Group 14 elements Carbene complexes

The first group 13 (IIIA) element-carbene complex to be reported was an imi-dazol-2-ylidene-alane complex (Scheme 8.23). Based on NMR data, it was suggested that the imidazole fragment has an electronic structure that is intermediate between those of the free carbene and imidazolium ion. The use of an imidazol-2-... 

Finally, the possibility of building the M=C bond into an unsaturated metallacycle where there is the possibility for electron delocalization has been realized for the first time with the characterization of osmabenzene derivatives. For these reasons then, it seemed worthwhile to review the carbene and carbyne chemistry of these Group 8 elements, and for completeness we have included discussion of other heteroatom-substituted carbene complexes as well. We begin by general consideration of the bonding in molecules with multiple metal-carbon bonds. 

A wide variety of cyclopropenylium metal cations has been prepared and studied over the past two decades, ranging from main group elements (Li, Mg) through transition metal -complexes (Fe, Mn, Pd, Pt). Extensive NMR studies suggest that the metal carbene... 

Derivatives of the Group 15 Elements Arsenic, Antimony, and Bismuth Aumann, Rudolf, (1 -Alkynyl )carbene Complexes (= l-Metalla-l-buten-3- 39 325... 

The cyclic variety of MACs are called cyclic alkyl amino carbenes (CAACs) and have an excellent steric element [198]. With a cyclohexyl wingtip group on the carbon end of the carbene - known as a wall of protection - they can sterically block one side of the transition metal centre in a transition metal carbene complex effectively introducing outstanding regioselectivity and stereoselectivity in catalytic processes [198] (see Figure 3.70). 

Once the hydroxy functionalised imidazolium salt is formed, it can be deprotonised and reacted with various metal complexes to form (transition) metal carbene complexes. The hydroxy group ensures that the ligand can be coordinated even to metals that are normally reluctant to form stable carbene complexes. A good example is the deprotonation of a hydroxyethyl functionalised imidazolium salt with potassium hydride [36]. The potassium cation coordinates to the oxygen atom of the alkoxide sidechain and forms cubes as structural elements (see Figure 4.6). The carbene end then coordinates to the respective... 

Recently, significant progress has been made in the characterization of main group adducts of these carbenes. This chapter will deal only with carbene compounds of the main group elements reported since 1993 and excludes transition metal carbene compounds (such compounds may be found in the article Carbene Complexes). 

Electrophilic transition-metal-carbene complexes (Fischer carbene complexes) serve as formal carbene transfer reagents in reactions with alkenes to give functionalized cyclopropanes. This reaction behavior is well documented for alkoxycarbene complexes of elements of group In contrast, aminocarbene complexes exhibit a different reactivity over a wide range of conditions and [2 + 1] cycloadditions to alkenes represent exception. 

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