Imidazolidinone Catalysis: Asymmetric Alkylation of Indole

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Conjugate addition mechanism

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The chiral cataylst reacts with the carbonyl to form an imine. This shields one side of the molecule, forcing the indole to attack from one face only via a simple conjugate addition mechanism. The imine can then be hydrolysed, regenerating the catalyst and giving the carbonyl product.

Press the Substrate Spacefill On button to judge for yourself which side the nucleophile is most likely going to attack

J. F. Austin and D. W. C. MacMillan, J. Am. Chem. Soc., 2002, 124, 1172–1173.