Tungsten complexes carbyne

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 addition to the reaction shown in Scheme 53, some other related reactions that are thought to proceed via cyclic carbometallation have also been reported (Scheme 54). In the cyclization reaction of 2-ethenyl-2 -ethynylbiphenyl, both Cr and W carbyne complexes must undergo the same cyclic carbometallation as that shown in Scheme 53 to give the corresponding metallacyclohexadiene intermediates, but the final products obtained were different.256 Some tungsten-carbyne complexes have been shown to undergo a stepwise [2 + 2 + 2]-cyclization via formal cyclic carbometallation that can be followed by reductive elimination to produce cyclopentadiene-tungsten complexes.2... 

PPEs by Acyclic Diyne Metathesis (ADIMET) Utilizing Schrock s Tungsten Carbyne Complex 11... 

Wilkinson et al. (64) found that irradiation of this complex in the presence of PMej at -20°C gave CH4 and the tungsten-carbyne complex... 

The tetrahedrally coordinated tungsten-carbyne complex (j/ -C5H5)Fe[t -C5H4-CsW(CO)2(j -C5H5)] also undergoes one-electron oxidation [E° = -1-0.64 V) in DME solution [34],... 

The general equation and mechanism for alkyne metathesis is depicted in Scheme 31. Alkyne metathesis is considerably less well developed in comparison to alkene metathesis. Garbyne complexes or carbyne complex precursors are among the most effective alkyne metathesis catalysts representative catalysts are depicted in Scheme 32. Tungsten carbyne complex 276 is one of the earliest alkyne metathesis catalysts, and has frequently been employed to initiate... 

Tungsten carbyne complexes are synthesised from alkynide complexes by double addition of electrophiles. Thus addition of FS03Me or Et30 to [W(CCBu )C0)5] leads to the vinylidene W(=C=CButR)(C0)5 (R = Me or Et). Addition of CF3S03H/Me4NI to this vinylidene or directly to the alkynide allows the high yield isolation of t ran -WI (CO ) [ C (CHBu R ) ]. ... 

Synthesis of Schrock s W tungsten-carbyne complex, the prototype of active alkyne metathesis catalysts... 

Acyclic diyne metathesis (ADIMET) is the method of choice for the preparation of PPEs. After the first successful attempts with a tungsten carbyne complex [148],... 

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