Porphyrin synthesis normally requires several kilograms of organic solvent for production of a few grams of product. This requirement has hindered industrial-scale synthesis of this useful class of molecules. We are studying the viabiliy of porphyrin syntheis and purification under solvent-free conditions. We have found that the four-fold acid-catalysed condensation of aldehyde and pyrrole followed by oxidative aromatization to yield a <i>meso</i>-tetrasubstituted porphyrin is possible through mechanochemical techniques, that is by grinding, without a solvent present. For example, upon grinding equimolar amounts of pyrrole and benzaldehyde in presence of an acid catalyst, cyclization takes place to give reduced porphyrin precursors (reversible), which upon oxidation form tetraphenylporphyrin (TPP). The approach has been found suitable for synthesis of a variety of <i>meso</i>-tetrasubstituted porphyrins. When oxidation of the mechanochemically-cyclized porphyrin precursor is carried out by using an oxidizing agent in solution, yields comparable to those published for traditional methods of porphyrin synthesis are obtained. When oxidation is done through mechanochemical means lower yields are obtained, but the porphyrin is still isolated. Results will be presented of mechanochemical oxidation optimization studies where oxidizing agent, filling ratio, milling medium, milling frequency, milling time, and milling ball size were varied. Results from optimization studies of solvent-free porphyrin purification by entrainment sublimation will also be presented. In these studies flow rate, temperature, and sample size were varied. These solvent-free methods of porphyrin synhesis will be compared with traditional methods using Green Chemistry metrics, namely, E factor and EcoScale. It will be shown that these techniques hold promise for the development of mechanochemical synthetic protocols for porphyrins and related classes of compounds, leading to a possibility of reduction or elimination of the need for large amounts of organic solvent when producing these useful molecules on a large scale.

Porphyrin synthesis by grinding

249th National Meeting (2015)

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