Tungsten complexes metathesis catalysts

An obvious drawback in RCM-based synthesis of unsaturated macrocyclic natural compounds is the lack of control over the newly formed double bond. The products formed are usually obtained as mixture of ( /Z)-isomers with the (E)-isomer dominating in most cases. The best solution for this problem might be a sequence of RCAM followed by (E)- or (Z)-selective partial reduction. Until now, alkyne metathesis has remained in the shadow of alkene-based metathesis reactions. One of the reasons maybe the lack of commercially available catalysts for this type of reaction. When alkyne metathesis as a new synthetic tool was reviewed in early 1999 [184], there existed only a single report disclosed by Fiirstner s laboratory [185] on the RCAM-based conversion of functionalized diynes to triple-bonded 12- to 28-membered macrocycles with the concomitant expulsion of 2-butyne (cf Fig. 3a). These reactions were catalyzed by Schrock s tungsten-carbyne complex G. Since then, Furstner and coworkers have achieved a series of natural product syntheses, which seem to establish RCAM followed by partial reduction to (Z)- or (E)-cycloalkenes as a useful macrocyclization alternative to RCM. As work up to early 2000, including the development of alternative alkyne metathesis catalysts, is competently covered in Fiirstner s excellent review [2a], we will concentrate here only on the most recent natural product syntheses, which were all achieved by Fiirstner s team. 

In retrospect it is not surprising that the niobium and tantalum alkylldene complexes we prepared are not good metathesis catalysts since these metals are not found in the "classical" olefin metathesis systems (2). Therefore, we set out to prepare some tungsten alkylidene complexes. The first successful reaction is that shown in equation 6 (L = PMe3 or PEt3) (11). These oxo... 

Prior to the first examples of the cross-metathesis of functionalised alkenes using these catalysts, however, was a report on the use of a lesser known tungsten complex 5 [18,19]. 

Probably the first isolated tungsten alkylidene complex active in metathesis and completely characterised is the one shown in Figure 16.10 reported by Wengrovius and Schrock the analysis included an X-ray structure determination by Churchill and co-workers [18], The alkylidene was transferred from a tantalum complex to yield the hexacoordinate tungsten complex containing two PEt3 ligands. One of these can be removed by the addition of half an equivalent of palladium chloride. The total turnover number of these catalysts with Lewis acids added was 50 in 24 hours. 

Although the real species of this solution for alkyne metathesis is not clear, this complex is an excellent tool from a preparative point of view. It is very active for the formation of cycloalkynes of different ring sizes from diynes. In contrast to tungsten alkylidyne complex 1, catalyst I4O/CH2CI2 is sensitive toward acidic protons such as amide proton and exhibited remarkable tolerance toward many polar functional groups (Table 6). ... 

Immobilized complexes anchored to the surface of oxide supports may serve as active metathesis catalysts. In fact, Mo(CO)6 on alumina was reported in the first paper on metathesis by Banks and Bailey.1 Molybdenum and tungsten hexacarbo-nyls and alkylmolybdenum and alkyltungsten complexes have been studied most.6,24,26 Anchored organomolybdenum complexes are particularly effective catalysts, even at temperatures below 0°C however, they quickly lose activity.41-43... 

Fig. 3 Olefin metathesis catalysts Schrock tungsten (Cl) and molybdenum (C2) alkylidene complexes, Grubbs first- (C3) and second-generation (C4) catalysts, Hoveyda-Grubbs second-generation catalyst (C5), and Grubbs third-generation catalyst (C6)...

As regards the metathesis polymerisation of cyclic trienes, it has been carried out in an attempt to find alternative routes for preparing soluble and meltable precursors of polyacetylene [149, 150], Hence, several substituted or unsubstituted tricyclic or other polycyclic trienes were subjected to polymerisation in the presence of metathesis catalysts such as WCl6-SnMe4 [151-154] and the tungsten neopentylidene complex [Me(F3C)2CO]2W(=NAr)(=CHCMe3) [155]. A successful solution of the problem is outlined below [125,150] ... 

A variety of isolated pentacoordinate tungsten-carbene complexes are known to be active metathesis catalysts [53]. At least one of these systems has been proposed to be living based primarily on 1H NMR identification of the propagating alkylidene [53 e]. To date, verification of the living nature of these catalysts through GPC determination of polydispersities are still pending. The solution NMR studies do confirm the mechanism of the metathesis reaction, but do not insure that all of the requisite factors for a living system are met. 

The dark red dicarbyne complex W(OBu )3 2(p-C2) (50) was first reported from metathesis of W2(OBu% and EtC=CC=CEt (Scheme 14).167 Two structure determinations revealed that the three OBu groups on each tungsten are staggered (D3d). The structure of the toluene hemisolvate has rather longer W-C and shorter C-C bonds both are consistent with the W=C-C=W formulation.168-170 This complex is not a metathesis catalyst... 

Most metathesis catalysts involve molybdenum or tungsten and in view of our speculation over a possible role for p-carbenes in the process it was of interest to attempt the preparation of such complexes of these metals. In an adaptation of the successful route described earlier, the p-ethyne complex (12) (and analogous... 

The metathesis of symmetrical aliphatic carbodiimides is also catalyzed by tungsten imido complexes above 140 Other carbodiimide metathesis catalysts include Cr(ll)/Si02/ iminophosphoranes, imido circonocenes, guanidine supported titanium complexes and group 14 amide complexes. ... 

To date, low volumes of materials have been produced commercially from norbomene and cyclo-octene. Numerous products are expected to result from the materitd produced by the ROMP of dicyclopentadiene in a RIM (reaction injection molding) process. In a RIM process, two streams of a monomer are mixed in the mold where it is polymerized to the final part. In this case, one of the monomer streams contains a tungsten complex while the second contains an alkyl aluminum activator. When the two streams of dicyclopentadiene are mixed, the metathesis catalyst is formed and the monomer is ROMP polymerized (equation 12). 

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