Carbynes tungsten

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

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... 

An unusual reaction of W111 with nitriles has been reported by Schrock et al. to provide metal a nitrido tungsten complex [46], Treatment of 98 with acetonitrile or benzonitrile provides the tungsten carbyne 99, along with the nitrido complex 100 (Eq. (37)). Additional studies resulted in the identification of reaction conditions that... 

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

Scheme 8.14 Preparation of neutral and cationic tungsten carbynes 53-55 from the carbene complexes 15a,b as the precursor (R = Me, Ph for X- and L see text)...

Molybdenum and tungsten carbyne (alkylidyne) complexes frequently undergo 2+2 cycloaddition reactions with alkynes to give the corresponding metallacyclobutadiene... 

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],... 

Scheme 17. Formation of poly(arylenethynylene)s via a tungsten-carbyne metathesis catalyst.

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