Chiral Lewis Acid
Chiral Lewis acids (CLAs) are a novel class of Lewis acid catalyst used in enantioselective asymmetric synthesis reactions which produce optically active products from optically inactive or impure starting materials. This type of preferential formation of one enantiomer or diastereomer over the other is formally an asymmetric induction. The electron accepting atom of CLAs are typically metal, such as indium, zinc, lithium, aluminium, titanium, or boron – and the chiral ligands employed for synthesizing CLAs most often have multiple Lewis basic sites (often diols or dinitrogen structures) to allow forming a ring structure involving the metal atom.
While the ability of achiral Lewis acids to promote the synthesis of racemic mixtures in myriad different reactions has been known for quite some time, starting in the 1960s, CLAs have been used to induce the enantioselectivity in these reactions. Common reaction types include Diels-Alder reactions, the ene reaction, cycloaddition reactions, hydrocyanation of aldehydes, and most notably, Sharpless expoxidations.
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