsaturated monomers to polyenes containing 10 to 20 double bonds. However, this route has some potential for the synthesis of new substituted polyacetylenes. Since most of the monomer unit is preformed before polymerization, it is possible that substitution patterns which cannot be incorporated into an alkyne or a cyclic olefin can be built into an ADMET monomer. 

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The direct metathesis polymerization of acetylenes is not the only route to polyacetylenes using olefin metathesis chemistry. Below are summarized some of the other methods that have been developed in recent years. 

Olefin metathesis, an expression coined by Calderon in 1967,1 has been accurately described in Ivin and Mol s seminal text Olefin Metathesis and Metathesis Polymerization as the (apparent) interchange of carbon atoms between a pair of double bonds (ref. 2, p. 1). This remarkable conversion can be divided into three types of reactions, as illustrated in Fig. 8.1. These reactions have been used extensively in the synthesis of a broad range of both macromolecules and small molecules3 this chapter focuses on acyclic diene metathesis (ADMET) polymerization as a versatile route for the production of a wide range of functionalized polymers. 

Olefin metathesis has proved to be a powerful synthetic tool in organic synthesis.5 The advent of well-defined metal carbene complexes with remarkable functional group tolerance has rendered metathesis as an efficient route to the synthesis of new C-C bonds. Examples of widely used ruthenium metathesis catalysts include [Ru-1],6 [Ru-2]7 and [Ru-3] 8 (Figure 1). 

Castanospermine is a polyhydroxylated alkaloid found in the plant Castanospermum australe.1 Its ability to function as a selective inhibitor of a and p glycosidases has made it the focal point of much synthetic activity in recent years.2 One particularly elegant synthesis of ( + )-castanospermine is that of Pandit and Overkleeft.3 It features a remarkable intramolecular olefin metathesis reaction for indolizidine ring assembly. We now analyse this interesting route, which showcases many important reagents and reactions used in contemporary organic synthesis. 

Interesting results have been obtained in the synthesis of biologically active compounds such as insect pheromones. Conventional synthetic routes to these pheromones are often multistep sequences, which make many pheromones too expensive for widespread use [23]. Metathesis offers a shorter, alternative route to pheromone synthesis, generating these compounds in a few steps only. The use of insect sex pheromones is an environmentally friendly, effective, and selective method of pest control. Kiipper and Streck [24] synthesized insect sex pheromones by cross-metathesis reactions between linear olefins. In the presence of the catalyst Re207/Al203, 9-tricosene was synthesized by cross-metathesis of the readily available aUcenes 2-hexadecene and 9-octadecene (Eq. 8). 

Olefin metathesis is a versatile reaction for the production of fine chemicals. Through metathesis, many different products, which are otherwise difficult to obtain, can be produced from readily available olefins in only a few reaction steps. With heterogeneous catalysts metathesis can be performed under mild reaction conditions and with high selectivity. Metathesis routes that use cheap raw material, such as esters from natural sources, and accessible heterogeneous catalysts are technologically viable. 

The olefin metathesis reaction clearly provides a route for the establishment of equilibrium between all species formed by exchange of alkylidene moieties. Thermodynamic data may therefore be used to predict the ultimate equilibrium position alternatively, experimental equilibrium concentrations may be used to provide new thermodynamic data and to check existing data. 

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