Porphyrin Synthesis

CHEM3412

Synthesis of a Porphyrin

Porphyrins are hetereonuclear, macrocyclic compounds that play an important role in living organisms. Examples include the non-protein heme portion of hemoglobin in animals whereby the lone pairs on nitrogen can bind to the oxygen-carrying iron. In plants, chlorophyll uses the vast conjugation to allow absorption of light for photosynthesis. The characteristic colors of porphyrins is also due to the conjugation.

An examination of the porphyrin structure reveals that individual pyrrole units comprise the overall structure. As an aromatic compound, pyrrole can participate in electrophilic aromatic substitution reactions to form the porphyrin (your report should discuss the mechanism in great detail, including the regiochemistry). The initial porphyrinogen product is not fully aromatic, but oxidation from atmospheric oxygen in the presence of the Lewis acid silica gel allows formation of the fully conjugated porphyrin product. Using benzaldehyde as the electrophile will result in a phenyl substituent on each carbon that links the pyrrole units. The porphyrins synthesized in this experiment will not be bound to metals.

Several features of this synthesis are worthy of comment. A recent trend known as "Green Chemistry" seeks to synthesize materials while having a minimal impact on the environment. This includes concepts such as atom economy, less toxic reagents, catalysis, and alternative energy sources. You will notice, for instance, that this reaction does not use a solvent. Consider also the amount of energy needed to microwave the reaction for 10 minutes vs. the traditional reaction conditions of heating to 250 oC for 20-30 minutes.

The principle of atom economy attempts to avoid "wasting" reactant atoms. A calculation of atom economy will show the efficiency of the reaction, and not just the chemical yield. The calculation take the molecular weight of the product and divides it by the sum of the molecular weights of the reactants. At one extreme is the addition of bromine to ethylene which has an atom economy of 188 g/mol / (160 g/mol + 28 g/mol) = 100%. A Wittig reaction such as:

has an atom economy of 56.1/(58.8 + 276.1) = 16.8% Very wasteful

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