Ruthenium alkylidene water-soluble

Water-Soluble Ruthenium Alkylidenes Synthesis, Characterization, and Application to Olefin Metathesis in Protic Solvents, D. M. Lynn, B. Mohr, R. H. Grubbs, et at, J. Am. Chem. Soc. 2000, 722, 6601-6609. 

This finding is a significant improvement over aqueous ROMP systems using aqueous ROMP catalysts. The propagating species in these reactions is stable. The synthesis of water-soluble block copolymers can be achieved via sequential monomer addition. The polymerization is not of living type in the absence of acid. In addition to eliminating hydroxide ions, which would cause catalyst decomposition, the catalyst activity is also enhanced by the protonation of the phosphine ligands. Remarkably, the acids do not react with the ruthenium alkylidene bond. 

Recent developments include the synthesis of new water-soluble ruthenium alkylidenes and their application to olefin metathesis in water [47, 48]. It is interesting to note that the addition of acid made the polymerization rate up to 10 times faster than without acid (Eq. 21). 

Furthermore Grubbs et al. have published water-soluble as well as chiral ruthenium alkylidene complexes based on 16 for ARCM and AROM, whereas Schrock, Hoveyda and coworkers have synthesized a variety of asymmetric molybdenum alkylidene complexes, e.g. (5)-17 17,27 addition Hoveyda et al. have synthesized the achiral ruthenium complex 18 and the chiral complex 19 for ARCM and AROM. 

Water-soluble derivatives of alkylidenes 8 and 9 were prepared via phosphine ligand substitution reactions. Exchange of the phosphines in 8a for PhP(p-C6H4S03-Na)2 afforded a water-soluble vinyl alkylidene [20]. This alkylidene was soluble in water, but the triarylphosphine ligands were too small and insufficiently electron-donating to produce an active catalyst [48], Analogous substitution of the phosphines in 9 a for more sterically demanding, electron-rich, water-soluble phosphines yielded ruthenium alkylidenes 10 and 11 (Scheme 2), which were soluble in both water and methanol [49]. 

Applications of the water-soluble catalysts to initiate methathesis of less strained alkenes are limited by the instability of some of the alkylidenes in water. For example, diethyldiallylmalonate, a standard metathesis substrate in organic solvents, does not ring close in either methanol or water/methanol mixtures. In this case the chain carrying species is the ruthenium methylene complex. As seen above, this complex is unstable in water. However, when the reaction is redesigned to use substrate 17 so that the methylene is not the chain carrying intermediate, ring closing can be carried out under aqueous conditions (Eq. 12). 

The lessons learned from these complexes were eventually applied to the synthesis of well-defined ruthenium alkylidenes 8 and 9. Although they were insoluble in water, these alkylidenes could be used to initiate the living ROMP of functionalized norbornenes and 7-oxanorbomenes in aqueous emulsions. Substitution of the phosphine ligands in 9 for bulky, electron-rich, water-soluble phosphines produced water-soluble alkylidenes 10 and 11, which served as excellent initiators for the ROMP of water-soluble monomers in aqueous solution. These new ruthenium alkylidene complexes are powerful tools in the synthesis of highly functionalized polymers and organic molecules in both organic and aqueous environments. 

These observations led to the catalytic application of well-defined ruthenium alkylidenes, some of them freely soluble and sufficiently stable in water (Scheme 7.9) although their stability was found somewhat less in aqueous solutions than in methanol [21,27,28], With these catalysts a real living ROMP of water-soluble monomers could be achieved, i.e. addition of a suitable monomer to a final solution of a quantitative reaction resulted in further polymerization activity of the catalyst [28], This is particularly important in the preparation of block copolymers. 

Although the well-defined water-resistant metal-alkylidene complexes do not dissolve in water, they can be used in emulsion polymerizations. Small size polymer particles and polymer latex can be prepared with this method in aqueous media in the presence of cationic surfactants (e.g., Dode cyltrimethylammonium bromide (DTAB)). Water-soluble, biologically active glycopolymers have been also synthesized with ruthenium-alkylidene complexes in... 

For the ring-opening metathesis polymerization (ROMP) in heterophase [81], a water-soluble ruthenium alkylidene was employed for the emulsion polymerization of norbornene, and an oil-soluble catalyst for the miniemulsion polymerization of norbornene and 1,5-cyclooctadiene. Similar to the polymerization of ethylene in heterophase, an organic solution of the catalyst was first miniemulsified in water, after which the monomer was added to the miniemulsion. This resulted in a high monomer conversion for norbornene ( up to 97%), and a particle size of 250nm. 

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