A New Mode of Action for Porphyrins – Nonplanar Porphyrins As Organocatalysts

Tuesday, 30 May 2017: 10:00
Churchill A2 (Hilton New Orleans Riverside)
M. O. Senge (Trinity College Dublin)
Porphyrins are nature’s cofactors par excellence. Next to oxygen transport and storage, their role in electron transport and as photosynthetic pigments, they catalyze a multitude of chemical reactions. All these catalytic functions depend on the presence of a central metal which is intricately involved in the catalytic processes. Thus, all known catalytically active porphyrins are metalloporphyrins. However, we have discovered that, upon appropriate manipulation of the porphyrin macrocycle conformation, the core nitrogen atoms in free base porphyrins can be involved in catalysis as well.

Distorted porphyrins are able to activate small molecules via general base catalysis by participation of the imine groups or via hydrogen bonding catalysis involving the pyrrole units. To make the “active center” more accessible to substrates, the degree of outwards orientation of the pyrrole groups can be fine-tuned depending on the peripheral substitution pattern using peri-interactions or core substitution. Various distortion modes exist and saddle distorted porphyrins have been shown to have an increased basicity which qualifies these tetrapyrroles for catalytic screening. Additionally, it is possible to enhance the basicity via electronic modulation of the β- and meso-substituents through the introduction of electron-withdrawing groups. An extensive screening of a number of nonplanar porphyrins and substrates, as well as reaction conditions, has been used to evaluate the catalytic properties of planar and distorted porphyrins. Notably, certain free base porphyrins have been found to function as organocatalysts in Michael addition reactions indicating a new mode of catalytic action for porphyrins which does NOT require a central metal ion.