From Porphyrin Isomers to Octapyrrolic “Figure Eight” Macrocycles

Abstract

Cyclooctapyrroles such as 1–3 tend to form during the condensation of dipyrrolic units if cyclization to cyclotetrapyrroles is hindered for conformational reasons. These macrocycles are chiral and might afford interesting complexes with metal ions. Their most intriguing feature is their unusual figure‐eight conformation.

Synthetic Expanded Porphyrin Chemistry

“…workers, [109,110] that expanded porphyrins could adopt figureeight conformations that were the complete antithesis of the flat structures one might have expected for conjugated heteroannulenes. Not surprisingly, therefore, the synthesis of these higher order systems provided an important stimulus for the field, both in terms of understanding the basis of these strange conformational features, and defining an implicit new challenge.…”

“…It thus resembles most other higher order expanded porphyrins which for the most part display similar nonplanar conformations. [33,109,110] The monoprotonated TFA salt of nonaphyrin (121·HTFA) was found to adopt a twisted helixlike conformation with a near mirror plane and a large cleft wherein the TFA counterion is bound.…”

“…The octaphyrins prepared previously by Vogel et al (113-116; shown with the b substituents omitted for clarity) [109,110] were the subject of recent studies targeting their electrochemical behavior as well as their ability to form metal complexes. [113] Similar redox patterns were observed for the two free bases 114 and 115: Both undergo five distinct redox steps, two reductions and three oxidations.…”

Giant Porphyrinoids: From Figure Eights to Nanomolecular Cavities

“…In related studies, [11] acid-catalyzed condensation of 13 with 10 failed to give corrole (14), the ª22º condensation product, but instead generated the ª2222º product figure eight conformation as demonstrated by NMR spectroscopy and X-ray crystallography. A third octapyrrolic system 16 was prepared [12] by the ª44º condensation of tetrapyrrole 17 with the related dialdehyde 18 in the presence of TFA, although better yields were subsequently obtained by treating bipyrrole 19 with 10 (Scheme 4).…”

“…Instead of isolating the expected tetrapyrrole 8, an octapyrrolic ª2222º condensation product 11 was obtained. [11] Dehydrogenation with Pd/C in refluxing toluene afforded the fully conjugated albeit nonaromatic [36]octaphyrin 12 (Scheme 2) in 85 % yield. The NMR spectra for 11 and 12 indicated the presence of diastereotopic CH 2 groups and figure eight conformations were postulated for these structures.…”