Tungsten complexes triple bonds

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. 

Protonation of 12 yields a compound best described as a face-protonated methylidyne complex, the tungsten-carbon bond length lying in the range observed for a triple bond (28). Protonation of the osmium compound 13 yields a true carbene complex, which for R = Ph has been characterized by X-ray crystallography (see Sections IV and VI). 

These compounds are the parents of the most important class of complexes containing an Mo—Mo triple bond not only do they exhibit a rich coordination chemistry, but also a versatile reactivity.7 810164 Many of the compounds of this class have been characterized by X-ray crystallography and a selection of these is presented in Table 2, together with their Mo—Mo separation. The chemistry of these systems has been developed in parallel to that of their tungsten analogues and the latter has helped illuminate the former. 

The reactions and nature of multiple bonds between two metal atoms form a lively area of research and debate (1,2). Many examples of triple bonds between metals are known, including the extensively studied molybdenum or tungsten alkoxide, M2(OR)6, and amido, M2(NR2)6, complexes (3). Classic examples of quadruple bonds are epitomized by [Re2Cl8]2 (4) and molybdenum acetate, Mo2(OCOMe)4 (5). 

Despite Schrock s extensive work on tantalum, molybdenum, and tungsten car-byne complexes (e.g., 47, 49, 50, 51, and 52) [96, 97, 99, 100], it must be recalled, however, that they were not the first compounds with a metal—carbon triple bond. 

E. O. Fischer, G. Kreis, C. G. Kreiter, J. Muller, G. Huttner, and H. Lorenz, trans-Halo[alkyl(aryl)carbyne]tetracarbonyl Complexes of Chromium, Molybdenum and Tungsten. A New Complex Type with Transition Metal-Carbon Triple Bond, Angew. Chem. Int. Ed. Engl. 12, 564-565 (1973). 

R. R. Schrock, M. L. Listemann, and L. G. Sturgeoff, Metathesis of Tungsten-Tungsten Triple Bonds with Acetylenes and Nitriles To Give Alkylidyne and Nitrido Complexes, J. Am. Chem. Soc. 104, 4291 1293 (1982). 

Fp C=CC2[Co2(CO)6]-moieties linked by a C-C single bond and is obtained from the reaction of the tetrayndiyl complex Fp i-(C=C)4 and Co2(CO)8.422 The two C2Co2 cores, which again involve the inner C=C triple bonds, induce a transoid conformation (S-shaped) in the carbon chain, analogous to that described above for the tungsten complexes. 

Here, the higher co-ordination number of the heteroatom tungsten is avoided by the assumed triple bond to the external tungsten atom. Indeed this complex shows only terminal CO frequencies in the IR spectrum. The compound decomposes rapidly and this is why attempts to investigate it by X-ray methods and to prove the structure proposed in Fig. 7 here failed. The spectroscopic investigations support the proposal. 

Many tungsten(III) complexes have been reported as W-W triple-bonded derivatives, which are described in Chapter 4.9. The other compounds are discussed here, including dimeric tungsten (III) complexes containing bridging ligands are described. 

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