Cyclic alkenes, polymerization

RING-OPENING METATHESIS POLYMERIZATION OF CYCLIC ALKENES... 

Since one of the substrates is a cyclic alkene there is now the possibility of ring-opening metathesis polymerisation (ROMP) occurring which would result in the formation of polymeric products 34 (n >1). Since polymer synthesis is outside the scope of this review, only alkene cross-metathesis reactions resulting in the formation of monomeric cross-coupled products (for example 30) will be discussed here. 

The ring-opening metathesis polymerization (ROMP) of cyclic alkenes yields polymers that still contain all double bonds [Eq. (13)]. 

Dienes. The bifunctionality of dienes makes their hydroboration more complex than that of simple alkenes. Competing hydroboration of the two double bonds may lead to mixtures of products arising from mono- or diaddition. Additionally, cyclic or polymeric organoboranes may be formed. Differences in the reactivity of the two double bonds and the use of appropriate hydroborating agents, however, may allow selective hydroboration.29,330... 

Cyano compounds liquid crystals, 12, 278 in silver(III) complexes, 2, 241 Cyanocuprates, with copper, 2, 186 Cyano derivatives, a-arylation, 1, 361 Cyanosilanes, applications, 9, 322 Cyclic acetals, and Grignard reagent reactivity, 9, 53 Cyclic alkenes, asymmetric hydrosilylation, 10, 830 Cyclic alkynes, strained, with platinum, 8, 644 Cyclic allyl boronates, preparation, 9, 196 Cyclic allylic esters, alkylation, 11, 91 Cyclic amides, ring-opening polymerization, via lanthanide catalysis, 4, 145... 

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. 

R and R refer to organic groups that contain some number of carbon atoms and may or may not be different from each other. With cyclic alkenes, metathesis leads to polymerization (specifically, ring-opening metathesis polymerization [ROMP]), shown here for cyclopentene ... 

When cyclic alkenes are utilized as starting materials the metathesis reaction will lead to long chain polymers and/or cyclic oligomers [103, 104, 107, 108]. If a strained cyclic alkene is employed the reaction is effectively irreversible. Industrially cyclooctene (polymer Vestenamer), 2-norbornene (polymer Norsorex), and dicyclopentadiene (polymer Telene, Metton, Pentam) are used as monomers. Upon polymerization cyclooctene and 2-norbornene yield straight chain polymers while dicyclopentadiene also allows cross-linking (Scheme 5.56). 

Unsaturated polymers can be produced by means of ring-opening metathesis polymerization (ROMP) of cyclic alkenes. These unique polymers can also be produced via intermolecular Acyclic Diene Metathesis (ADMET). Dienes can also react intramolecularly via Ring Closing Metathesis (RCM) to afford cyclic products. RCM is often applied to synthesis of compounds for fine chemical and pharmaceutical application. Generic examples of these reactions are shown in Figure 2. 

In the context of ROMP chemistry, living polymerization reaction conditions have only been observed when well-defined carbene complexes are used as the catalysts. The first catalyst to behave in this fashion was the titanocene complex (4), while more recently, complexes containing Ta, W, and Mo have been shown to be catalysts for the living ROMP of a variety of cyclic alkenes. The Mo complex (5b) is an especially promising catalyst since it is compatible with a number of functional groups and thus can be used to synthesize a variety of functionalized polymers. 

W(NAr)(=CHBuO(OR)2 (R = CMe(CF3)2, 50) are highly active catalysts for the metathesis of internal alkenes (equation 16), and also effect the stereoselective olefmation of hydroxy ketones (equation 17). The reactivity of these catalysts can be tuned by varying the aUcoxide ligands for example, when R = Bn, the complex acts only upon strained cyclic alkenes and is a highly effective ring-opening metathesis polymerization (ROMP) catalyst (see Metathesis Polymerization Processes by Homogeneous Catalysis). 

The alkene metathesis reaction represents a unique carbon skeleton rearrangement in organic chemistry. This reaction, which grew out of the studies of catalysts related to alkene polymerization, results in the cleavage of the carbon-carbon double bond of an alkene and the redistribution of the resulting alkyl-idene fragments (equation 1). The same reaction carried out with cyclic alkenes generally results in the formation of polymers (equation 2). 

Related molybdenum catalysts appear to show even more functional group tolerance. To date, the major test of functional group compatibility has been in the synthesis of polymers however, it is anticipated that this activity will persist into acyclic metathesis. Later transition metals are active in the metathesis polymerization of highly functiondized cyclic alkenes. These catalyst systems, which appear to tolerate almost all functional groups, show very low activity for acyclic alkene metathesis. If these systems can be activated, the problems associated with the use of alkene metathesis in the synthesis of multifunctional organics will be solved. 

Ring-opening metathesis polymerization (Problem 26.28) An olefin metathesis reaction that forms a high molecular weight polymer from certain cyclic alkenes. 

Acyl halides have been added to many alkenes, in the presence of Friedel-Crafts catalysts, although polymerization is a problem. The reaction has been applied to straight-chain, branched, and cyclic alkenes, but to very few containing functional... 

In the course of time it appeared that many olefinic substrates could undergo this reaction in the presence of a transition metal compound, such as substituted alkenes, dienes, polyenes, and cyclic alkenes, and even alkynes. Calderon et al. were the first to realize that the ring-opening polymerization of cycloalkenes, which they observed with their tungsten-based catalyst system [4], and the disproportionation of acyclic olefins are, in fact, the same type of reaction. They introduced the more general name metathesis [2], The metathesis reaction has now become a common tool for the conversion of unsaturated compounds. In view of the limited space this intriguing reaction is reviewed only briefly more information can be found in a detailed and extensive monograph [5]. 

The ring-opening metathesis polymerization (ROMP, cf. Section 2.3.3) of strained-ring cyclic alkenes has attracted considerable attention in recent years due to the discovery that well-characterized metallacyclobutane [24] and metal alkylidene [25] complexes catalyze the living polymerization of monomers such as norbomene. Unfortunately, these catalysts often sulfer deactivating side reactions... 

---