Mechanistic Insights into a BINOL-Derived Phosphoric Acid-Catalyzed Asymmetric Pictet-Spengler Reaction

The reaction of tryptamine and (2-oxocyclohexyl)acetic acid can be catalyzed by 3,3'-bis(triphenylsilyl)-1,1'-bi-2-naphthol phosphoric acid to give an asymmetric beta-carboline. This reaction was first studied by Holloway et al. (Org. Lett. 2010, 12, 4720-4723 ), but their mechanistic work did not explain the high stereoselectivity achieved. This study uses density functional theory and hybrid quantum mechanics/molecular mechanics calculations to investigate this reaction and provide a model to explain its outcome. The step leading to diastereo- and enantioselectivity is an asymmetric Pictet-Spengler reaction involving an N-acyliminium ion bound to the catalyst in a bidentate fashion. This interaction occurs via hydrogen bonds between the two terminal oxygen atoms of the catalyst phosphate group and the hydrogen atoms at N and C-2 of the substrate indole group. These bonds hold the transition structure rigidly and thus allow the catalyst triphenylsilyl groups to influence the enantioselectivity.