Reactivity ambiphilic ligands

As discussed in the previous section, the coordination properties of PBs and related ambiphilic ligands have been quite extensively studied over the last decade. Comparatively, the reactivity and catalytic application of the ensuing complexes have only been scarcely explored. The few investigations reported in this area are summarized in this section. They nicely illustrate the various possibilities offered by the presence of the Lewis acid in the coordination sphere of a metal or at proximity. 

Ambiphilic Ligands Unusual Coordination and Reactivity Arising from Lewis Acid Moieties... 

By design, ambiphilic ligands combine donor and acceptor sites on the same skeleton (Figure 1). The basic idea is to use donor sites as anchors to introduce Lewis acids in the coordination sphere of transition metals. Fine tuning of the structure of the ambiphilic ligands (coordination sites and linker) gives the possibility to control the position of the Lewis acid moiety and the way it participates in bonding and/or reactivity. 

Reactivity of metallic complexes deriving from ambiphilic ligands... 

So far, the potential of ambiphilic ligands in reactivity and catalysis has only been scarcely investigated. The kinetic, thermodynamic and even the outcome of metal-mediated transformations can be influenced by the presence of a Lewis acid in the coordination sphere. As discussed hereafter, the first studies performed in this area are very promising and further developments are certainly to be expected. 

The research onphosphine-boranes and related ambiphilic ligands is still in its infancy, but their coordination properties are particularly appealing and open interesting possibilities in reactivity. Depending on the role of the Lewis acid, four different situations can be distinguished ... 

The ambiphilic character of JT-allylmthenium complexes is in remarkable contrast to palladium chemistry [29]. A series of (jt-C3H5)RuX(CO)3 (X = Br, OAc or OTf) complexes prefer the attack of electrophiles such as aldehydes as well as the attack of nucleophiles such as NaCH(C02Me)2, while Jt-allylpalladium complexes react exclusively with nucleophiles. Thus, stoichiometric reactions of Jt-allylmthenium complex with benzaldehyde and the sodium salt of diethyl malonate afford the corresponding homoallyl alcohol and allylmalonate, respectively (Scheme 5.1). The carbonyl ligand plays a very important role, and ambiphilic reactivity is realized only in ruthenium complexes bearing a carbon monoxide ligand. 

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