Investigation of porphyrin-forming reactions. Part 2. Examination of the reaction course in two-step, one-flask syntheses of meso-substituted porphyrins

Abstract

The reaction course leading to meso-substituted porphyrins was examined for reversibility (formation of oligomers, formation of α- and β-pyrrole linkages), inactivation of the acid catalyst, homogeneity of the reaction medium, and the pathway of oligomer formation. The methodology employed enabled characterization of the oligomer composition (LD-MS), yield of porphyrin (UV–Vis), yield of N-confused porphyrin (HPLC), and level of unreacted aldehyde (TLC). Experiments were performed with benzaldehyde and pyrrole with catalysis by TFA or BF₃–Et₂O. Key observations include the following. (1) Reactions with BF₃–Et₂O exhibited reversible exchange of oligomers throughout the reaction. With TFA, the oligomer exchange processes were reversible at short reaction times, but became largely irreversible over the course of several hours. (2) The BF₃–Et₂O activity declined during the course of the reaction, whereas that of TFA was little changed. (3) The reaction medium remained homogeneous at 10 mM pyrrole + aldehyde. (4) Dipyrromethanes comprised of α- but not β-linkages underwent cleavage with either TFA or BF₃–Et₂O. (5) Condensations with carbinol intermediates (pyrrole-carbinol, dipyrromethane-monocarbinol, dipyrromethane-dicarbinol) provided rapid reactions, lower yields of porphyrin, and longer oligomers than typical in reactions of pyrrole + benzaldehyde. Higher porphyrin yields were obtained with BF₃–Et₂O than TFA, which is attributed to the more facile recovery from longer oligomers with the former versus the latter catalyst. Collectively, these and other observations lead to a model for the aldehyde + pyrrole condensation comprised of a combination of irreversible and reversible reactions in oligomer formation, irreversible side reactions (formation of dipyrrins, β-linkages), and slow inactivation of the catalyst (BF₃–Et₂O).