Catalysts Schrock-type

In particular, Schrock-type catalysts suffered from extreme moisture and air sensitivity because of the high oxidation state of the metal center, molybdenum. Due to the oxophilicity of the central atom, polar or protic functional groups coordinate to the metal center, poisoning the catalyst and rendering it inactive for metathesis. Since late transition metal complexes are typically more stable in the presence of a wide range of functionalities, research was focused on the creation of late transition metal carbene complexes for use as metathesis catalysts. 

Scheme 5 syn-anti interconversion of Schrock-type catalysts... 

Fig. 8 Schrock-type catalysts successfully used in regioselective cyclopolymerization...

Fig. 5.3 Mo-based Schrock-type catalyst. Ar = phenyl, 2,6-Me2-Cf,H3, 2,6-i-Pr2-C6H3, etc. R = ethyl, phenyl, trimethylsilyl, CMejPh or t-butyl and R = CMe3, CMe2CF3, CMe(CF3)2, C(CF3)2, aryl, etc.

In contrast to unsubstituted acetylene, the polymerization of differently substituted 1-alkynes and di-l-alkynes may be carried out conveniently using Schrock-type catalysts. Dipropargylmalonate and derivatives thereof may be cyclopolymer-ized in a living manner using Mo(N-2,6-i-Pr2-CgH3)(CH-t-Bu)[OCMe(CF3)2]2. The... 

The high tolerance of Schrock-type catalysts of the general formula Mo(NAr )(CHCMe2Ph)(OR)2, NAr ... 

SCHEME 6.8 Synthesis of dihexyl-PPE by a Schrock-type catalyst. 

Pathways to deactivation of the Schrock-type catalysts are thought to proceed by association of two catalysts to form bridged structures. Thus, it might be possible to improve the catalyst activities and lifetimes by isolating the catalysts on a solid support, such that two catalysts do not interact with each other. Thus, Schrock, Coperet, and Basset have worked to support the Schrock hmgsten and molybdenum catalysts on a partially dehydrated silica support. " Tliis work has led to more active catalysts, partly from site isolation and partly from the unsymmetrical structure of the supported catalyst. 

See e.g. Ivin, K.J., Mol, J.C. Olefin Metathesis and Metathesis Polymerization, Academic Press 1997. Very rarely, some order of tacticity was observed in certain ROMP polymer structures but - so far -only with Schrock-type catalysts. 

Figure 5.1 Representative examples of Grubbs and Schrock-type catalysts.

Figure 5.15 Various well-defined, silica-supported, Schrock-type catalysts.

Figure 5.17 Supported, Schrock-type catalysts immobilized through the imido ligand.

Two further reports on Schrock-type catalysts are worth mentioning Feher et al. used sesquisiloxanes as ligands [148] and Stelzer et al. heterogeneized them on a y-Al203 support using hexafluorobisphenol-A linkers [70]. 

Figure 5.1 Alkene metathesis catalysts Grubbs-type first generation, second generation, and third generation. (From left side, top row to right side, second row) Grubbs-Blechert-Hoveyda catalyst, Grela s nitro derivative, and Schrock-type catalyst [7-11].

Polymerization of a-norbornenyl-polystyrene macromonomer, a-NBPS, with the Schrock-type catalyst Mo(NAr)(CHtBu)(OC(CH3)(CF3)2)2 in toluene at room temperature gave polynorbornene-polystyrene diblock copolymer (118) in a quantitative yield [86] [Eq. (50)]. 

As polymers that contain both hydrophilic and hydrophobic components aroused keen interest from theoretical and practical points of view over the past years, synthesis of amphiphilic branched copolymers by ring-opening metathesis polymerization of miscellaneous macromonomers is an important goal of the actual research. Thus, in order to obtain globular shape macromolecules that would present the same features as those exhibited by certain assemblies of molecules such as the micelles or the latices, with a bulk part different from the external surface, polymerization of norbornyl polystyrene-poly(ethylene oxide) macromonomer has been conducted in the presence of Schrock-type catalyst Mo(NAr)(CH/Bu)(OC(CH3)(CF3)2)2 in toluene at room temperature to produce poly-norbornene-polystyrene-poly(ethylene oxide) block copolymers (120) [88] [Eq. (52)]. 

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