Carbyne complexes of tungsten

G. A. McDermott, A. M. Domes, and A. Mayr, Synthesis of Carbyne Complexes of Chromium, Molybdenum, and Tungsten by Formal Oxide Abstraction from Acyl Ligands, Organometallics 6, 925-931 (1987). 

When hydroxycarbene complexes of tungsten are reacted with dicyclohexylcarbodi-imide (DCCD), a novel carbyne complex is formed, having a metal acylate moiety ligated trans to the carbyne ligand . Reaction of the corresponding complexes of chromium with DCCD yields different (noncarbyne) products. 

CHEMISTRY AND ELECTROCHEMISTRY OF ALKYNE- AND ISOCYANIDE-DERIVED CARBYNE COMPLEXES OF RHENIUM, MOLYBDENUM OR TUNGSTEN... 

Dicarbonyl(r -cyclopentadienyI)carbyne Complexes of Molybdenum and Tungsten... 

ABSTRACT. Dicarbonyl(t 5-cyclopentadienyl)carbyne complexes of molybdenum and tungsten prove to be a valuable synthetic tool Reaction with phosphines provides substituted carbyne complexes and leads via an intramolecular CC-coupling to t 1- or Tj -ketenyl complexes respectively. Electrophiles attack the metal carbyne triple bond forming hetero- and acyclic carbene complexes, r 2-acyl compounds, T -ketene complexes and metalla-dithia-bicyclobutane cations. Dithio-carboxylates are formed in reaction of these dicarbonyl(Ti5 cyclo-pentadienyl)carbyne complexes with sulfur or cyclohexene sulfide. 

Under specific reaction conditions dicarbonyl-(ri -cyclopentadienyl)carbyne complexes of molybdenum and tungsten (R = CH3, C6H5, C6H4CH3) react with trimethylphosphine via substitution... 

If, however, carbonyl(cyclopentadienyl)(trimethylphosphine)carbyne complexes of molybdenum and tungsten are allowed to react with carbon monoxide, carbonylation at the carbyne carbon takes place forming r l-ketenyl complexes in nearly quantitative yields [3,9]. 

Fenske s theoretical studies on cyclopentadienyl-substituted carbyne complexes indicate a remarkable charge on the carbyne carbon atom [25]. This existence of a nucleophilic carbyne carbon becomes evident in the protonation of different carbyne complexes of molybdenum [26], tungsten [27-29]) and osmium [30]. 

DICARBONYL(ti5-CYCLOPENTADIENYL)CARBYNE complexes of MOLYBDENUM AND TUNGSTEN AS BUILDING BLOCKS F. R. KreiBl, J. Ostermeier, W. Schiitt, C. M. Stegmair, N. Ullrich,... 

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... 

Metallacyclopent-l-enes have been obtained by formal [2 + 3] cycloaddition of ethylene to Re(CtBu)(CHtBu)(OR)2 (a carbene/carbyne complex) [612]. Nucleophilic tungsten(II) complexes can react directly with cyclopentanone to yield cyclopentylidene complexes [613]. 

A series of cyclopropyl substituted carbyne complexes was photolyzed in order to follow the regio- and stereochemical outcome of the photooxidation. Irradiation of the deuterated molybdenum complex gave cyclopentenone exclusively labeled at C3 (equation 114). In the analogous tungsten complex only 90% of the deuterium resides on C3 the remaining 10% labeled the 2-position158. 

It was also of interest to us how changes in the organic residue of the carbyne ligand influence the stability and the behavior of carbyne complexes. Hence, we treated with boron tribromide a series of pentacarbonyl-[methoxy (aryl) carbene tungsten (0) complexes which were substituted at... 

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