Grubbs initiator

With the C12,C13 disconnection producing an effective solution to the synthesis of epothilone A (4), it would seem likely that the metathesis approach could be extended readily to the preparation of epothilone B (5). However, installation of the desired C12 methyl group requires ring-closure of a diene precursor in which one of the olefins is disubstituted. Recently, such reactions have been shown to be problematic for Grubbs initiator 3 but more successful with Schrock s molybdenum initiator 1 [19]. Consistent with these reports, Danishefsky demonstrated that triene 38 would not undergo RCM with 3, whereas 1 was effective in promoting the transformation of 38 into a 1 1 mixture of 39a and 39b in good yield [14b] (Scheme 8). 

Better control of the reaction is achieved by using stable, isolable metal-carbene complexes, such as the Schrock initiators based on molybdenum and tungsten (LXIV, Mt = W, Mo) and the Grubbs initiators based on mthenium (LXV). The Schrock initiators... 

ROMP of the SCLCP block was carried out with the Grubbs -initiator 6 in a monomer to initiator ratio of about 25. The polymerization was terminated by reaction with 4-(2-bromopropionyloxy)-but-2-enyl 2-bromopropionate, leading to a macroinitiator for ATRP. The radical polymerization was carried out with CuCl, 4,4 -di(n-nonyl)-2,2-bipyridine, and butylacrylate (BA), giving a poly-XXVII-fo-BA copolymer. The homopolymer of poly-XXVII had a narrow PDI of 1.06, while the diblock copolymer showed a PDI of 1.32. 

AcycUc diene metathesis polymerization (ADMET) is a related polymerization in which an unconjugated diene polymerizes with loss of ethene [Lehman and Wagener, 2002, 2003 Schwendeman et al., 2002]. ADMET is carried out using the Schrock and Grubbs initiators at about 40-80°C. The process is a step polymerization, not a ROP chain reaction. The reaction is reversible, and high pol3Tner MW is achieved by removal of ethene (usually by reduced... 

Allaert and Verpoort et al. reported on Schiff-base-substituted Ru compounds [94] and second-generation Grubbs initiators [95], and their use for the ROMP of ds-cyclooctene [96] and NBE [97] (Figure 19.6). However, the reactivity was lower than that of the parent second-generation catalyst. 

Grubbs eial. reported on the generation of a ROMP-active, first-generation Grubbs initiator-derived catalytic species from an inactive precursor via chloride... 

Scheme 19.9 UV-triggered formation of a first-generation Grubbs initiator.

A systematic procedure based on norbornene, norbornadiene and their 7-methyl derivatives is described for the determination of the cis content, blockiness and tacticity of ring-opened metathesis polymers (ROMP). In this way the factors determining tacticities and stereoselectivities can be established and applied to the rapidly growing families of well-defined Schrock and Grubbs initiators. 

Table 2. GPC data (CHCI3, polystyrene standards) for polymers produced by ROMP of monomer (5) with Grubbs initiator in organic solvents.

Figure 9. NMR study of the polymerisation of monomer (6) in water initiated by the Grubbs initiator...

For this set of three monomers we conclude that for (4) and (6), which are amphiphilic and form micelles in water, the polymerisation probably starts in the organic droplet in which the water-insoluble Grubbs initiator is introduced to the system. Alternatively the organic droplet may dissolve in the micelle carrying the initiator to the... 

During a recent systematic survey [1] of the cis contents, tacticities, and other stereoselective features of a variety of poly(NBEs) and poly(NBDs) made using the Grubbs initiator, (PCy)2RuCl2CHPh, (I), we were surprised to find that it was very poor... 

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... 

SYNTHESIS, IMMOBILISATION AND CHARACTERISATION OF A SHIFF BASE MODIFIED GRUBBS INITIATOR (SCHEME 1)... 

Scheme 11.2 "Grafting-from" approaches to functional surfaces using both Schrock and Grubbs initiators.

Following this seminal work, Arisawa and Nishida investigated the behaviour of A-tosyl diallyl amine in the presence of the second-generation Grubbs initiator (10) and different catalyst modifiers. Trimethyl(vinyloxy)silane afforded the best yields of cycloisomerised versus RCM product, allowing for the formation of diverse exo-methylene benzofurans and indolins via diene cycloisomerisation (Equation (7.7)). The method was also successfully applied to the synthesis of a novel indole alkaloid that displayed antifungal activity. " As mentioned previously (see Section 7.3.2.2), it is known that in the presence of the vinyl trimethylsilyl ether, compound 10 decomposed into hydrido-car-bonyl complex 33, which was believed to be the actual catalyst for the cycloisomerisation process. 

COE and a COE-based cross-linker, that is, tris(cyclooct-4-en-l-yloxy)methylsilane (TCOMS). This development was guided by the idea that the polymer backbone of poly (COE)-derived materials consists of sec-allylic carbons and, therefore, presents a viable alternative to NBE-based systems. However, compared to NBE-based monomers, COEs are characterized by a significantly reduced ring strain, which makes the use of a more active initiator than the commonly used first-generation Grubbs initiator inevitable. These changes in monomer, cross-linker, and initiator required a comprehensive redesign of monolith synthesis. As a direct consequence of... 

Hoveyda-Grubbs initiator [Ru]-III nicely solved this problem, presumably because ruthenium loading adjacent to the oxirane was now viable via the relay of 21 to 22. This constituted the first report of a successful application of an RRCM reaction in the context of a natural product total synthesis. 

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