Isophlorinoids: The Antiaromatic Congeners of Porphyrinoids

Abstract

Ever since the discovery of the porphyrin ring in "pigments of life", such as chlorophyll and hemoglobin, it has become a prime synthetic target for optoelectronic properties and in the design of metal complexes. During one such early expedition on the synthesis of porphyrin, Woodward proposed that condensing pyrrole with an aldehyde under acidic conditions yields the "precursor" porphyrinogen macrocycle. Its four-electron oxidation leads to the "transitory" 20π isophlorin, which undergoes subsequent two-electron oxidation to form the 18π "porphyrin". Due to its fleeting lifetime, it has been a synthetic challenge to stabilize the tetrapyrrolic isophlorin. This macrocycle symbolizes the antiaromatic character of a porphyrin-like macrocycle. In addition, the pyrrole NH also plays a key role in the proton-coupled, two-electron oxidation of isophlorin to the aromatic porphyrin. However, a major aspect of its unstable nature was attributed to its antiaromatic character, which is understood to destabilize the macrocycle upon conjugation. Antiaromaticity in general has not gained significant attention mainly due to the lack of stable 4nπ systems. In this regard, a stable isophlorin and its derivatives provide a glimmering hope to peek into the world of antiaromatic systems. This review will focus on the attempted synthesis of antiaromatic isophlorin ever since its conception. Based on recent synthetic advances, the chemistry of isophlorins can be expected to blossom into expanded derivatives of this antiaromatic macrocycle. Along with the synthetic details, the structural, electronic, and redox properties of isophlorin and its expanded derivatives will be elaborated.

Formation and Isolation of a Four‐Electron‐Reduced Porphyrin Derivative by Reduction of a Stable 20π Isophlorin

“…[1] In addition, porphyrins with aphenolic moiety as aredox-active site at the periphery have been reported to show reversible multi-electron redox behavior by external stimuli. [3] However,there is no report of areversible redox system comprised of am ulti-electron pool based on aporphyrin framework without redox-active sites because of the instability of multi-electron reduced porphyrinoids. [3] However,there is no report of areversible redox system comprised of am ulti-electron pool based on aporphyrin framework without redox-active sites because of the instability of multi-electron reduced porphyrinoids.…”

“…[3][4][5][6][7] Some isophlorin derivatives have been isolated through the reduction of the corresponding porphyrins with highly positive central ions (Si IV ,G e IV ) [4] or core-modified porphyrins, [5] such as tetraoxaporphyrin, with 20p anti-aromatic character.However,noinvestigation on further reduction of isophlorin has been reported, in spite of the fact that ap orphyrin-based redox system is expected to act as areversible multi-electron redox system. [3][4][5][6][7] Some isophlorin derivatives have been isolated through the reduction of the corresponding porphyrins with highly positive central ions (Si IV ,G e IV ) [4] or core-modified porphyrins, [5] such as tetraoxaporphyrin, with 20p anti-aromatic character.However,noinvestigation on further reduction of isophlorin has been reported, in spite of the fact that ap orphyrin-based redox system is expected to act as areversible multi-electron redox system.…”

Facile Redox‐Induced Aromatic–Antiaromatic Interconversion of a β‐Tetracyano‐21,23‐Dithiaporphyrin under Ambient Conditions

“…Porphyrinoids, specifically expanded porphyrins, are undoubtedly key compounds in this field and a considerable amount of stable antiaromatic porphyrinoids was synthesized . Isophlorins are two‐electron‐reduced products of basic porphyrins (18 π aromatic), and therefore they potentially have 20 π antiaromatic properties . The basic isophlorins are generally unstable and difficult to isolate.…”

Triggering Redox Activity in a Thiophene Compound: Radical Stabilization and Coordination Chemistry

“…[6] Ligandcentered oxidation was also seen in Ni complexes of a similar bis(phenolate)dipyrrin ligand, with the one-electron oxidation product characterized as a ligand-centered radical. [7] In contrast to the nitrogen-containing heterocycles found in dipyrrins, porphyrinoids and sub-porphyrins, [8] studies on the redox activity of sulfur-containing heterocycles such as thiophene are more limited, despite their use in tuning the electronic properties of molecular compounds [9] and polymeric materials. [10] Expanded porphyrinoids featuring five thiophene units undergo single-electron oxidations, from the aromatic mono-anion to an isolable, air-stable neutral radical, and further to an anti-aromatic mono-cation.…”