Olefin cyclic, metathesis

All the above cascade alkene metathesis reactions are based on the ROM of a cycloalkene moiety. Harrity and co-workers have described the synthesis of functionalized spiro cyclic systems by cascade selective olefin ringclosing metathesis reactions from an acyclic tetraalkene. The selectivity for five-membered ring closure over seven-membered ring closure would be the result of a kinetically favored cyclization process [42] (Scheme 20). The syn-... 

Metathesis-type reactions of acetylenes fall into two categories true metathesis, in which the triple bond is completely broken, and olefin-type metathesis, in which only two of the three bonds are broken. The latter results in polymerization and the formation of cyclic oligomers, especially trimers. The overall reactions are illustrated by eqns. (l)- 3). 

Okada Y, Chiba K (2011) Electron transfer-induced four-membered cyclic intermediate formation olefin cross-coupling vs.olefin cross-metathesis. Electrochim Acta 56 1037-1042... 

Stereospecific ring-opening polymerization of cyclic olefins by metathesis catalysts... 

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. 

Monomers 24 and 25 behave differently when exposed to catalyst 14, shown in Fig. 8.15. Divinyltetramethyldisiloxane 24 is found to be metathesis inactive due to similar steric inhibitions experienced with divinyldimethylsilane. Monomer 25 is synthesized with one additional methylene spacer unit between the silicon atom and the olefin moiety, which then is reacted with Schrock s [Mo] catalyst. Here, metathesis occurs quite readily, exclusively forming a seven-membered cychc molecule (26) instead of polymer. The formation of the cyclic product can be explained by tire Tliorpe-Ingold effect.15... 

Diol-functionalized telechelic polymers have been desired for the synthesis of polyurethanes however, utilizing alcohol-functionalized a-olefins degrades both 14 and 23. Consequently, in order for alcohols to be useful in metathesis depolymerization, the functionality must be protected and the oxygen atom must not be /3 to the olefin or only cyclic species will be formed. Protection is accomplished using a/-butyldimcthylsiloxy group, and once protected, successful depolymerization to telechelics occurs readily. 

In 2001, Furstner reported the preparation and characterisation of the NHC-Ru complex 22 containing iV,iV -bis[2,6-(diisopropyl)phenyl]imidazolidin-2-ylidene (SIPr) [29] (Fig. 3.6), which is the congener of complex 20. Subsequently, Mol and co-workers revealed that complex 22 was a highly active metathesis initiator [30]. More recent comparative studies showed that catalyst 22 could catalyse the RCM of 1 faster than any other NHC-Ru catalyst, while it was not stable enough to obtain complete conversion in the RCM of 3 and was inefficient for the construction of the tetrasubstituted double bond of cyclic olefin 6 [31]. 

ROMP is without doubt the most important incarnation of olefin metathesis in polymer chemistry [98]. Preconditions enabling this process involve a strained cyclic olefinic monomer and a suitable initiator. The driving force in ROMP is the release of ring strain, rendering the last step in the catalytic cycle irreversible (Scheme 3.6). The synthesis of well-defined polymers of complex architectures such as multi-functionaUsed block-copolymers is enabled by living polymerisation, one of the main benefits of ROMP [92, 98]. 

The application of olefin metathesis to the synthesis of piperidines continues to be widely employed. The use of ring closing metathesis (RCM) in the synthesis of fluorovinyl-containing a,P-unsaturated lactams 148 and cyclic amino acid derivatives 149 is shown below. A key improvement in these reactions is the addition of the Grubbs 2nd generation catalyst (G2) in small portions during the reaction to compensate for catalyst decomposition that occurs at elevated reaction temperatures <06EJOl 166>. 

The diverse steric course of the metathesis of cyclic vs. acyclic olefins poses a dilemma. It stands to reason that once a cycloolefin monomer... 

A variety of cyclic olefins (5-, 6-, and 7-membered) that contain nitrogen have been prepared via ring-closing metathesis, for example as shown in Eq. 33 [209]. Other examples are shown in Eqs. 34 [210] and 35 [211]. A variety of pyrrolizi-dines, indolizidines, quinolizidines, pyrrolidinoazocines, piperidinoazocines, and other fused nitrogen heterocycles have also been prepared via ROM (e.g., Eq.36 [212,213]). 

Grubbs has reported a similar tandem olefin metathesis-carbonyl olelination process for the preparation of cyclic olefins [31]. In this case, treatment of a keto-olefin with the molybdenum alkylidene 1 at 20°C generates an intermediate alkylidene complex. Under these conditions, competing intermolecular olelination does not occur. However, intramolecular carbonyl olelination of the initially formed alkylidene complex can occur and this results in the formation of a cyclic olefin. This tandem sequence is illustrated by the transformation of keto-olefins... 

Scheme 17. Titanium-mediated metathesis strategy for the conversion of olefinic esters (118) to cyclic enol ethers (123) (Nicolaou et al.) [34]...

Scheme 19. Titanium-mediated metathesis strategy for the conversion of olefinic esters to 6- and 7-mem -bered cyclic enol ethers, (a) 4.0 equiv of Tebbe reagent (93), THF, 25°C, 20 min then reflux, 2-8 h, 64% (129), 45% (131a), 32% (131b), 45% (133) (Nicolaou etal.) [34a]...

Catalytic ring-closing metathesis makes available a wide range of cyclic alkenes, thus rendering a number of stereoselective olefin functionalizations practical. The availability of effective metathesis catalysts has also spawned the development of a variety of methods that prepare specially-outfitted diene substrates that can undergo catalytic ring closure. The new metathesis catalysts have already played a pivotal role in a number of enantioselective total syntheses. 

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