Ruthenium metathesis

Synthetic routes to active ruthenium metathesis catalysts are classified according to the ruthenium precursor used. 

Table 5 Phosphine-free ruthenium metathesis catalysts...

The metathesis of ene-ynamides has been investigated by Mori et al. and Hsung et al. [80]. Second-generation ruthenium catalysts and elevated temperatures were required to obtain preparatively useful yields. Witulski et al. published a highly regioselective cyclotrimerization of 1,6-diynes such as 98 and terminal alkynes using the first-generation ruthenium metathesis catalyst 9... 

As part of an investigation into new synthetic routes to the important acyclic nucleoside class of antiviral drugs, the cross metathesis of 9-allyl-6-chloropurine with 2,2-dimethyl-4-vinyl-l,3-dioxolane was attempted <2003TL9177>. The reaction was confounded by the coordination of the ruthenium metathesis catalyst with the purine heterocyclic nitrogens. This was overcome to some extent by using the /i-toluenesulfonic acid or hydrogen chloride salts of the... 

Observations The next generation of ruthenium metathesis catalysts have been prepared... 

Additional Schiff bases used in forming ruthenium metathesis catalysts, (I), are provided by Schaubroeck et al. (1). 

D.M. Lynn, E.L. Dias, R.H. Grubbs, and B. Mohr, Acid activation of ruthenium metathesis catalysts and living ROMP metathesis polymerization in water, US Patent 6 486 279, assigned to California Institute of Technology (Pasadena, CA), November 26,2002. 

Olefin metathesis has proved to be a powerful synthetic tool in organic synthesis.5 The advent of well-defined metal carbene complexes with remarkable functional group tolerance has rendered metathesis as an efficient route to the synthesis of new C-C bonds. Examples of widely used ruthenium metathesis catalysts include [Ru-1],6 [Ru-2]7 and [Ru-3] 8 (Figure 1). 

Olefin metathesis has become a very important reaction in polymer chemistry and natural product synthesis [47-49]. Garber et al. have used the physical properties of dendrimers in order to improve the separation between the dendritic metathesis catalyst and products on silica gel column chromatography [50]. The Van Koten group has reported on the synthesis of different generations of carbosilane dendrimers functionalized with ruthenium metathesis catalysts [51]. 

Scheme 8 Synthesis of Van Koten s dendritic ruthenium metathesis catalyst applied in ring-closure metathesis...

Diver has recently reported new entries for the assembly of tetracyclic derivatives [89]. Interestingly, ruthenium metathesis-type catalysts have also given birth to tricyclic derivatives incorporating a cyclopropane from di-enynes [90]. Cationic gold-based catalysts have proven to be even more reactive promotors of various reactions resulting from a preliminary electrophilic activation [91]. They also allow the formation of tetracyclic derivatives 140 from acyclic precursors 139 at low temperature and as single diastereomers. In one case, the minor metathesis diene 141 was isolated. Tetracyclic products... 

An azo-free method for preparing the ruthenium metathesis catalysts was developed by Nolan [6]. 

Ruthenium metathesis catalysts have been the focus of much recent attention. Two recent papers describe efforts to recycle such catalysts. In the first of these reports, Yao described a PEG-bound version of a ruthenium catalyst de-... 

Tandem GM-RGM (see Scheme 20) can be employed for the synthesis of complex ring systems from simple starting materials. A very noteworthy example of this reaction serves as the cornerstone for the total synthethis of the natural product cyclindrocyclophane. The GM-RGM reaction of diene 182 was completely selective for formation of the head-to-tail isomer 183. The selectivity was attributed to thermodynamic control the head-to-head dimer 184 is considerably less stable. In an effort to synthesize pyrenophorin derivatives, the formation of cyclic dimers (e.g., 186) and trimers (e.g., 187) from treatment of acrylate ester-alkene derivatives (e.g., 185) with various ruthenium metathesis catalysts was examined.The product distribution was very time and concentration dependent higher concentrations favor the trimer over the dimer. 

In this case, ADMET was chosen based on the mild reaction conditions involved and the functional group tolerance of the ruthenium metathesis catalyst. The results of this study highlighted the use of the many forms of olefin metathesis in the fabrication of complex structures that previously were synthetically unobtainable. 

Matron and Grubbs formed block copolymers by combining ring opening metathesis polymerization with ATRP [437]. Use was made of fast initiating ruthenium metathesis catalyst to form three different monotelechelic poly(oxa)norbomenes. The ends were functionized and ATRP polymerizations of styrene and ferf-butyl acrylate followed. 

Tentagel, described above, is in wide use today in solid phase syntheses. Polyethylene glycol has also been attached to various other polymers to form support resins. For instance, Frechet and coworkers [37] used cross-linked methacrylate esters of ethylene glycol oligomers in a suspension polymerization to synthesize hydroxyl group functionalized beads. These beads swell well in a variety of polar solvents. Another example is that of Grubbs attaching a ruthenium metathesis catalyst to polyethylene [38] ... 

This method allows the quantification of the olefin isomerization that may occur in the course of ADMET polymerization using second generation ruthenium metathesis catalysts. 

Further, it was demonstrated that the addition of benzoquinone to the polymerization mixture prevents the olefin isomerization. Therefore, second generation ruthenium metathesis catalysts can be used for the preparation of well defined polymers via an ADMET technique causing little isomerization (32). 

The chelating ether motif used in Hoveyda-type catalysts has become a widely used structural platform in ruthenium metathesis catalyst development [3, 22]. 

Ruthenium metathesis catalysts are known to react with electron-rich olefins Fl2C=C(H)ER (where E is N, O, or S) to give Fischer-type, carbene complexes. These Fischer carbenes are considerably more stable and less active in olefin metathesis than the Grubbs systems, with the order of metathesis activity (E = C>N>S>0) being the reverse of that of complex stabilities [82]. Thus, the development of ruthenium catalysts capable of efficiently catalyzing the... 

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