Mechanistic Insights in the Mode of Action of Bifunctional Pyrrolidine-Squaramide-derived Organocatalysts

Abstract

The modes of catalytic action of three squaramide-derived bifunctional organocatalysts have been investigated using DFT methods. The [5+2] cycloaddition between oxidopyrilium ylides and enals has been used as model reaction. Two primary modes are possible for the different catalysts studied. The preference for one mode over the other is due to the possibility of additional favourable π,π-interactions between the H-bond activated pyrilium ylide and an electron-deficient aromatic ring bonded to a squaramide NH-group. The model can be extended to other reactions catalyzed by the same catalysts such as formal [2+2] cycloadditions between nitroalkenes and α,β-unsaturated aldehydes. The computational results are in excellent concurrence with the available experimental reports on the observed total enantioselectivity and differences in diastereoselectivity depending on the substrate and the reaction