Syntheses and Redox Chemistry of Antiaromatic Core-Modified Isophlorinoids

Abstract

During the attempted total synthesis of chlorophyll, Woodward hypothesized the formation of tetrapyrrolic 20π isophlorin as a transient antiaromatic intermediate which provides a plausible template to synthesize stable antiaromatic molecules. Despite its structural similarity with the 18π aromatic porphyrin, it significantly differs in its electronic and chemical properties. However, due to its unstable nature under ambient conditions it immediately gets oxidized to stable 18π aromatic porphyrin. Similar macrocyclic structures with β-substituted heterocycles, such as furan/thiophene/selenophene, have been synthesized, which undergo facile oxidation to yield the 18π porphyrin dication. Attempts to synthesize stable tetrapyrrolic isophlorin and its metal complex have remained unaccomplished till date. Strategies to synthesize stable core-modified 20π isophlorin and its confused isophlorin derivative have met with considerable success. Predominantly, they are synthesized by replacing three or four pyrroles with furan/thiophene. The 20π systems either with four furan units or with a ‘pair’ of furan and thiophene units were sufficiently stable enough to resist oxidation towards the corresponding porphyrin dication. The 20π isophlorins displayed noncovalent interactions with the curved π surface of fullerene which predominantly rises due to van der Waals attraction between the dissimilar π systems. This antiaromatic isophlorin-fullerene complexes were obtained and successfully characterized by single-crystal X-ray diffraction studies. Replacing only three pyrroles by furan rings yielded the first stable pyrrole derivative of antiaromatic 20π isophlorin, which can be reversibly oxidized to 18π aromatic porphyrin without deprotonating the inner pyrrole NH. In addition, replacing all the pyrrole units of N-confused porphyrin with thiophene yielded the first derivative of confused isophlorin. Further, its two-electron oxidation led to the formation of 18π aromatic cation with enhanced aromaticity. The structure and electronic properties of the oxidized and neutral species were unambiguously determined from a combination of spectroscopic techniques, X-ray crystallography, and computational methods. These studies reveal that antiaromatic systems like isophlorin and its confused isophlorin derivative can be stabilized under ambient conditions and they offer potential to explore the chemistry of 4nπ systems. This Account focuses of recent advances in 20π antiaromatic isophlorins, confused isophlorins, and nocorroles along with their redox chemistry. 1 Introduction2 Stable 20π Isophlorins2.1 Furan-/Thiophene-Based Isophlorins2.2 Pyrrole Isophlorins3 Confused Isophlorins3.1 Isophlorin-Fullerene3.2 meso-meso-Bridged Tetraoxaisophlorin Dimer4. Norcorroles5. Conclusion and Outlook