Reactivity of transition metal-carbene complexes

Olefin cyclopropanation and carbonyl olefination are generally mediated by tra- 

Transition Metal-Carbene Complexes in Olefin Metathesis 

The reactivity patterns of the various formal representations of metal-carbene polarization. 

Transition metal-carbene complexes undergo many more reactions in addition to these three and have been widely applied to the synthesis of organic molecules. Because this material is outside the scope of the chapter, we direct the interested reader to reviews on other carbene-mediated transformations [19-21]. 

The usual representation of Schrock-type nucleophilic carbenes as electron rich at carbon can be especially misleading in the case of the Tebbe reagent and related complexes. These high oxidation state complexes are electron-deficient and electrophilic at the metal center, and it is unlikely for polarization of the metal-carbon bond to remove even more electron density from the metal under these circumstances. Thus, the reactivity of the Tebbe reagent is more closely related to the electrophilicity and oxophilicity of the metal center than to the nucleophilicity of a polarized carbene carbon that is, the reactivity is due to carbonyl polarization upon complexafion, not attack of the alkylidene carbon on an unactivated, electrophilic carbonyl carbon. 

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Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

There has been considerable recent interest in the reactivity of transition-metal carbene complexes and other species containing metal-carbon multiple bonds because of their involvement in olefin metathesis and the... 

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