Fischer tungsten-carbene complex

The first [3S+2C] cycloaddition reaction using a Fischer carbene complex was accomplished by Fischer et al. in 1973 when they reported the reaction of the pentacarbonyl(ethoxy)(phenylethynyl)carbene complex of tungsten and diazomethane to give a pyrazole derivative [45]. But it was 13 years later when Chan and Wulff demonstrated that in fact this was the first example of a 1,3-dipolar cycloaddition reaction [46,47a]. The introduction of a bulky trime-thylsilyl group on the diazomethane in order to prevent carbene-carbon olefi-nation leads to the corresponding pyrazole carbene complexes in better yields (Scheme 15). 

The potential of Fischer carbene complexes in the construction of complex structures from simple starting materials is nicely reflected in the next example. Thus, the reaction of alkenylcarbene complexes of chromium and tungsten with cyclopentanone and cyclohexanone enamines allows the di-astereo- and enantioselective synthesis of functionalised bicyclo[3.2.1]octane and bicyclo[3.3.1]nonane derivatives [12] (Scheme 44). The mechanism of this transformation is initiated by a 1,4-addition of the C -enamine to the alkenylcarbene complex. Further 1,2-addition of the of the newly formed enamine to the carbene carbon leads to a metalate intermediate which can... 

Tungsten alkynyl Fischer carbene complexes are excellent dienophile partners in the classical Diels-Alder reaction with 1-azadienes (see Sect. 2.9.2.1). On the contrary, the chromium-derived complexes exhibit a different behaviour and they react through a [4S+3C] heterocyclisation reaction to furnish azepine derivatives [116] (Scheme 68). The reaction is initiated by a 1,2-addition of the nitrogen lone pair to the carbene carbon followed by a [l,2]-Cr(CO)5 shift-pro-... 

Reactions of Cjq with metal carbene complexes also yield the [6,6] methano-fullerenes [392]. These adducts are probably not formed via a carbene addition, but via a formal [2-1-2] cycloaddition under formation of a metalla cyclobutane intermediate. The Fischer carbene complex [mefhyl(methoxymethylene)]pentacarbonyl chromium can be utilized to prepare l,2-mefhyl(methoxymethano)-fullerene in 20% yield [392]. A tungsten carbene complex was primarily used to initiate the formation of a polyacetylene polymer, but it was discovered that addition of to the complex-polymer-mixture improves the polymerization and dramatically increases the catalytic activity of the carbene complex [393]. can be integrated into the polymer via carbene addition. 

E. O. Fischer, and A. Maasbol, Tungsten Carbonyl-carbene Complex, Angew. Chem. Int. Ed. Engl. 3, 580 (1964). 

Intramolecular cyclopropanations of pendant alkenes are more favorable. Heteroatom-substituted 2-aza- and 2-oxabicyclo[3.1.0]hexanes, together with 2-oxabicyclo[4.1.0] heptanes, can be prepared from chromium and tungsten Fischer carbenes having a tethered alkene chain. An interesting carbene formation via a cationic alkylidene intermediate, nucleophilic addition (see Nucleophilic Addition Rules for Predicting Direction), and intramolecular cyclopropanation is shown in Scheme 59. An intramolecular cyclopropanation via reaction of alkenyl Fischer carbene complex (28) andpropyne was used in a formal synthesis of carabrone (Scheme 60). 

Dihydropymn formation. Cyclization of 4-alkynols is induced by the tungsten version of a Fischer carbene complex without the need of UV irradiation, its application to a synthetic approach to altromycin-B disaccharide has been demonstrated. ... 

A number of Fischer carbene complexes of chromium and tungsten have been prepared by conventional and microwave heating (YIF = 0.79-1.1). Mono- and dimethylureas were reacted with alkynyl alkoxy Cr or W carbenes using THF as the solvent (Scheme 7.22). The resultant complexes contained a uracil-moiety. A sol-vent-free approach to the reaction was also attanpted, but this gave somewhat lower yields of the desired products. 

The cycloaddition reactions of chiral 2-amino-l, 3-dienes with Fischer-type carbene complexes have been examined by Barluenga et al. Reactions with tungsten vinyl carbene complexes A and with boron-nitrogen-chelated chromium complexes B lead very selectively to cyclohexanone derivatives. However, when employing chromium vinyl carbene complexes in acetonitrile at room temperature in these reactions, cycloheptadienes were obtained selectively by cyclopropanation and subsequent Cope rearrangement. 

Evidence for the tetrahedral intermediate has been gained in some cases by the reaction of the Fischer carbene complex with a Lewis base. Some of these reactions generate Lewis acid-base adducts in which the the carbene carbon acts as the Lewis acid. For example, addition of quinuclidine to the classic Fischer carbene complexes [M(CO)5=CPh(OMe)] (M = Cr or W) generated stable adducts, as shown for the tungsten example in Scheme 13.7. ... 

The electrophilic nature of Fischer carbene complex of Cr and W featuring an alkoxy or a thiomethyl substituent has been assessed in the nucleophilic substitution of the carbene heteroelement by the cyanamide anion.Alkoxy carbene complexes were found to be expectedly more reactive. Tungsten carbene were found to exhibit higher kinetic constants as a consequence of the higher electronegativity of W compared to Cr. 

A colloidal palladium nanoparticle prepared from a Fischer carbene complex of tungsten with K2PdCl4 as the reductant and PEG as the capping agent, efficiently catalyzed the Hiyama cross-coupling reactions in air. ... 

The first report of using RCM to synthesize aromatic compounds seems to be the 1976 report by Katz and Rothchild, who investigated the mechanism of olefin metathesis [3], They synthesized phenanthrene (7) from a 1 1 mixture of 2,2 -divinylbiphenyl (5) and the deuterated derivative 6 by RCM using a Fischer carbene complex (molybdenum catalyst or tungsten catalyst) in one of the experiments to reveal whether metathesis occurred by a pairwise or a nonpairwise mechanism [4] (Scheme 26.1). Although the yield of 7 was only 1 to 2% [5], this experiment clearly demonstrated the possibility of using RCM to synthesize aromatic compounds. 

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. 

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