Tungsten triple bonds

An interesting, but currently unique, reaction to form W(0-tBu)3N (probably tetrameric) involves the cleavage of the tungsten-tungsten triple bond of [W2(0-tBu)fi] by alkyl nitriles as shown in Eq. (68). 

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). 

Chemistry of compounds containing metal to metal triple bonds between molybdenum and tungsten. M. H. Chisholm and F. A. Cotton, Acc. Chem. Res., 1978,11, 356-362 (66). 

The cr2 tta triple bond between molybdenum and tungsten atoms developing the chemistry of an inorganic functional group. M. H. Chisholm, Angew. Chem., Int. Ed. Engl., 1986,25,21 (70). 

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). 

Despite early reports on the addition of hydroxyl groups to tungsten-alkylidyne bonds [5-7], recent work has showed that the triple-bond remains untouched, at least in the cases involving the reaction of [Mo(=CEt)(N BuPh)3] [13], [W(=CCMe3) (CH2Me3)3] [26, 27] and [Mo(=CCMe3)(CH2Me3)3] with silica [28]. 

Alkyne metathesis is a curious reaction in view of the fact that two alkyne triple bonds are cleaved and reconstructed simultaneously leading to different triple bonds. The first reported effective catalyst is a heterogeneous mixture of tungsten oxide and silica. Then Mortreux found that a catalytic system that consisted of Mo(CO)6 and resorcinol was effective for alkyne metathesis. As reported, the added alkynes come into equilibrium with different product... 

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. 

Cp2Mo2(CO)4] is an important member of a group of cyclopentadienyl compounds and, like its chromium and tungsten analogues, this species is considered to involve a metal-metal triple bond.7,220 These and related compounds manifest a rich and novel chemistry,221 but this falls outside the field of this review. 

The tungsten-carbon single, double, and triple bond lengths in this compound are 225.8, 194.2. and 178.5 pm, respectively, and the accompanying W—C—C bond angles are 124°. 150°. and 175 , all of which is quite consistent with tungsten-carbon bond orders of 1,2. and 3. 

Chisholm and Cotton have summarized their successful efforts characterizing metal-metal triple bonds in molybdenum(III) and tungsten(III) compounds containing no bridging ligands (46). This realm of strong homo-nuclear metal-metal bonds was unknown prior to the 1970s, and the rapid development and exploitation of the chemistry of these dimers provide an... 

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