Non-planar porphyrins: An innovative approach for organocatalysis

Porphyrin is a macrocycle containing four pyrrole rings with methine bridges connecting the pyrrolic α-carbon position. In catalytic terms, this tetrapyrrolic macrocycle is usually metalated, giving it the role of a co-factor and catalyst in chemical reactions. This is attributed to the metal centre’s ability to accommodate a substrate molecule in its co-ordination sphere. However, non-metalated free base porphyrins have been overlooked catalysts. In a planar porphyrin, the porphyrins nitrogenous lone pairs and N-H units are shielded and are unable to participate in non-covalent interactions i.e., hydrogen bonding. Recently, we have shown, by distorting the conformation of the macrocycle by N-substitution and/or increasing steric bulk in the porphyrin periphery, that the inner pyrrolic N/NHs are available for use in organocatalysis. The non-planarity of these distorted porphyrins is critical for the catalytic activity. Non-planarity opens the porphyrin core for non-covalent interactions by pushing the pyrrolic NHs out of plane. We demonstrated that non-planar saddled porphyrin is highly effective in catalysing the Sulfa-Michael reactions to yields greater than 98%. By introducing more complex substituents in the periphery of the porphyrin we hope to catalyse more complex chemical reaction, through multiple non-covalent binding sites. Further increasing the macrocycles’ ability to bind substrate and efficiency as a catalyst.