Cooperative activation electrophile

The secondary amine of the AEP group is responsible for the supported enamine formation with acetone (aldol reaction), the deprotonation of nitromethane (Henry reaction) and the generation of a potential nucleophile from trimethylsilyl cyanide through hypervalent silicate formation (cyanosilylation reaction). Therefore, the presence of both AEP and UDP groups in close proximity can cooperatively activate the electrophile (through hydrogen bond) and the nucleophile by enamine formation, thus enhancing the reaction rate. 

Mechanistic investigations have shown that reduction of Ni(II) to Ni(0) and deprotonation of the hydroxyphosphine ligand leads to a nickel-magnesium bimetallic species. Oxidative addition occurs through transition state TS2 wherein the bimetallic species performs a push-pull cooperative activation of the electrophile (Figure 5.2). This activation differs from the one observed when using... 

Brpnsted bases have been explored in cooperative catalysis to deprotonate substrates that subsequently add to metal-activated electrophiles. Regarding the use of these organocatalysts in cooperative catalysis, the combination of quinine and quinine-derived thiourea organocatalysts with Cu(II)-PhBox and Cu(OTf) has been employed in aza-Henry [23] and... 

Scheme 26.4 (a-c) Selected examples of cooperative enamine addition into metal-activated electrophiles. See insert for color... 

Due to the ability of imidazolium compounds to form metallic Af-heterocyclic carbene complexes, imidazolium-based ionosilicas have widely been studied for the formation of silica-supported NHC species and found wide applications in organometallic catalysis. However, ionic species recently found to promote a large variety of reactions due to their ionic nature. Cooperative nucleophilic-electrophilic activation is a widely accepted concept in catalysis, and due to their ionic nature, ionic liquid should be considered as bifunctional catalysts. 

In the catalytically active complex 4-Ba the negative poles and the polyether bridge act as working units that perform cooperatively in providing the driving force for the formation of the complex itself, whereas the metal ion serves as an electrophilic catalyst both in the acylation and deacylation steps. The crucial importance of the polyether bridge is demonstrated by the disappearance of any catalytic activity upon replacement by two methoxy groups. 

Are heterogeneous catalysts appropriate platforms to construct cooperativity Cooperative catalysis can occm in homogeneous catalysis. For example, Kanemasa and Ito [3] have reported a two-catalyst system that leads to the dual activation of an electrophile and a nucleophile by a Lewis acid and amine base, respectively. 

Many reactions in organic synthesis are reactions of nucleophiles and electrophiles, in other words they are acid-base reactions. Usually, the nucleophilicity or the electrophilicity of a substrate is enhanced by strong acid or base to promote a reaction. However, a reaction can be also promoted when weak acid and base activate both a nucleophile and an electrophile at the same time. In a homogeneous medium, acid and base could not coexist because of neutralization. Therefore, it is not expected to promote a reaction by acid and base cooperation. However, acid and base supported on an inorganic solid make it possible to promote a reaction by acid and base cooperation. 

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