Lewis acids transition-metal cooperative

Lewis Acid/Transition-Metal Cooperative Catalysis. 179... 

Beyond the concept of combined acid catalysis, there are also many other bifunctional acid catalysts interacting with nucleophiles and electrophiles simultaneously, and thus benefiting through such cooperative effect. Selected examples on Lewis acid/hydrogen bonding cooperative catalysis and Lewis acid/transition-metal... 

In the simplest case, heterodinuclear complexes will react as pairs of metal electrophiles and nucleophiles. Cooperative reactivity is particularly likely with polar substrates. In this case, the electropositive early transition metal center may react with the more Lewis-basic part of the substrate, while the nucleophilic late transition metal complex fragment will attack the more Lewis-acidic part of the substrate molecule. For metal- metal single bonds between the two metal centers, this interaction wiU go along with the cleavage of the metal-metal bond. In many cases, it is difficult to establish whether the scission of the metal-metal bond precedes the transformation of the substrate or whether it occurs at a later stage. 

To resolve this reactivity problem, we explored the possibility of employing a Lewis acid to activate the iV-acyl hydrazone by lowering its LUMO energy (Scheme 22) [145-147]. If successful, such an achievement would expand the possibilities for the development of new NHC reactions. The most important challenge to this idea is the known ability of carbenes to act as ligands and form stable complexes, particularly with late transition metals such as palladium and copper [148-151]. We hypothesized that early metals would react reversibly with NHCs, allowing for the development of a cooperative catalytic system. After... 

More examples of these types of cooperative use of transition metal catalysts with Lewis acid catalysts can be found in a recent review [9]. And we expect many more asymmetric reactions of this type could be developed in the near future. 

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