Roucan, M., M. Kielmann, S. J. Connon, S. S. R. Bernhard, and M. O. Senge. “Conformational Control of Nonplanar Free Base Porphyrins: Towards Bifunctional Catalysts of Tunable Basicity.” Chemical Communications 54, no. 1 (2018): 26–29.
For the first time, free base and N-methylated porphyrins have been utilized as bifunctional organocatalysts in Michael additions and it was found that distortion of the macrocycle is a vital prerequisite for their catalytic activity. Conformational design has been used to tailor the properties of nonplanar porphyrins with regards to availability of the N–H units for hydrogen bonding (distortion-dependent hydrogen bonding) and the basicity of the heterocyclic groups. NMR spectroscopic- and catalyst screening studies provided insight into the likely mode of catalyst action. This unprecedented use of free base and N-substituted porphyrins as organocatalysts opens a new functional role for porphyrins.