Synthesis and characterization of a new [4.0.4.0] porphyrin‐like antiaromatic macrocycle

Abstract

The antiaromatic macrocycle 2 has been prepared by the reaction of the bipyrrole dialdehyde 1 with hydrazine. In the crystal structure of 2·DMSO molecule 2 is nearly planar, the mean deviation of the 28 atoms of the core from the best least‐squares plane being 0.227 Å. AM1 geometric optimization of the core of 2 leads to a fully planar macrocycle. Attempts to oxidize 2 to 3 were largely unsuccessful, yielding only limited evidence for the formation of 3. The antiaromatic precursor 2 turns out to be much more stable than the putative aromatic product 3. Molecular orbital calculations at the AM1/PM3 level indicate the S0–T1 gap for 3 is only about 0.2 eV and hence is readily accessible via intersystem crossing from any visible transition. This small band gap of 3 contrasts starkly with the much larger S0–T1 band gap of 1.86 eV calculated for 2 itself. Paratropicity in antiaromatic systems is usually attributed to a low‐lying paramagnetic state, with band gaps ranging up to 2.1 eV. For the 2/3 system the S0–T1 band gap is substantially larger for the paratropic species than for the putative diatropic species. © 1997 John Wiley & Sons, Ltd.

Cyclo[8]pyrrole: A Simple-to-Make Expanded Porphyrin with No Meso Bridges

“…It has, however, been employed to aromatize macrocycles 31, 32. Herein, we report a highly efficient one‐step synthesis of cyclo[8]pyrroles ( 2 ) that is based on the use of FeCl 3 as the oxidant and readily accessible α , α ′‐unsubstituted bipyrroles33–35 as the only organic precursors (Scheme ).…”

Cyclo[8]pyrrole: A Simple-to-Make Expanded Porphyrin with No Meso Bridges This work was supported by the National Science Foundation (grant CHE 0107732 to J.L.S.). The authors would like to thank Dr. Paul Fleitz and Weijie Su (Wright Patterson Air Force Base) for recording the UV/Vis spectrum of 2 b.

“…It has, however, been employed to aromatize macrocycles 31, 32. Herein, we report a highly efficient one‐step synthesis of cyclo[8]pyrroles ( 2 ) that is based on the use of FeCl 3 as the oxidant and readily accessible α , α ′‐unsubstituted bipyrroles33–35 as the only organic precursors (Scheme 1).…”

“…All solvents and chemicals were obtained commercially and used as received. The bipyrrolic precursors were prepared as described previously 33–35…”

An Overall Comprehension of Anti‐Aromatic Porphyrinoids Using 3D‐Graphical Chemical Shielding Description

“…for 4 , [ 17 ] and (−4.7–−8.7 a.u.) for 5 , [ 18 ] respectively. More importantly, the IDR is also reduced from 1315 a.u.…”