Investigation of phthalocyanine catalysts for the aerobic synthesis of meso-substituted porphyrins

Abstract

The aerobic oxidation process for the synthesis of porphyrins, previously performed using 5 mol % p-chloranil (TCQ), 5 mol % iron(II) phthalocyanine (FePc) and stoichiometric amounts of O₂, has been refined using new phthalocyanine catalysts. Four phthalocyanine catalysts have been prepared, characterized by Mossbauer spectroscopy and examined for efficacy in the high concentration (0.1 M) synthesis of tetraphenylporphyrin at room temperature. Each phthalocyanine has been identified to be a μ-oxo dimer. Two catalysts are soluble (the μ-oxo dimers [(t-butyl)₄FePc]₂O and [(n-C₆H₁₃O)₄FePc]₂O) and enable homogeneous reactions, while two are insoluble (the μ-oxo(1) and μ-oxo(2) dimers of FePc, (FePc)₂O) and give heterogeneous reactions. These four phthalocyanine compounds provide efficient catalysis at the 0.3–1 mol % level using only 1 mol % quinone or hydroquinone (TCQ, DDQ, TCQH₂ or DDQH₂), affording ∼25% yields of tetraphenylporphyrin in 60 min of oxidation. There are no discernible advantages of the homogeneous versus heterogeneous catalysts. The μ-oxo dimers are active, but FePc is inactive, at the 0.3 mol % level. The activity of the FePc sample at the 5 mol % level is attributed to residual μ-oxo dimer impurity. This aerobic oxidation process is superior to stoichiometric oxidation with TCQ or DDQ and can be performed in the presence of BF₃•O(Et)₂, trifluoroacetic acid or under neutral conditions.