Tungsten heteroatom-substituted

Heteroatom-substituted (Fischer-type) carbene complexes are mostly used as stoichiometric reagents. For this reason only carbene complexes of reasonably cheap metals, such as chromium, molybdenum, tungsten, or iron have found broad application in organic synthesis. 

Conjugate hydride abstractions have also been used for the generation of carbon-metal double bonds. An interesting reaction sequence, in which a (thermally unstable) cationic, non-heteroatom-substituted tungsten carbene complex is prepared by conjugate hydride abstraction, is shown in Figure 3.9. 

Fig. 3.9. Preparation of a cationic, non-heteroatom-substituted tungsten carbene complex by conjugate hydride abstraction [435].

Also non-heteroatom-substituted tungsten [440,443,444,451,452,461], molybdenum [437], and chromium carbene complexes [440] have been prepared by a-abstraction of alkoxide. 

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). 

Synthesis of Five-Membered Carbocycles An annulation of chromium arylcarbenes by alkynes without CO-insertion leads to five-membered rings. It has been observed as a side reaction along the benzannulation which may become predominant depending on the nature of the carbene heteroatom substitution pattern, [72] the metal [40] and its coligands [73] and the solvent [43] used. In this respect, indene derivatives have been obtained from arylcarbene complexes of chromium and tungsten. [73,74]... 

The Schrock type tungsten (VI) carbyne complexes Cl3(dme)W=C Bu (A), ( BuO)3W=C Bu (B), Np3W=C Bu (C) and the heterogeneous catalyst Si02/Np2W=CH Bu (D) are very active olefin metathesis catalysts. They catalyze ROMP reactions of mono and polycyclic olefins and ADMET or ringclosure (RCM) reactions of a,co-dienes. They also tolerate many heteroatom substituted monomers. 

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