Ruthenium carbene complexes norbomene

Cross-metathesis of trialkoxy- and trisiloxy-substituted vinylsilanes [21] as well as octavinylsilsesquioxane [15] with vinyl sulfides proceeds efficiently but only in the presence of the 2nd generation Grubbs catalysts (IV) to offer a new and very attractive route for syntheses of [alkyl(aiyl)]sulfide-substituted vinylsilanes and vinyl-silsesquioxane with high preference for the -isomer, as illustrated by exclusive isolation of such isomers. The Fischer-type ruthenium carbene complex Ru(=CHSPh)Cl2(PCy3)2 has recently been reported as an effective catalyst in the ring opening/cross-metathesis of norbomene derivatives with vinyl sulfide [22], suggesting that these carbenes can be reactive in the cross-metathesis. 

Studies of this kind fall broadly into two groups as indicated in Table 15.1. Those labelled P are mainly concerned with the effect of small quantities of acyclic olefin M2 on the MW, yield, and cis content of the polymer produced from the cyclic olefin Mi in this case the ratio [M2]/[Mi] is usually 0-0.05. Those labelled T involve the use of much higher proportions of M2 (with [M2]/[Mi] 0.5-2), with the object of producing telomers with well-defined end-groups, in some cases for synthetic purposes. For example, highly substituted cyclopentane and tetrahydro-fiiran derivatives can be readily made by the cross-metathesis of substituted norbomenes or 7-oxanorbomenes with hex-3-ene or l,4-dimethoxybut-2-ene catalyzed by ruthenium carbene complexes (Schneider, M.F. 1996). If the acyclic olefin is unsymmetrical and represented by Q Q, where Q and are the alkylidene moieties, three series of telomers may be produced, Q (Mi) Q, Q (Mi) Q, Q (Mi) Q, where ti is the number of Mi units. The lowest members of the series (n= 1) are dienes, and it is sometimes possible to deteet, separate, and identify the cc, ct, and tt isomers. 

Again, utilisation of the N,N,C pincer framework results in the corresponding ruthenium complex. The C,N,C ruthenium(ll) pincer carbene complex showed some activity in the olefin metathesis of norbomene, norbomadiene (catalyst loading 1 mol %), 1,5-cod, cyclooctene and diallylmalonate (catalyst loading 10 mol %). The N,C,N ruthenium(II) pincer carbene complex was not tested. 

Turnover frequencies for a given ROMP catalyst are very dependent on the monomer type. Table 10. For example using 5 as catalyst, NBE polymerizes ca. 700 times faster than DCPD, 4000 times faster than the 2-norbomene-5-tert-butylester (NBE-TBE) and >10" times faster than the 2-norbornene-5,6-dimethyldiester (NBE-DME). The reason for this large reactivity differences of ruthenium carbenes towards different monomers is not yet completely understood but steric constraints and complexation of the catalyst with certain functional groups of the monomer (ester groups, double bonds, etc.), which is in large excess relative to the catalyst, may play an important role. 

Heterobimetallic ruthenium alkylidenes, which may be prepared by reaction of Cl2Ru(PCy3)2(CHR) with [Ru(/j-cymene)Cl2]2, [Os(/j-cymene)Cl2]2, and [Rh(terf-butylcyclopentadienyl)Cl2]2, respectively, were reported to possess significantly elevated activities in the ROMP of 1,5-cyclooctadiene (COD) and 2,2-bis(trifluormethyl)norbomene. The most active compounds obtained so far are the bimetallic complexes (/j-cymene)RuCl(M-Cl)2RuCl(CHPh)(NHC) and (Cp )RhCl( -Cl)2RuCl(CHPh)(NHC) (NHC = N-het-erocyclic carbene, Cp = pentamethylcyclopentadi-enyl) (Scheme 23). It is worth mentioning that... 

The second step involved the ROMP of the functionalised PEO macromer using the well-defined Schrock (hexa-fluorinated) or Grubbs initiating complexes. Each initiator had a metal core (molybdenum or ruthenium) with bulky ligands that controlled the approach and co-ordination of the reactive carbene bond to the double bond of the norbomene group... 

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