p-Nitrophenyl Ethylthioester in Enantioselective Organocatalytic Michael Additions: Different Behaviour of β-Aryl and β-Alkyl Enals

Although several nucleophiles derived from arylacetic acids have been described in catalysis via iminium activation, none have presented good enantioselectivity and reactivity with both β‐alkyl and β‐aryl enals. This study on the Michael addition of p‐nitrophenylacetic thioesters suggests that this behavior is due to the different reactivity and distinct tendency to reversibility exhibited by these types of enal. Independent optimization of the conditions for β‐alkyl and β‐aryl enals provide good yields and enantioselectivities with both substrates, affording Michael adducts that are versatile building blocks for the preparation of an interesting variety of arylacetic acid derivatives. S‐Ethyl 4‐nitrophenylthioacetate presents excellent reactivity and enantioselectivity in organocatalytic asymmetric Michael reactions with both aromatic and aliphatic enals under different conditions. The corresponding adducts have proven to be versatile intermediates.