Mechanochemistry, or chemistry by grinding of reagents, is becoming increasingly prevalent in organic chemistry synthesis. Mechanochemistry can allow the solvent-free production and purification of compounds with fewer waste products and less cost than traditional solvent chemistry. This makes mechanochemistry an ideal approach for new synthetic protocols for porphyrins. As a model reaction, benzaldehyde and pyrrole are ball-milled in the presence of an acid catalyst for ten minutes. This produces a cyclized precursor to a porphyrin, which is then oxidized to produce tetraphenylporphyrin (TPP) along with a number of byproducts. Porphyrins are usually isolated from solution-synthesis mixtures by column chromatography. This research focuses on purifying TPP by separating it from the mechanochemical reaction mixture via entrainment sublimation. During this processes, a nitrogen carrier gas passes over the sample in a Pyrex processing tube while being heated by a tube furnace. TPP is sublimed away from the impurities and subsequently deposited in a cooler area of the tube. Since TPP sublimes at around 400<b> °</b>C, this process has been carried out at various temperatures near this point with various nitrogen gas flow rates. Also, different aldehydes have been tested and used to form porphyrins with different R-groups attached to the methine bridges. The highest pure, isolated yield we have obtained is 6.4%, using 2-chlorobenzaldehyde as a starting material. The conditions will continue to be optimized for various aldehydes in an attempt to raise yields to levels that match or surpass traditional solvent-chromatography yields of about 30%.

Entrainment sublimation for purification of mechanochemically-synthesized porphyrins

249th National Meeting (2015)

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