Why are Aromatic Additives Effective in Asymmetric Organocatalysis?

The presence of an aromatic additive has been seen to enhance, often significantly, the yield and enantioselectivityof the desired product in asymmetric organocatalysis.Considering their success across a dizzying range of organocatalysts and organic transformations, it would seem unlikely that a common principleexists by which they function.However, the current investigations with DFT reveal that such a general principle indeed does exist: thearomatic additivesandwiches itself, through hydrogen bonding and π•••π stacking, between theorganocatalyst coordinatedelectrophile and nucleophile. This is seen to occur for a wide range of experimentally reported systems.That such complex formation leads to enhanced stereoselectivity isthen demonstrated for two cases: the cinchona alkaloid complex (BzCPD), catalysing thiocyanation(2-naphthol additive employed), as well as for the L-pipecolicacidcatalyst (2,4-dinitrophenol additive employed), catalysingtheasymmetric nitroaldol reaction.These findings are likely to have a significant impact on the field of asymmetric organocatalysis.