Tungsten vinylcarbene complex

Experimental Procedure 2.2.7. Diels-Alder Reaction of a Tungsten Vinylcarbene Complex Methyl 2-(2-Furyl)-5-methyl-4-oxo-l-cyclohexanecarboxylate [237]... 

The anion 117 prepared by deprotonation of the carbene complex [Cr(CO)s C(OEt)CH2CHS(CH2)3S ] (116) also rearranges spontaneously by carbonyl insertion to give a neutral six-membered chelate (118) after alkylation. The analogous tungsten carbene complex affords stereoselec-tively a vinylcarbene complex (119) by opening of the 1,3-dithiane ring (705). Reaction of 118 with methyl hydrazine produces mainly an NMe- or... 

Inter- and intramolecular trapping of vinylcarbene complexes generated in situ, including those of metals such as chromium, molybdenum and tungsten, have also been described. ... 

However, yields in the intermolecular cycloaddition reactions of vinylcarbene complexes, formed by intramolecular insertion of an alkynyl tethered metal carbene complex, are higher when molybdenum rather than chromium or tungsten carbene complexes are employed. Mild thermolysis (THF, 65 °C, 1 h) in the presence of ten equivalents of an electronically undemanding alkene directly leads to the 2-alkyl-2-(2-methoxycyclopentenyl)cyclopropanes 31. ... 

Cyclopropanation. These carbenes are particularly useful for cyclopropanation of electron-poor olefins. The reaction occurs under milder conditions and at a faster rate with molybdenum carbenes than with chromium- or tungsten-derived complexes. This cyclopropanation has been used to trap a molybdenum vinylcarbene generated by intra-... 

The diastereoselectivity of Diels-Alder reactions with acyclic sugar-vinylcarbene complexes depends on the nature of the diene. Whereas the tungsten vinylcar-bene 222 gives all four possible diastereomers of cydoadduct 296 upon reaction with cyclopentadiene, only one pair of diastereomers of 297 is observed with 2,3-dimethylbutadiene (Scheme 11.61) [110]. 

---