Ring-opening metathesis catalysts

The use of ROMP has been widely applied. Thus, cyclopentene can be polymerized by olefin metathesis to a linear pol5mier. In this reaction the cyclic hydrocarbon is opened up by the catalyst and joined together in a linear fashion (see Eq. 2.34). The catalysts that perform this operation are called ring-opening metathesis catalysts. 

Aqueous ring-opening metathesis polymerization (ROMP) was first described in 1989 (90) and it has been appHed to maleic anhydride (91). Furan [110-00-9] reacts in a Diels-Alder reaction with maleic anhydride to give exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3—dicarboxylate anhydride [6118-51 -0] (24). The condensed product is treated with a soluble mthenium(Ill) [7440-18-8] catalyst in water to give upon acidification the polymer (25). Several apphcations for this new copolymer have been suggested (91). 

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). 

Ring-opening metathesis polymerization (ROMP) of 1,4-cyelooctadiene was used to prepare poly(l,4-B) terminated with halo end groups.647 This was then used as a macroinitiator of ATRP with heterogeneous Cu bpy catalysts to form PS- >/ti /r-poly(l,4-B)-WoeA -PS and PMMA-Moc.T-poly(l,4-B)-Wof A-PMMA. 

Catalysts with an unsymmetrical NHC ligand featuring a vinylic side chain have the unique ability to metathesise their own ligand to form a metaUacycle as shown in Scheme 3.7 [119], Ring opening metathesis will then incorporate the monomers, e.g. cyclooctene, into that cycle until a cyclic polymer is cleaved by another intramolecular metathesis step. The catalyst is recovered and can restart this endless route to cyclic polymers [121]. 

Wolfe and Wagener have developed main-chain boronate polymers (59) (Fig. 38) by the acyclic diene metathesis (ADMET) polymerization of symmetrical ,oj-dienes, containing both methyl- and phenyl-substituted boronate functionalities using Mo and Ru catalysts.84 The ring-opening metathesis polymerization (ROMP) of several norbornene monomers containing methyl- and phenyl-substituted boronates into... 

In this chapter I will cover only well-defined or well-characterized compounds. Results will be included that have appeared since reviews in 1991 on alkylidene and metalacyclobutane complexes [41] and in 1993 on ring-opening metathesis polymerization [30], but an overview of prior results that are especially relevant to olefin metathesis in particular will also be included. (An excellent and comprehensive text also has been published recently [1].) The terms well-defined or well-characterized originally were meant to imply that the alkylidene complex is isolable and is essentially identical to that in a catalytic reaction except for the identity of the alkylidene. These terms have been watered down from time to time in the literature, even to the point where they are used to describe a catalyst that is formed from a well-characterized transition metal precursor complex, but whose identity actually is not known. In this article I... 

Many of the studies concerning ring-opening metathesis by well-characterized metathesis catalysts have employed substituted norbornenes or norborna-dienes. Substituted norbornenes and norbornadienes are readily available in wide variety, and they usually react irreversibly with an alkylidene. Norbornene itself is the most reactive, and the resulting polynorbornene probably is the most susceptible to secondary metathesis. Formation of polynorbornene often is used as the test reaction for ROMP activity. ROMP by well-defined species has been reviewed relatively recently [30], so only highlights and selected background material will be covered here. 

Another piece of mechanistic evidence was reported by Snapper et al. [14], who describe a ruthenium catalyst caught in action . During studies on ring opening metathesis, these authors were able to isolate and characterize carbene 5 in which a tethered alkene group has replaced one of the phosphines originally present in Id. Control experiments have shown that compound 5 by itself is catalytically active, thus making sure that it is a true intermediate of a dissociative pathway rather than a dead-end product of a metathetic process. 

It has been shown that [(r]6-arene)RuCl2]2 6 and [(r 6-arene)RuCl2] PR3 7 complexes can be activated in situ to afford active metathesis catalysts, either on treatment with diazoalkanes [15] or by UV irradiation [16]. The structure of the active species thus formed is unknown, but it initiates the ring opening metathesis polymerization reactions (ROMP) of various cycloalkenes very efficiently. Therefore these in situ recipes may also be useful in the context of preparative organic chemistry. 

Rolling ball viscometers, 21 738 Roll mills, 16 722 Rolls, high pressure, 16 612-613 Romanechite, 15 540 Romascone, 24 571 Romet-30/Romet-B. See Ormetoprim ROMP catalysts, 17 707, 708 ROM polymers. See also Ring-opening metathesis (ROM) polymers, 26 947-948 Roof coatings... 

Multiphase homogeneous catalysis (continued) hydroformylation, 42 483-487, 498 hydrogenations, 42 488-491 metal salts as catalysis, 42 482-487 neutral ligands, 42 481 82 organic reactions, 42 495 0X0 synthesis, 42 483-487 ring-opening metathesis polymerization and isomerization, 42 492-494 telomerizations, 42 491-492 diols as catalyst phase, 42 496 fluorinated compounds as catalyst phase, 42 497... 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

Although NHCs have to be considered as even more electron-donating than trialkylphosphines, the catalytic performance of 53 turned out to be just comparable to 52, but not significantly better. It depends on the substrate whether the one or the other catalyst performs slightly better in ring opening metathesis polymerization (ROMP). 

Attempted polymerization of 16, 17, and 18 by ring-opening metathesis polymerization (ROMP) was carried out using two different catalysts ... 

It is worth noting that 6,7-dihydro-2(3//)-oxepinone is an unusual lactone because it can be polymerized by two distinct mechanisms ROP of the cyclic esters by aluminum alkoxides, and the ring-opening metathesis polymerization (ROMP) of endocyclic olefins by the Schrock s catalyst (Fig. 28) [121]. 

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