Investigation of porphyrin-forming reactions. Part 4. Examination of the reaction course in syntheses of porphyrins via dipyrromethanecarbinols

The dipyrromethanecarbinol motif is a key component in rational routes to a wide variety of porphyrinic macrocycles. We have investigated the dipyrromethane-monocarbinol self-condensation and the dipyrromethane-dicarbinol+dipyrromethane condensation under four different acid-catalysis conditions and have characterized the oligomer composition (by LD-MS), yield of porphyrin (by UV-Vis), and yield of N-confused porphyrin (by HPLC). Under conditions giving no-scrambling, the condensations are rapid and afford an oligomer composition characterized by (1) absence of scrambling, (2) suppression of acidolysis, and (3) a disproportionate amount of long oligomers from which recovery does not occur. The irreversibility of the reaction and lack of recovery from the long oligomers may explain the lower yields of dipyrromethanecarbinol condensations (< 30%) compared with reversible reactions of aldehyde+pyrrole (similar to 50%). Modest levels of acidolysis and scrambling with the dipyrromethane-carbinols are obtained under reaction conditions that cause extensive scrambling in dipyrromethane+aldehyde condensations. Because both reactions give the same porphyrinogen, the scrambling processes must primarily involve the dipyrromethane or oligomer intermediates rather than the porphyrinogen. The dipyrromethane-monocarbinol self-condensation and dipyrromethane-dicarbinol+dipyrromethane condensation afford quite similar oligomer compositions, suggesting that both follow similar pathways to the porphyrinogen. Reaction conditions that suppress scrambling in the dipyrromethanecarbinol condensations were found to dramatically suppress formation of the N-confused porphyrin. Taken together, these experiments show how the greater reactivity of the dipyrromethane-carbinol unit (compared with the pyrrole+aldehyde reaction) facilitates synthesis of meso-substituted porphyrins without scrambling.