Metathesis Catalysts Molybdenum-based

Ruthenium-based metathesis catalysts versus molybdenum-based metathesis catalysts, 26 936-937 Ruthenium catalysts... 

Originally, the conversion of substituted bicyclo[2.2.2]octa-2,5-dienes into a precursor polymer of PPVs by ring opening metathesis polymerization (ROMP) has been shown by Grubbs using a molybdenum-based metathesis catalyst. The precursor polymer aromatizes in an inert atmosphere at 280 0, or in the presence of trioctylamine at 200°C. 

This facile solution for catalyst immobilisation follows a more complex approach reported by Mayr et al. [246], This research group attached the imidazolium salt to a nor-bomene unit that was then subjected to a ring opening metathesis polymerisation (ROMP) reaction to form the polymeric NHC precursor (see Figure 4.77). The ROMP reaction had to be carried out with a molybdenum based Schrock catalyst because the ruthenium based Grubbs catalyst did not provide an endgroup that could be quantitatively capped with a suitable final endgroup. 

Ring-Opening Polymerization. Ring-opening polymerization of cycloolefins in the presence of tungsten- or molybdenum-based catalysts proceeds by a metathesis mechanism (67,68). 

An area, in which catalytic olefin metathesis could have a significant impact on future natural product directed work, would be the desymmetrization of achiral molecules through asymmetric RCM (ARCM) with chiral molybde-num-(for a chiral molybdenum-based catalyst available in situ from commercial components, see Refs 156 and 156a-156c) or ruthenium-based catalysts. 

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

As part of a program exploring functionalized bicyclic /3-lactams, the ring-closing metathesis of the /3-lactam diene 46 in the presence of a molybdenum-based catalyst (5 mol%) was shown to afford access to the fused 1,3-thiazepine 47 in 78% yield (Equation 7) <1997CC155>. 

Schrock, Gibson et al. [52d] found that styrene and 1,3-pentadiene could be used as chain transfer reagents for the living ring-opening olefin metathesis polymerization of norbornene with molybdenum based catalyst 35a. Renewed norbornene addition to a polymerization mixture containing initiator 35a and 30 equivalents of styrene resulted in the formation of polynorbomene with a low polydispersity and a molecular weight controlled by the number of norbornene equivalents in each of the individual monomer solutions, Eq. (38). This method allows a more efficient use of the catalyst. 

In the early 1980s, Schrock prepared a series of tungsten- and molybdenum-based carbyne complexes, and demonstrated that they are viable catalysts for performing stoichiometric and catalytic alkyne metathesis [7]. With the defined carbyne complexes, he laid the foundation for the mechanistic understanding of alkyne metathesis, and was the first to demonstrate that vinyl-substituted carbyne complexes are stable [8] and that alkyne metathesis could be performed in the presence of C=C double bonds. 

Schrock and Hoveyda have reported the synthesis and activity of a number of chiral molybdenum-based catalysts for enantioselective olefin metathesis (for example, 97a, b) (Scheme 14) to date, the majority of successful... 

Scheme 14 Chiral molybdenum-based catalysts for asymmetric olefin metathesis...

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