Trapping of Stable [4n+1] π‐Electron Species from Peripherally Substituted, Conformationally Rigid, Antiaromatic Hexaphyrins

Abstract

Peripherally substituted antiaromatic naphthorosarins have been synthesized for the first time. The synthesis was accomplished by acid‐catalyzed condensation of naphthobipyrrole building blocks with aromatic aldehydes. The naphthobipyrrole building blocks were synthesized by simple oxidative coupling of the corresponding pyrrole substituted aromatics. Solid‐state structural analyses of the synthesized naphthorosarins revealed that the presence of meso‐2,6‐dichlorophenyl‐ and 5,6‐difluoro‐substitution substantially alter the geometry and properties of the naphthorosarins. The substituents affect the redox potentials as well and, in turn, the proton‐coupled electron‐transfer processes leading to the formation of one‐ and two‐electron reduced forms of the corresponding naphthorosarins. One particular naphthorosarin that bears both peripheral fluorine and meso‐2,6‐dichlorophenyl substituents forms a stable 25 π‐electron species upon treating with TFA that was characterized by single‐crystal X‐ray diffraction analysis. The current study underscores how structural modifications can be used to fine‐tune the electronic features of naphthorosarins, including stabilization of odd electron species.

Synthetic Applications of Oxidative Aromatic Coupling—From Biphenols to Nanographenes

“…[181] However, the efficient intramolecular coupling of pyrrole rings can be achieved under carefully optimized conditions. [182] Ap rominent example was provided in 2007 by Müllen and coworkers,who synthesized the rim-fused hexapyrrolylbenzene 177 (Figure 13). [183] Thee lectron-rich nature of pyrrole was evident by the fact that the FeCl 3 -mediated cyclization initially led to an overoxidized dicationic form of 177,w hich could be reduced to the neutral 177 with hydrazine.M ore recently,t he authors have extended this synthetic method to include analogues of 177 with one,t wo,o rt hree pyrrole moieties replaced by benzene units (e.g.…”

“…As a consequence of their high electron density, pyrrole‐based compounds are often cumbersome substrates for reactions involving strong oxidants and acids . However, the efficient intramolecular coupling of pyrrole rings can be achieved under carefully optimized conditions . A prominent example was provided in 2007 by Müllen and co‐workers, who synthesized the rim‐fused hexapyrrolylbenzene 177 (Figure ) .…”

Post Synthetic Modification of Planar Antiaromatic Hexaphyrin (1.0.1.0.1.0) by Regio-Selective, Sequential SNAr

“…Such a larger shift is unique in these classes of macrocycles considering the fact that the meso-substituents adopt nearly orthogonal conformation against the rigid macrocyclic core. For instance, peripherally modified rosarrin analogues displayed minimal changes in absorption spectra compared to the parent rosarrin 2 [10,11]. We envisaged that macrocycle core possesses a modified dioxine ring at meso position which alters its dihedral angle and results such a bathochromic shift.…”

“…Metallation of rosarrin 2 with Rh(I) has led to the coordination of two Rh(I) on opposite side of the macrocyclic core with Rh(I)-Rh(I) interaction [9]. Successful modifications at periphery and meso-substituents of the rosarrin enabled to isolate a 25π-electron species as air stable form [10]. Furthermore, upon varying the meso-substituents from pentafluorophenyl group to bis(trifluoromethyl)phenyl group displayed strong, intermolecular 'face-to-face', π-π interaction between antiaromatic molecules.…”

Syntheseanwendungen der oxidativen aromatischen Kupplung – von Biphenolen zu Nanographenen

“…Verbindungen auf Pyrrolbasis sind wegen ihrer hohen Elektronendichte häufig problematische Substrate für Reaktionen mit starken Oxidationsmitteln und Säuren . Unter sorgfältig optimierten Bedingungen kann jedoch eine effiziente intramolekulare Kupplung von Pyrrolringen erreicht werden . Ein prominentes Beispiel lieferten 2007 Müllen et al., die das randkondensierte Hexapyrrolylbenzol 177 synthetisierten (Abbildung ) .…”