One-flask synthesis of meso-substituted dipyrromethanes and their application in the synthesis of trans-substituted porphyrin building blocks

The reaction at room temperature of an aldehyde with excess pyrrole in the absence of solvent affords the meso-substituted dipyrromethane. The reaction is catalyzed with trifluoroacetic acid or with BF3ṡO(Et)2. The dipyrromethane is purified by crystallization or by flash chromatography on silica with eluants containing 1% triethylamine. The reaction is compatible with aliphatic or aromatic aldehydes, including 2,6-disubstituted benzaldehydes. Nine dipyrromethanes have been prepared in this manner in yields of 47–86%, indicating the broad scope of the reaction. The dipyrromethanes are stable in the purified form in the absence of light and air. Similar reaction with terephthalaldehyde and pyrrole affords the corresponding bis-dipyrromethane. The reaction of a meso-substituted dipyrromethane with an aldehyde under the conditions of the two-step one-flask porphyrin synthesis affords a direct route to trans-substituted meso-porphyrins. Acidolysis of the dipyrromethane is negligible under the conditions of the porphyrin-forming reaction. Four porphyrins bearing peripheral functional groups and facially-encumbering groups have been prepared which serve as key building blocks in the synthesis of linear porphyrin arrays.