INVESTIGATION OF A SYNTHESIS OF MESO-PORPHYRINS EMPLOYING HIGH-CONCENTRATION CONDITIONS AND AN ELECTRON-TRANSPORT CHAIN FOR AEROBIC OXIDATION

The room-temperature synthesis of meso-porphyrins has been investigated at aldehyde and pyrrole concentrations greater than or equal to 10.1 M using tetraphenylporphyrin (TPP) and tetramesitylporphyrin (TMP) as models. The decline in yield that occurs at-higher reactant concentrations can be offset partially with higher acid catalyst concentrations. Reactions of 0.1 M benzaldehyde and pyrrole afford 20-30% yields of TPP at appropriate concentration of three acids, trifluoroacetic acid, BF3.OEt(2), or methanesulfonic acid, using the two-step process of acid-catalyzed condensation followed by quinone oxidation. A one-step synthesis of porphyrins has been investigated where aldehyde, pyrrole, acid catalyst, and oxidant are present simultaneously. Among five quinones examined only p-chloranil (TCQ) provides successful oxidation. The one-step synthesis performed at concentrations of 0.1 M affords TPP in 10-20% yields. Both the one- and two-step syntheses at high concentrations involve slurries due to the amount of TCQ required for oxidation. An electron transport chain employing TCQ and iron(II) phthalocyanine (FePc) in catalytic amounts and O-2 as the terminal electron acceptor has been adapted for the porphyrin synthesis. FePc activates O-2 and recycles reduced TCQ at room temperature. This electron transport chain provides mild aerobic oxidation. The aerobic oxidation process is cleaner than oxidation with stoichiometric amounts of TCQ and can be implemented as part of a one-step or two-step porphyrin synthesis. Reaction of 0.1 M benzaldehyde and pyrrole with acid catalysis in the presence of 5 mol% TCQ and 5 mol% FePc with gentle aeration affords TPP in 11% (one step) or 24% (two step) yield. Mesitaldehyde is converted to tetramesitylporphyrin upon reaction at a concentration of 0.1 M with BF3-ethanol cocatalysis and aerobic oxidation. The two-step synthesis with aerobic oxidation proceeds under mild ambient conditions, affords similar to 100 mg porphyrin from 50-mL reactions, and should prove generally useful for preparative scale syntheses meso-porphyrins.