Tungsten Fischer carbene

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). 

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-... 

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. 

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. 

In 1994, Quayle et al. reported the application of this cyclic Fischer-carbene synthesis from 3-butynols to spirolactone synthesis, although the process was stepwise and a stoichiometric amount of the complex was employed [17]. The key transformation was the chromium or tungsten carbene complex formation followed by the CAN oxidation of the complex to give y-lactone. The reaction was further applied to the synthesis of andirolactone and muricatacin, the former being shown in Scheme 5.14. 

Aminothiophenol 37 and 277 formed an isolable adduct 278 (X = S) that on heating was transformed to a mixture of complex 283 (10%) and the demetallated benzothiazepinone 284 (51%) (Scheme 50) <2003CEJ4943>. Similarly, monocyclic 1,4-oxazepinone derivatives of tungsten and chromium have been prepared by a domino [4+2]/[2+2] cycloaddition reaction of 1-alkynyl Fischer carbenes with oxazolines <20050M302>. 

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

Several useful inter- and intramolecular reactions of chromium, molybdenum, tungsten, iron, or manganese Fischer carbenes with aUcenes to form cyclopropanated products have... 

Stable chromium and tungsten Fischer dienyl carbenes bearing a 2,3-dihydroisoxazole ring have been treated with isocyanides to give highly functionalized 2,3-dihydro-l,2-benzisoxazoles and indazoles. Treatment of chromium isoxazoline 110 with benzyl isocyanide afforded the tricyclic compound 111 selectively in 82% yield (Equation 10) <2001CEJ5318>. 

While these rhodium and copper carbenoids are unstable, some transition metals such as tungsten and chromium form stable, isolable carbenoids, called metallocarbenes or Fischer carbenes. 

Scheme 41. Examples of (a) tungsten and (i ) chromium Fischer carbene-mediated enyne cycloisomerization reactions.

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 series of stable chromium and tungsten Fischer dienyl carbenes 66 were prepared by 1,3-DC of alkenylethynyl carbene complexes 65 with nitrones. Treatment of isoxazolines with isocyanides afforded highly functionalised 2,3-dihydro-l,2-benzisoxazoles 67 in a completely selective fashion and with high yields <01CEJ5318>. 

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. 

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