Rational Synthesis of Antiaromatic 5,15‐Dioxaporphyrin and Oxidation into β,β‐Linked Dimers

Abstract

5,15-Dioxaporphyrin was synthesized for the first time by an ucleophilic aromatic substitution reaction of an ickel bis(a,a'-dibromodipyrrin) complex with benzaldoxime,f ollowed by an intramolecular annulation of the a-hydroxy-substituted intermediate.T his unprecedented molecule is a2 0 p-electron antiaromatic system, in terms of Hückelsr ule of aromaticity,b ecause lone pair electrons of oxygen atoms are incorporated into the 18p-electron conjugated system of the porphyrin. Atheoretical analysis based on the gauge-including magnetically induced current method confirmed its antiaromaticity and ad ominant inner ring pathway for the ring current. The unique reactivity of 5,15dioxaporphyrin forming a b,b-linked dimer upon oxidation was also revealed.

Aromatic Pathways in Porphyrinoids by Magnetically Induced Ring Currents

“…The pyrrolic rings sustain a values were contradicting. 134 The aromaticity of Ni(II) 5,15-dioxaporphyrin deduced from the current density agrees with the accepted notion, whereas its aromatic pathway is somewhat unexpected, which deserves a more thorough discussion.…”

“…This was recently demonstrated by synthesizing Ni(II) 5,15-dioxaporphyrin, which was obtained by replacing two meso CH groups of Ni(II) porphyrin with oxygens. 134 The assessment of its aromatic character was challenging, because the electronic absorption spectra showed typical features of an antiaromatic porphyrin, whereas the 1 H NMR signals of the protons at the β carbons indicated that the molecule is weakly antiaromatic or nonaromatic. Calculations of the current density revealed that Ni(II) 5,15-dioxaporphyrin is antiaromatic sustaining a paratropic ring current of -10.9 nA/T along the inner-cross pathway as shown in Figure 21.…”

“…It highlights the strong paratropic ring current along the inner-cross pathway. Integrated current-density strengths (in nA/T) obtained for selected bonds are also given 134.…”

Enhancement of Antiaromatic Character via Additional Benzoannulation into Dibenzo[a,f]pentalene: Syntheses and Properties of Benzo[a]naphtho[2,1-f]pentalene and Dinaphtho[2,1-a,f]pentalene

“…The calculation of current patterns and ring current strengths in polycyclic πconjugated systems can precisely characterize the (anti)aromatic features of the individual rings and distinguish the electron-delocalization pathways of the global and local πconjugated circuits. 107,109,110 The magnetically induced current densities of the neutral and dianion species of 2a, 4a, and 5a are summarized in Figure 4. The GIMIC calculations perfectly supported the observed NMR results and NICS(1) calculations.…”