Solvent-free synthesis of meso-tetraarylporphyrins in air: product diversity and yield optimization

The scope and optimization of a solvent-free method for the rapid preparation and facile purification of technologically important meso-substituted aryl porphyrins, such as 5,10,15,20-tetraphenylporphyrin is presented. This one-step method involves heating the aromatic aldehyde to similar to 200 degrees C in a vial fitted with a septum-lined cap, followed by addition of the pyrrole and maintaining the temperature for about 20 minutes. The dioxygen in air serves as the oxidant. Present results show that the addition of benzoic acid as a catalyst improves the yield of 5,10,15,20-tetraphenylporphyrin from 22% to 32% and of para halogenated phenylporphyrins from 10% to similar to 25%. Herein is also presented an examination of the many factors that influence the yield, the ease of purification, and the ability to scale up the reaction. Since the tarry by-products from this method are much less soluble than in most other synthetic strategies, much less solvent is required for purification; simple extraction is often sufficient. This method can be scaled in the lab to >300 mg, and provides an attractive route to many meso-substituted porphyrins because of its minimal waste generation in terms of both solvent and chromatography support.