Investigation of Streamlined Syntheses of Porphyrins Bearing Distinct Meso Substituents

The use of porphyrins in fundamental studies and diverse applications requires facile access to ample quantities of material in pure form. The existing conditions for the condensation of a dipyrromethane plus a dipyrromethane−dicarbinol employ 2.5 mM reactants and afford ∼30% yields with no detectable scrambling. Large-scale syntheses require condensation and oxidation conditions that function at higher concentrations. Thirty-one acids (plus additives) have been examined for reactions at 25 mM reactants using the synthesis of a trans-A₂B₂-porphyrin as a model. The porphyrin was formed in ∼20% yield upon condensation in CH₂Cl₂ at room temperature using (1) Sc(OTf)₃ (3.2 mM) + 2,6-di-tert-butylpyridine (32 mM), or (2) Zn(OTf)₂ (10 mM). Nine porphyrins were prepared in this manner in yields of 15−22% with no detectable scrambling, whereas three other porphyrins afforded low levels of scrambling and/or lower yields (8−14%). Conditions for the oxidation also have been investigated. The reaction of 5-mesityldipyrromethane and the dicarbinol derived from 1-(4-methoxybenzoyl)-9-(4-methylbenzoyl)-5-phenyldipyrromethane (18 mmol each in 720 mL of CH₂Cl₂; 25 mM) with catalysis by Sc(OTf)₃/2,6-di-tert-butylpyridine and aerobic oxidation [(t-Bu₄FePc)₂O and DDQ, 2.5 mol % each with a stream of O₂] afforded 2.88 g (22.8% yield) of the corresponding ABCD-porphyrin. The present synthesis (25 mM), at 4.5-times larger scale than the largest prior analogous synthesis (2.5 mM reactants with stoichiometric use of DDQ), afforded 4.0 times as much porphyrin on a molar basis while employing about one-half the amount of solvent and <¹/₂₅ the amount of DDQ. A three-step one-flask process also was developed that employs (i) condensation at 25 mM reactants, (ii) aerobic oxidation, and (iii) metal insertion to afford the metalloporphyrin [Mg(II), Ni(II), Cu(II), Zn(II), Pd(II)] in a streamlined manner. Taken together, the various improvements facilitate gram-scale syntheses of diverse porphyrins.