Venkataramanarao G. Anand

An Amphoteric Switch to Aromatic and Antiaromatic States of a Neutral Air‐Stable 25π Radical

Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)π open-shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)π and antiaromatic (4n)π electronic circuits. The synthesis and characterization of an air- and water-stable neutral 25π pentathiophene macrocyclic radical is now described. It undergoes reversible one-electron oxidation to a 24π antiaromatic cation and reduction to a 26π aromatic anion, thus confirming its amphoteric behavior. Structural determination by single-crystal X-ray diffraction studies revealed a planar configuration for the neutral radical, antiaromatic cation, and aromatic anion. In the solution state, the cation shows the highest upfield chemical shift ever observed for a 4nπ system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.

Antiaromatic Supramolecules: F⋅⋅⋅S, F⋅⋅⋅Se, and F⋅⋅⋅π Intermolecular Interactions in 32 π Expanded Isophlorins

jugated systems with structural features akin to annulenes, and have contrasting ring current effects. 4np macrocycles can react quickly to lose their antiaromatic character. The isophlorin/porphyrin redox couple further illustrates the unstable nature of isophlorin relative to porphyrin under ambient conditions. Similar to the isophlorin p framework, the inherently antiaromatic tetraepoxy annulenes are susceptible to oxidation/reduction to form the corresponding aromatic macrocyclic ions. Their large ring size induces structural flexibility leading to inseparable conformational isomers. Interestingly, steric hindrance favor macrocycles to undergo structure-induced loss of p delocalization. Giant porphyrinoids are well studied examples for macrocycles which adopt figure-eight conformations. Such nonplanar cyclic conjugated systems lack ring current effects and are considered to be nonaromatic in nature. Relative to aromatic systems, the experimental evidence reported for antiaromaticity is found to be far less than satisfactory. There are very limited reports to bridge theoretical predictions with experimental data, particularly for large antiaromatic systems. In this context, the synthesis of stable quintessential planar 4n p molecules is crucial not only to understand the electronic effects of p conjugation but also imperative to the development of novel materials for applications in molecular electronics. Expanding the p framework of either 1 or the modified isophlorin 2 is an attractive strategy for generating a variety of expanded isophlorins (3 and 4, respectively). Its effective conjugation and rigid structural skeleton affords a unique opportunity to explore the chemistry of antiaromatic molecules. But the synthesis of stable antiaromatic isophlorinoids is hindered by the inherent unstable nature of 4np systems. Amongst a few isophlorins known so far, the synthetic strategy employed for 2 is probably the simplest to obtain stable antiaromatic macrocycles from easy-to-make precursors. Adopting a similar synthetic strategy, with modified precursors, we describe the syntheses and structural features of novel and stable 32p vinylogous expanded isophlorins which display unprecedented short F···X (X=p/S/Se) and p–p intermolecular contacts for antiaromatic systems. Replacing two methine carbon bridges in 3 by an equal number of ethylene bridges can convert a (4n+ 2)p system into a 4np system. McMurry coupling is widely used to couple dialdehydes of thiophene oligomers into macrocycles with ethylene bridges. We adopted a modified Rothemund-type synthesis, (Scheme 1), wherein diols of five-membered heterocycles were condensed with bis(thiophene)s under

Reversible Redox Reaction Between Antiaromatic and Aromatic States of 32π‐Expanded Isophlorins

32π-Antiaromatic expanded isophlorins with a varying number of thiophene and furan rings adopt either planar, ring-inverted, or twisted conformations depending on the number of furan rings in the macrocycle. However, they exhibit identical reactivity with respect to their oxidation to aromatic 30π-dicationic species under acidic conditions. These 32π-antiaromatic macrocycles can also be oxidized with [Et3O(+)SbCl6(-)] and NOBF4 to generate dications, thus confirming ring oxidation of macrocycles. Furthermore, they can be reduced back to their parent 32π-antiaromatic state by triethylamine, Zn, or FeCl2. Single-crystal X-ray diffraction analysis confirmed a figure-eight conformation for a hexafuran system, which opens to a planar structure upon oxidation.

Controlled Core-Modification of a Porphyrin into an Antiaromatic Isophlorin

Partial core-modification of a porphyrin can be employed to synthesize the 20π antiaromatic isophlorin. Unlike the tetra-, tri-, and dipyrrole derivatives of a porphyrin, a monopyrrole porphyrin exhibits antiaromatic characteristics. It undergoes a two-electron reversible ring oxidation to yield the 18π aromatic dication. (1) H NMR analysis provides distinct evidence of the altered electronic characteristics through typical paratropic and diatropic ring current effects for the 4n and the (4n+2) π-electron systems, respectively.

“To Twist or Not to Twist”: Figure-of-Eight and Planar Structures of Octaphyrins

Amongst the various porphyrinoids, octaphyrin has attracted significant attention owing to its diverse syntheses, conformations, and metal-ligation properties. Octaphyrin is a higher homologue of porphyrin and is formed by linking together heterocycles such as pyrrole, furan, thiophene, and selenophene through α-α or α-meso carbon bonds. The planar conformation is mainly achieved through inversion of the heterocyclic units from the center of macrocycle; avoiding meso-bridges; introducing a para-quinodimethane bridge; employing a neo-confusion approach; protonation; and by generating dianionic species. In this Focus Review, recent synthetic advancements in the field of octaphyrins are summarized. The twisted conformation of the octaphyrin binds to two metal ions in a tetracoordinate geometry. The diphosphorus complex of octaphyrin represents the first example of a stable expanded isophlorin.

Two-electron Oxidation of a Twisted Non Anti-aromatic 40π Expanded Isophlorin

AbstractExpanded isophlorins are typical examples for stable anti-aromatic systems. Paratropic ring current effects are observed in their NMR spectra mainly due to their planar conformation. Herein we report the synthesis of the first twisted 40 π expanded isophlorin and also its two-electron oxidation to a 38 π dication. It sustains the twisted conformation for the 4n π and (4n + 2) π electrons. Due to the non-planar conformation, they do not display ring current effects in their respective1H NMR spectrum. NICS calculations reveal the non-(anti)aromatic features for the neutral 40 π and the 38 π dication species. Graphical AbstractA non-planar 40π expanded isophlorin with four thiophene and four furan sub-units has been synthesized from simple precursors. It can be oxidized to the corresponding 38π macrocycle with unaltered topology.