Grubbs’catalysts immobilization

Scheme 8.4 Grafting of isocyanate-telchelic poly(l, 3-di(l -mesityl)-4- [(bicyclo[2.2.1 ]hept-5-en-2-ylcarbonyl)oxy]methyl -4,5-dihydro-l H-imidazol-3-ium tetrafluoroborate) on silica and generation of the immobilized second generation Grubbs catalyst.

CM has also been applied in the immobilization of biologically active molecules. Thus, [230] Reetz and co-workers supported chiral phosphonate as a potential suicide enzyme inhibitor by reacting alkenyl phosphonates with either allyl-modified SIRAN (a porous glass) or allyl-modified TentaGel in the presence of the Grubbs catalyst in CH2CI2 at reflux. 

Irrespective of the reaction conditions used (/. e. ultrasound, microwave, changing reaction times, temperature and solvents), the maximum turnover number (TON) that was achieved was 75. In principle, second generation Grubbs-type initiators immobilized on non-porous silica should behave similar to those immobilized on monolithic supports[16]. In fact, catalysts immobilized onto monolithic supports give similar maximum TONs (< 65) in the absence of any chain transfer agent (CTA). Ruthenium measurements by means of ICP-OES revealed quantitative retention of the original amount of ruthenium at the support within experimental error ( 5%), thus otfering access to metal free products. 

Scheme 16.86. Ring closing metathesis utilizing an immobilized Grubbs catalyst.

Scheme 11.12 Synthesis of a monolith-supported Grubbs second-generation ruthenium catalyst immobilized via the NHC ligand.

Figure 3 (a-c) Examples of Grubbs-type metathesis catalysts immobilized via the NHC ligand. 

Even with immobilized catalysts being developed, removal of ruthenium by-products remains an important challenge. Georg and coworkers found that addition of 50 equiv (relative to ruthenium) of dimethyl sulfoxide or triphenylphosphine oxide brought ruthenium levels in reaction mixtures down from 50 to l-2qgmg-. The ruthenium levels in purified products are similar to those reported by Grubbs, where the metal was removed as trishydroxymethylphosphine complexes, " and those from the Pb(OAc)4 oxidation of ruthenium reported by Paquette. 

Week [203] developed a monomer salen complex linked to a norbomene via a stable phenylene-acetylene linker and its subsequent polymerization by means of the controlled ROMP method using 3 generation Grubb s catalyst (Scheme 137). This polymerization methodology led to fully functionalized immobilized metal-salen catalyst. By this way, the supported catalyst showed catalytic activities and stereoselectivities similar to the nonsupported Jacobsen catalyst. Moreover, activities and selectivities seemed to depend on the density of the catalytic moieties homopolymer 324 were less selective than their copolymer analogs 325. For example, AE of 1,2-dihydronaphtalene led in both cases to total conversion and 76% ee for the homopolymer 324 vs 81% ee for copolymer 325a. Recycle was possible and after 3 recyles a drastic decrease in ee was observed. AE of dihydronaphtalene led to 81% ee for the first cycle vs 6% ee for the third one. 

Metathesis catalysts including Gmbbs ruthenium catalyst (see Chap. 5) are hard to separate from the reaction products. Chemists therefore sought ways to overcome this problem by immobilizing the catalyst on various supports. Buchmeister recently reviewed various polymer-supported metathesis catalysts [54]. A few are reported here. Early, Grubbs described a phosphine-derivatized polystyrene-supported catalyst [55]... 

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