Syntheses and Redox Chemistry of Antiaromatic Core-Modified Isophlorinoids

Abstract

During the attempted total synthesis of chlorophyll, Woodward hypothesized the formation of tetrapyrrolic 20 pi isophlorin as a transient antiaromatic intermediate which provides a plausible template to synthesize stable antiaromatic molecules. Despite its structural similarity with the 18 pi 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 18 pi aromatic porphyrin. Similar macrocyclic structures with beta-substituted heterocycles, such as furan/thiophene/selenophene, have been synthesized, which undergo facile oxidation to yield the 18 pi porphyrin dication. Attempts to synthesize stable tetrapyrrolic isophlorin and its metal complex have remained unaccomplished till date. Strategies to synthesize stable core-modified 20 pi 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 20 pi 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 20 pi isophlorins displayed noncovalent interactions with the curved pi surface of fullerene which predominantly rises due to van der Waals attraction between the dissimilar pi 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 20 pi isophlorin, which can be reversibly oxidized to 18 pi 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 18 pi 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 4n pi systems. This Account focuses of recent advances in 20 pi antiaromatic isophlorins, confused isophlorins, and nocorroles along with their redox chemistry.