Identification of Stable Porphomethenes and Porphodimethenes from the Reaction of Sterically Hindered Aldehydes with Pyrrole

Use of pivalaldehyde in mixed acid-catalyzed condensations of an aryl aldehyde with pyrrole allows the isolation and structural characterization of stable porphomethenes (5,10,15,22-tetrahydroporphyrins) and porphodimethenes (both 5,10- and 5,15-diphydroporphyrins) as intermediates of porphyrin synthesis. Crystal structures reveal the importance of the absolute configuration at the sp3-hybridized centers for the oxidation and stability of these (bio)synthetic intermediates.