Porphyrin synthesis under solvent-free conditions represents the "greening" of a traditional synthesis that normally requires large amounts of organic solvent, and has hindered industrial-scale synthesis of this useful class of molecules. We have found that the four-fold acid-catalysed condensation of aldehyde and pyrrole to yield a tetra-substituted porphyrin is possible through mechanochemical techniques, without a solvent present. This represents one of the still-rare examples of carbon-carbon bond formation by mechanochemistry. Specifically, upon grinding equimolar amounts of pyrrole and aldehyde in presence of an acid catalyst, cyclization takes place to give reduced porphyrin precursors (reversible), which upon oxidation form meso-tetraarylsubstituted porphyrin. The approach has been found suitable for synthesis of a variety of meso-tetraarylsubstituted porphyrins from the corresponding aromatic aldehyde. Here we will present progress on mechanochemical synthesis of a series of such porphyrins, with special focus on those that are most difficult to synthesize in solution (reported yield &lt; 5%) due to the bulky nature of the aryl group. Products were characterized by UV-Vis spectroscopy and proton NMR. These results hold promise for the development of mechanochemical synthetic protocols for porphyrins and related classes of compounds.

Mechanochemical synthesis of porphyrins with bulky meso-substituents

255th National Meeting (2018)

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