30π Aromatic Meso-Substituted Heptaphyrin Isomers:  Syntheses, Characterization, and Spectroscopic Studies

Abstract

The syntheses of new aromatic 30pi heptaphyrins either through a [5 + 2] or a [4 + 3] acid-catalyzed condensation and oxidative coupling reactions of easily available and air-stable precursors are reported. The methodology followed is not only simple and efficient but also allows synthesis of a range of heptaphyrins with different heteroatoms in the core. The oxidative coupling reactions of modified tripyrranes 11 and tetrapyrranes 12 were found to be dependent on the acid concentration used and as well as the substituents present on the meso position. The change of meso aryl substituents in 11 and 12 to meso mesityl substituents gave a new heptaphyrin 18. The structural characterization has been done with extensive 1H and 2D NMR studies. The heptaphyrins reported here show rich structural diversity when the connections of the heterocyclic rings are altered, and accordingly, one ring and two ring inversions have been observed. By a judicious choice of the precursors it has been possible to control the site of ring inversion either in the bithiophene unit or in the tripyrrane unit. Theoretical calculations performed on three different heptaphyrins, 4, 5, and 17, also reveal that the inverted structures are approximately 35-40 kJ lower in energy relative to the corresponding noninverted structures. Furthermore, one of the heptaphyrins 10c shows the presence of two conformers in solution in the ratio 1:2 and no interconversion between the conformers have been observed in the temperature range of 343-228 K. On protonation, the aromaticity and the ring inversions are retained and the deltadelta values vary in the range 10.07-20.59 ppm. The energies of the Soret maxima and the HOMO-LUMO gap vary linearly with the increase in pi electrons further justifying the aromatic nature of the heptaphyrins.

Synthetic Expanded Porphyrin Chemistry

“…[124] Continued advances in the area of heteroatom-containing expanded porphyrin chemistry were also reported by Chandrashekar and co-workers. [125] They obtained, among other species, the new heptapyrrolic macrocycles 135 and 136, and their conformational behavior was studied in detail. In a separate study, Hung et al reported that it was possible to obtain novel C-N bridged systems such as 137 and 138 by starting with an aryl-substituted tristhiophene-containing hexaphyrin analogue.…”

“…Unfortunately, the associated characterization chemistry is still in its infancy and it may thus be some time before the properties and potential applications of these new expanded porphyrins become fully appreciated. (125,Scheme 38). [117] A crystal- Figure 43.…”

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

“…A family of heteroheptaphyrins containing up to five thiophene or selenophene rings and four meso bridges in various patterns were characterized in solution revealing a variety of diatropic quasi-planar conformations containing at least one inverted subunit. [60] Representative examples of these systems include the uniconcave trithiaheptaphyrin 60b-H 2 (T0 B , t FI = 1.28), biconcave tetrathia species 59b-H (T0 A,D , t FI = 1.06), and uniconcave pentathiaheptaphyrin 58b (T0 C , t FI = 1. 19).…”

Figure‐Eight‐Strukturen, Möbius‐Bänder und mehr: Konformation und Aromatizität von Porphyrinoiden

“…Die Makrocyclen zeigten diatrope quasiplanare Konformationen mit mindestens einer invertierten Untereinheit. [60] Beispiele sind das unikonkave Trithiaheptaphyrin 60b-H [250] Symmetrie und Linienverbreiterung im Raumtemperatur-1 H-NMR-Spektrum lassen auf einen dynamischen Prozess schließen, aber das Konformationsverhalten von 66 e wurde bislang nicht untersucht.…”