Lowest unoccupied molecular orbital Lewis acid catalysts

The Catalysis Concept of Iminium Activation In 2000, the MacMillan laboratory disclosed a new strategy for asymmetric synthesis based on the capacity of chiral amines to function as enantioselective catalysts for a range of transformations that traditionally use Lewis acids. This catalytic concept was founded on the mechanistic postulate that the reversible formation of iminium ions from a,p-unsaturated aldehydes and amines [Eq. (11.10)] might emulate the equilibrium dynamics and 7i-orbital electronics that are inherent to Lewis acid catalysis [i.e., lowest unoccupied molecular orbital (LUMO)-lowering activation] [Eq. (11.9)] ... 

By definition, Lewis acid catalysts involve a metal center as an electron pair acceptor that accepts the electron pair from a nucleophile. This property makes them effective in many organic reactions and indispensable for the production of a large category of chemicals from simple alkylated compounds to complicated polymers or pharmaceuticals [6]. Also, the differences between the energy levels of the highest occupied molecular orbital (HOMO) of the reactant (nucleophile) and the lowest unoccupied molecular orbital (LUMO) of the Lewis acid (electrophile) make Lewis acid catalysis more complicated than the corresponding Br0nsted catalysis [7]. 

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