Carbene Complexes with Group 15 Elements

Carbene Complexes with Group 1 and Group 2 Elements... 

Structural Characterization of Carbene Complexes with Heavy Group 13 Elements... 

Carbene complexes with heavier Group 13 elements have recently been explored in the activation of small molecule substrates and in a variety of catalytic reactions. NHC-AlH Cl3 complexes were used as stoichiometric reagents in the hydroalumination of carbonyl- and epoxide-containing sub-strates. " More importantly, however, substoichiometric amounts (5 mol%) of NHC-AlEta complexes were shown to induce the ring-opening of epoxides with addition of an ethyl nucleophile in modest to excellent yields. ... 

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. 

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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