Diarylporphyrins have substituents in only two of the four meso positions and are fascinating synthetic targets due to their unsymmetrical nature, the possibility of forming isomers, and the possibility of “scrambling”; a process whereby substituents migrate to neighboring meso-positions. They are also useful precursors to porphyrins with unsymmetrical substitution patterns (A2B2-type porphyrins), used in photodynamic therapy, as supramolecular building blocks, toxicology and other applications. Previously, we have shown that the four-fold acid-catalyzed condensation of aldehyde and pyrrole yielded a tetra-substituted porphyrin using mechanochemical techniques, without a solvent present. In this study, we grind equimolar amounts of dipyrromethane and benzaldehyde in a ball mill in presence of an acid catalyst. Following this, the reaction mixture is oxidized using DDQ to yield porphyrinic products, as evidenced by UV-Vis and proton NMR spectroscopy. The products can be purified using small plug of alumina, followed by silica. Our results suggest that with each replication of the experiment we are obtaining 5,15-diphenylporphyrins and never 5,10-DPP which suggests that scrambling is not taking place. The results also show that we are getting an average percent yield around 12% compared to literature yield of 9.5%, and EcoScale scores of -20.08 and -109.25 respectively. Studies are ongoing and being further extended as we are now trying to use different aldehydes such as 4-fluorobenzaldehyde, 2-napthaldehyde, and 3 4-dihydroxybenzaldehyde to test the generalizability of this mechanochemical reaction.

Mechanochemistry leading to a series of diarylporphyrins

2019 Spring National Meeting

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